How you get electric shocks

I have sent a lengthy article to this blog some time back on how a person can happen to get electrical shock. However, some readers complained that it was too long that they have difficulties finding what they want to read about. Therefore, I have decided to write a few short articles on subtitles covered by that post for the benefits of casual readers who are looking for only certain very specific issues. This post is the first of those subtitles.

Look at the four beautiful diagrams that I just created below.

Diagram 1 – Touching live electrical wire and the neutral wire



THE SIMPLE LOGIC OF ELECTRIC SHOCKS

Electric shock happens when a certain amount of electrical current happen to flow through a person’s body for whatever reason. The human body has not been designed to conduct electricity above a certain current level or miliamperes. The above-mentioned post on electric shock protection gives a detailed description on the levels of current that can cause injuries and electrocutions. You can read it here, ELCB - Home Electrical Shock Protection, if you wish to know them in more details.
What happen when the electrical current flow through the body? At a lower level, the current interferes with the nervous system. That is why when someone accidentally holds a live wire with her hand, she may not be able to let go of the wire. In fact, her fingers may even grip the live wire tighter, resulting in a much better contact between the hand and the wire. This will result in lower contact resistance that will further increase the current flow and therefore more serious injuries. If you visit this post, Electrical injury pictures, you can see the pictures of how serious the electrical injuries can be.

The higher the shock current, the more severe the injuries

If the shock current is higher, then the body tissue where the current flows will get damaged. The higher the current, the more serious the damage. At 10mA, the victim may still be able to control his arm and instinctively release the electrified object or wire he is holding. However that is about the maximum shock current that he can handle. If the shock current is higher, he would lose control of the arm. The hand grip on the tool may get stronger, resulting in higher current flow.

At 100 mili-amperes (that is 0.1A), the victim faces a certain death if he sustains that level of shock current flow for 2 seconds or more.

The degree of injuries depends on which body parts the shock current flow through

The path where the shock current travels through the body also will have some effects on how serious the injuries can be. If a person touches a live wire by his right hand while standing on a grounded metal part, the electric current flow through from the live wire through the hand, the arm and shoulder, the chess, legs and down to the metal part. In this case, the current can damage the victim’s heart while flowing through the chess. At 30 mili-amperes, the victim may even stop breathing (respiratory paralysis).

The higher the voltage, the more serious the injuries

The higher the voltage, the more serious the resulting injuries. At 600 volts, the electric shock current can be as high as 4 amperes. This level of current can seriously damage the victim’s heart and other internal organs. The skin where the body makes contact with the live wire can be burned. The tissue damages are so serious that a limb on the path of the flow of the shock current can come off the victim’s body.

The longer the current flow through the victims body, the more serious the injuries

As I said above, at 100 mili-amperes, the victim faces a certain death if that level of current continues to flow through the body for more than 2 seconds. This is very useful information for those who work on live electrical parts either by occupation or just a do-it-yourself (DIY) working on a live house wiring.

The way you stand or the way you access the live parts should be in such a way as to make the duration of contact with the live part as short as possible in the event of accidental contact. Of course, you must at all times de-energize the faulty part before starting on the repair works. However this advise very often are just ignored by some people either because they feel they are skilled enough to work on live parts, or they just don’t bother much with their own safety.

Whatever the reasons are, you can actually position yourself such a way that at the moment of accidental contact, your reflexes will cause your body to break contact immediately. This quick contact break will mean life or death.

About a week ago, a news broke out about a fatal accident involving electrical works has occurred at a project not far from the jobsite I was working on. After some checking, I finally found out what actually has happened that led to the immediate death of the worker. Actually he was trying to cut an insulated electric wire inside a recently completed building. The building has just been handed over to the client and most of the tenants were busy with their own renovations works before they actually move in to the new place.

So the worker was trying to cut a live internal wiring that was supposed to have been dead and isolated by a colleague. As it happened, his colleague isolated a wrong circuit breaker on the electrical DB.

We can talk about isolation procedures in other posts. What I wish to point out here is the way the tool is handled in the case of a wire cutting. The cutter is held with the palm of the hand and a few fingers forming a strong grip. This form of hand action is difficult to let go in the case of an accidental contact.

Some other forms of hand action will more likely result in breaking contact by the body reflexes as the electric shock is sensed. One example is the turning of a termination screw at a circuit breaker terminal using a screwdriver. This sort of issue can be quite subjective and different persons may handle a tool a bit differently. But I think all experienced technicians agree that there is a room there to make the injuries less severe if it has to happen.

The higher the skin contact resistance, the lower the shock current

What does this mean in layman terms? I know these electrical terms can be intimidating to some people. However, electrical safety should be an issue for concern to everyone who use electricity on his or her houses or at their workplaces.

Look at Diagram 1 again. In this case, the girl is almost touching two electrical wires. When both hands touches the wires, the shock current can travel from the left hand holding the red live wire to the right hand holding the black neutral wire.

How much current will actually flow?

That depends on three things. 1. How many volts of voltage at the red live wire; 2. How many volts of voltage at the neutral black wire; 3. How much resistance is presented by the body against the current flow from left hand to the right hand.

Let’s answer this question one by one. For question 1, if you live in London, that voltage may be 220 volts. (Note: Do not get confused by the “volts” terminology. A volt is like speed. 3A or 3 amperes is like 3 meter per second, not like 3 apples per basket.) If you live in Kuala Lumpur, that voltage level may be 240 volts.

However if you live in Los Angeles, your house power sockets will have 110 volt supply. So the red wire there will have 110 volts of voltage.

For question 2: The voltage there will probably be zero volts in all the three cities mentioned above. However there are situations where the voltage at the neutral wire there is not zero. So the neutral wire is not always safe. Always keep this in mind when working on electrical wiring or equipment.

Before we go to question 3, let us do a little bit of calculations first. Since the voltage at the left hand is 240 volts (assuming you are visiting Kuala Lumpur), and 0 volt at the right hand, then the voltage different between the two points is

240 – 0 = 240 volts.

An electrician will call this a 240 volts of potential difference. If the black neutral wire is injured somewhere on the floor, and it touch a live faulty equipment which is at 200 volts, then the potential difference is

240 – 200 = 40 volt, which is a lower value (and hopefully less dangerous).

So how much shock current will flow through the girl’s body if the potential difference is 240 volts (assuming 240 – 0 volts)?

That will depend on how much resistance is presented by the girl’s body against current flow from the live wire and the neutral wire.

So how much is the normal body resistance? This is actually Question No 3 above.

Well, when the skin is dry, the body can present a resistance as high as 100,000 ohms (that is 100 kilo-ohms) against the current flow.

The magnitude of current is lower as the body resistance goes higher. So in this case, the magnitude of the shock current will be:

Shock current = voltage difference divided by body resistance
= 240 volt / 100,000 ohm
= 0.0024 ampere

That is 2.4 thousandth of an ampere of electric current flow, or 2.4 mili-amperes, or 2.4 mA.

How much damage will this amount of current do to the human body? Again you can see the detail list of the injuries in the above-mentioned post. However I listed a few of them here for easy comparison and to help you appreciate the size of this shock current from the viewpoint of injuries that it can cause.

At 1 mA – a normal person would feel a slight tingling sensation.

At 5 mA - A light shock will be felt, but most persons will be able to “let go”. Not a painful feeling, but definitely disturbing. However, a strong reflexive movement by the victim can cause further accidents and other type of injuries.

At 6 to 30 mA - The victim can be paralyzed, or the muscles will freeze (will not be able to release a tool, wire, or other object).

Painful, and my not be possible to let go.

At high voltage (above 600 Volt), this current can already cause severe burns.

As you can see above, at 5 mili-amperes or below, a normal person should still be able to let go the electrified object the he accidentally came into contact with. So a person with dry skin would be less likely to suffer severe injuries if he accidentally comes into contact with a 240 volts live wire. A 110 volts domestic voltage at American house will be less than half of that shock current, so it would be safer theoretically (the American safety follows a different standard so we cannot really compare them apple to apple).

Now let’s see what will happen when the skin is wet. The wet condition can result in a very much lower skin resistance to electric shock current. It can be as low as 1,000 ohms. Therefore the shock current is

Shock current = 240 divided 1000
= 0.24 ampere, which is 240 mili amperes!

From the list that I copied above, at 30 mA the victim would surely not be able to let go of the electrified object he was holding. Therefore the shock current through the body will eventually rise to the maximum that is limited by the body resistance which is 240 mA.

So the victim can suffer a maximum level of injuries. The list on the other post says:

(At 75 mili-Amperes and above – The victim undergo ventricular fibrillation (very rapid, ineffective heartbeat). This condition can cause death within a few minutes. The only way to save the victim is by a special device called defibrillator.

So there you have it. The comparison of the level of resulting injuries between the lowest and the highest skin contact resistances.

BASIC CASES OF ELECTRIC SHOCKS

I started out this post with the intention of explaining about how someone can get electric shocks. But I seem to have spent this far explaining more on the logics of electric shocks.

However, the understanding of the working principles, or the mechanics, behind these accidents will be more useful to you in achieving the most important objective: that is to prevent all serious injuries that result from electrical shocks.

It would be nice to be able to prevent electrical accidents altogether. However as any construction man would testify, it is near to impossible to prevent accidents at construction sites.

The rough environment that a temporary electrical installation is subjected to, the nature of the electrical users at the jobsite, and the difficulties faced by persons responsible on electrical safety on large construction sites to ensure adequate level of safety habits are actually practiced throughout the construction grounds and floors. These three constraints make it near impossible to totally prevent electrical accidents on large construction sites with thousands of workers, especially those with small site areas such as high rise construction at city centers.

What we can do is to minimize the degree of electrical injuries when they do occur.

However, electrical accidents at home or office can still be prevented totally. That is my opinion.

Now THE how to get electric shock No 1. As shown in Diagram 1 at high up in the beginning of this post, that can happen when two locations on the human body came into contact with conductive parts at two different voltages. I have used this diagram to explain the whole upper part of this post above. So that should be enough.

Diagram 2 -Touching live and earth wire


In this case, the right hand came into contact with the earth wire instead of the neutral wire shown in Diagram 1. The result can be similar. I said “can be” instead of “will be”. Why? Because in this case, the live red wire can be installed with an electric shock protection device that will sense this shock current.

The devise is the ELCB (earth leakage circuit breaker). Other names are also used to explain this function in an electrical installation such as RCD (residual current devices), GFCI (ground fault circuit interrupters), etc. Read this post ELCB Circuit to know more how this device functions.

Diagram 3 – When the hand touches a live wire and the person is not separated at the feet from the ground with purpose-made electrical insulation means like rubber shoes, the rubber mat in front of the switchboards in electrical rooms, etc.



Diagram 4 – When there is simultaneous contacts with fault electric motors and ground.



The electric motor can be any electric equipment or appliances. The same process of electric shock current flow will happen.

There are more points that I wish to write down here. However it’s already almost three AM and I have to go to work tomorrow. So I will continue this some other time, in another post.

Stay safe. Electricity kills.

RELATED ARTICLES: a) Home electrical wiring, symbols and checking;

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ELCB – Home Electrical Shock Protection

This post discusses in some details the home electric shock protection, and the ELCB as one of the most important components in the system.
I have also added into this post some details on the dangers of electric shocks and the type on injuries may be suffered by the victims. However, if you like you can also go straight to the article on shock injuries. But if you are actually looking for electrical injury pictures, just go straight there.
This is a long post, so I noted below the content list to guide you as you scroll down to the bottom.

Content:

A. What is an ELCB
(Image: picture of home electrical panel)

B. Why do we need an ELCB
(a) The danger of electrical shocks
(b) Injuries caused by electric shocks

C. How electrical accidents (i.e. electrical shocks) happen
(a) General
(b) How house wiring works
(c) How electrical shocks happen

D. The working of ELCB
(Image: typical ELCB circuit)
(a) What does ELCB need in order to operate properly
(b) Grounding

E. Other variations of ELCB
F. How to test ELCB

A. What is an ELCB

The earth leakage circuit breaker or commonly called the ELCB is located inside the home electrical panel or distribution board. This component of the home electrical installation is designed to detect any leakage of electrical current.
This so-called leakage current occurs when there are some defects in the performance of some part of the installation, which can be caused by faulty components or by injuries to the insulation of the wiring, cables, electrical appliances or other accessories such as the switches and socket outlets.
When the current leakage occurs, the ELCB then trips the electricity supply within a fraction of a second of the leakage being detected, before the magnitude of the current reaches a lethal level.
Therefore any possibility of serious injuries due to electric shock to persons who are in contact with the electrical installation at that particular moment is minimized.
The use of ELCB in house wiring is required by law and omitting it is a serious offence.
Shown below is a picture of a typical home electrical panel. The ELCB is the second component from the left, with the letters CLIPSAL on it. The left-most component is the main switch (in black color), while all other components on the right of the ELCB are the outgoing circuit breakers.

B. Why do we need an ELCB

(a) The danger of electrical shocks

Whenever you use electrical appliances and equipment whether at home or at work, there is always a risk of hazards, especially of getting electric shock.
The refrigerator, the washing machines, the space heaters or even electric toys and entertainment equipment; they are all electrical hazards to users and household members at some point in appliances’ life cycles.
Eventually these harmless machines and toys will become liabilities and accidents waiting to happen. You need to watch out for them.
Of course, workers face a much higher risk of electrical injuries and electrocution and this not only apply to workers who use electrical tools.
Haphazard site work conditions usually have temporary electrical supply cables and electrical extension cords running all over the place on the work floors. All these situations present very high risks of electrical accidents to all workers regardless of their trades and disciplines. These are how you get electric shocks.

Workers who work on live electrical equipment and electrical distribution and wiring may find the second part of this article too familiar and therefore an unnecessary reminder for them. However, accidents usually happen when we least expect it. Ever heard of a circus tiger killing its own master?
So do not take shortcuts when safety procedures at work are concerned. All safety procedures are there for a reason even if sometimes we are all very reluctant to follow or even to accept some parts of them.
Lastly, let us not forget about the creative do-it-yourselfers. From statistics released by many organizations, many house electrical fires have been a result of imperfect electrical jobs carried out by the not-so-skilled DIY hobbyists. You can bet electrical shocks and electrocutions are part of the lists.

(b) Injuries caused by electric shocks

Let us go directly to what actually happens to a human body when electric current flow through by accident (or by intention…).

Effects of the shocks

Listed below are the effects of electric shocks starting from the lowest amount of current flow to the highest for a duration of one second at typical household voltages.

1 mA - A normal person will feel a slight tingling sensation.

5 mA - A light shock will be felt, but most persons will be able to “let go”. Not a painful feeling, but definitely disturbing. However, a strong reflexive movement by the victim can cause further accidents and other type of injuries.

6 to 30 mA - The victim can be paralyzed, or the muscles will freeze (will not be able to release a tool, wire, or other object

Painful, and my not be possible to let go.

At high voltage (above 600 Volt, this current can already cause severe burns)

Women start to suffer the effect at lover current levels (6-26mA), while men can sustain until a bit higher (10 to 30 mA)

30 mA - Will cause respiratory paralysis
(The victim stops breathing for a period of time)

30 mA - This is the most sensitive rating of Earth Leakage Circuit Breakers (ELCB) normally installed in residential home in this country.

50 to 150 mA - The victim get an extremely painful shock.
The breathing stops (respiratory arrest).
Severe muscle contraction: flexor muscles may cause holding on, extensor muscles may cause intense pushing away.
Death is possible.

(At 75 mili-Ampere and above – The victim undergo ventricular fibrillation (very rapid, ineffective heartbeat). This condition can cause death within a few minutes. The only way to save the victim is by a special device called defibrillator.)

1 A and above - Uneven heartbeats occurs (Ventricular fibrillation).
The muscles will contract.
Damage to the nerves.
Death is likely.

4 A - The victim gets heart paralysis, which means the heart stops pumping.

(Highlight: How much is 4 amperes? If you connect a 1KW portable space heater to a wall socket outlet, and your house supply from the electricity company is 240 Volt, then that’s about 4.1 amperes running inside the wires from the socket to the space heater.)

5 A and above - Human tissues get burned.

10 A and above - Cardiac arrest and severe burns.
Death is probable.

Note: The above medical data has been obtained from the National
Institute for Occupational Safety and Health (NIOSH)

13 A - The lowest current a typical plug fuse will blow in a socket – plug supply connection.

15 A - Lowest level of current a normal circuit breaker or fuse will trip at a home distribution board, or a house electrical panel.
Further explanations on the electric shock injuries
The higher the current, the longer the time of the shock current, the more severe the injuries

(a) As you can see above, the higher the current that flow through a human body, and the longer it flows, the more serious the injuries.
If the shock is short in duration, it may only be painful. A longer shock (lasting a few seconds) could be fatal if the level of current is high enough to cause the heart to go into ventricular fibrillation.
100 mili-ampere current flow (that is one tenth of an ampere, or 0.1 Ampere) through the body will kill a person in just 2 seconds. Maybe he does not die immediately, but death is almost certain after sustaining 100 mA for 2 seconds.
(b) A person can only withstand less that 10 mili-amperes and still have control of his arm muscles. Beyond that, he no longer has control over his arms. That is the reason he cannot let go of the faulty tool he is holding (the hand may even tighten the grip on the electric tool), resulting in longer flow of shock current through the body thereby making the injuries more serious.
This situation when prolonged will lead to respiratory paralysis (the muscles that control breathing cannot move.)
That is part of the reason for the requirements to have install Earth Leakage Circuit Breakers (ELCB) for circuit supplying electrical tools. The ELCB can detect very small amount of leaked electrical current and trip that circuit within a fraction of a second thereby saving lives.
A severe shock can cause much more damage to the body than is visible. A person may suffer internal bleeding and destruction of tissues, nerves, and muscles.
Sometimes the hidden injuries caused by electrical shock result in a delayed death.
If a shock current is maintained long enough at a relatively high current, death is probably not avoidable.
But if somehow the contact area to the electrified object is broken fast enough and the victim’s heart has not yet been damaged, his normal heartbeat may return, even though this type of recovery is rare.

The severity of injuries depends on which part of the body does the shock current flow through

The most serious effect is when the current flow through the heart.
If a live wire accidentally touches the body by contact at the head, the nervous system will be severely damaged.
If during the accident the victim’s right hand touches the LIVE wire, while the left hand is holding the metal casing of the washing machine, the electrical current will flow through the chest. Then the lungs and heart will probably be injured.
Of course how severe will also depend of how many mili-amperes and how long the shock current flows.
If the current only flow through the arm portion, then the injuries can be as bad as the arm coming off while the victim still survive (not dead). There have been actual cases like these in high voltage accidents.
If the current does go through the chest, the person will almost surely be electrocuted.
A large number of serious electrical injuries involve current passing from the hands to the feet. Such a path involves both the heart and lungs.
This type of shock is often fatal.

A higher skin resistance will lower the shock current.

(a) Again the current is inversely proportional to the resistance. If the victim’s body is dry, then the shock current through his body will be lower. Then the injury will be less severe.
The resistance of a dry skin is can be 100,000 ohm or more. While that of a wet skin is only approximately 1,000 ohm.
At 600 volts, the dry skin resistance will only allow 6 mA at the most, while the wet skin can allow 600 mA to flow through the body.
Compare this to the list of injuries above and you can appreciate the extreme importance of dryness in the effort to avoid electrical shock.
Even at 240 volt, the wet skin will allow 240 mA to flow through the body, making very severe injuries and even death possible.
(b) Other wet skin, wet working conditions will have the effect because they can make the skin wet and reduce resistance. Likewise, a damaged or broken skin.
(c) The resistance will also be reduced in direct proportion of the cross-sectional area of the path current. This means that when the contact made to an electrified object with an applied force as opposed to touching it with the tip of the fingers, the contact area will be larger. Therefore, the resistance to the current flow will be lower and the shock current will be higher.

Very Low Voltage also can kill

(a) The severity of the injury can increase the longer the victim is exposed to the shock current. Because of that, even low voltages can be extremely dangerous because the degree of injury depends not only on the amount of current but also on the length of time the body is in contact with the circuit.
Some victims have stopped breathing when shocked with currents from voltages as low as 49 volts.
For example, a shock current of 100 mA applied for 3 seconds can cause injuries as severe as a current of 900 mA applied for a fraction of a second.
(b) The victim’s muscle structure also plays a factor. People with less muscle tissue are typically affected at lower current levels.

The higher the voltage, the more serious the injuries.

(a) A current flow is directly proportional to the voltage supplying the current. That is why the higher the voltage, the higher the shock current flowing through the victim’s body. Therefore, he injuries will be more severe.
(b) At high voltage (i.e. 600 volts), the shock current can be as high as 4 amps. That amount of shock current will damage the hearts and other internal organs. In addition, internal blood vessels may clot, and the nerves in the area where the skin touches the electrified object may be damaged.
(c) High voltages can also cause severe tissue burns. A strong shock at the limb can cause the limb to come off.

Higher voltage can cause further accidents, therefore additional non-electrical injuries.

(a) Sometimes high voltages can lead to additional injuries. High voltages cause violent muscular contractions. The victim may lose his balance and fall, which can cause further injury or even death if he falls into machinery that can crush him.
(b) Bones can be fractured as a result from extreme muscle contractions during the shock, or cause by falling from working height.

C. How electrical shocks happen

(a) General

A person gets an electric shock when an electrical current passes through the body. This happens when he comes into contact with live metal or live electrical conductors, which will cause the current to flow through our body.
A human body is not supposed to receive this sort of electrical current. Therefore, it is a shock and the body will suffer injuries. These injuries can be serious and can even be fatal.

(b) How the house wiring works

House electrical system in this country normally has three wires with one of them usually green color. The other two black may be black.
Another possible combination may be one black and the other red, yellow, or blue.
The green wires are connected to the earth through steel rods half an inch in diameter driven a few feet into the ground. The same ground wires at electrical substation or at power plants (i.e. diesel electric generators) are also connected to the earth mass the same way.
So the house green earth wire and the electrical plant’s earth wires are actually connected to each other via the earth mass. (Remember that earth mass contain water and minerals. Therefore, it is a good conductor of electricity, just like the electrolyte water inside the car battery).
So these green wires are at 0 volts.
One of the other two black wires, or the colored wires, in the house is at 240 volts and it is normally called the LIVE wire.
The last wire is the return path (also called the NEUTRAL wire) for the electrical current in normal operation. Remember that the current flow must return back to the substation (or the generator) for the electricity to work.

Visit this post, Home electrical wiring, symbols and checking, to know more about how a house wiring works.

(c) How electrical shocks happen

(i) When two wires are at two different voltages and they come into contact with each other, electric current will pass through them.
If they are not in contact, but your body connects the two wires by touching both of them at the same time, then electric current will pass through your body and you will get the electric shock.
(ii) If your body is in contact with the LIVE wire, while another part is touching a grounded object, you will also get an electric shock.
(Notes: A grounded object is:
1. Anything that in good contact with earth mass, like water pipes. Most authorities require that water pipes be grounded.
However, even if some pipes are not grounded, the pipes that are laid partly or wholely underground (i.e. the mater mains coming to the house) are actually in good contact with earth mass.
2. Or anything that is purposely connected to the earth mass, like the street lamp posts in front of the house.
Do not be fooled by the innocent-looking lamp posts, the compound lighting posts or the short bollard lighting posts at the park or by the street in front of the house. Many grown adults have actually been electrocuted and died just by leaning to the metal posts.
3. Or anything that is electrically connected to the house earth wires, like the metal conduit, the metal casing of the washing machines, etc.)
(iii) There is a much higher risk of electric shock if you are standing in a pool of water. For example when your kitchen floor is all wet with water from the overflow at the kitchen sink, or while you are cleaning your kitchen floor with water.
Worst still if the wet floor is at the living room because the a few power outlets there are normally installed at lower level (approximate 12 inches above floor level).
(iv) The chances of being electrocuted will be much higher under certain conditions like wet clothing, high humidity and perspiration.
(v) Electrical equipment that is not properly grounded also can cause electrical shock. A typical example of this scenario is when an electrical appliance cord has a 3-pin plug, but you attach a plug adapter to it so you can plug it into a 2-pin socket.
(vi) If you touch a person who is receiving an electrical shock, you will also get the shock. So, if you suspect that a person is down because of electric shock, do not rush to go and touch him.

D. How an ELCB operates

A (maybe) more technical name for the ELCB is Residual Current Device or RCD. This is because ELCB detects a current leaking to earth and uses this current to operate a tripping mechanism which then open the circuit breaker, stopping the incoming power supply. The current leaking to earth is a residual current so that gives the device its name.
The Figure shown below illustrates a typical schematic construction of an ELCB. A close observation of the schematic will reveal the principles of it operation.


On a healthy circuit the same current passes through the phase coil, the load and then return back through the neutral coil.
Both the phase and neutral coils are wound on a common transformer core such that they will produce opposing magnetic flux. With the same current passing both coils, their magnetic effect will then cancel out under a healthy circuit condition.
In a faulty circuit, the line current will be higher than the neutral currents, so the the line coils produce a stronger flux than the neutral coils. So the magnetic flux does not cancel out and there is a resultant or residual magnetic flux in the common transformer core.
Since this magnetic field is alternating and it crosses with the turns of the search coil (in the middle), a an electrical voltage develops in the search coil. This voltage then drives the current through the trip coil, which then trip the circuit breaker.
Observe that if a fault develop between the phase and neutral wires, the ELCB will not trip. This fault is seen by the devise just like a normal load with maybe a different wattage.
In order to safeguard the person touching an energized metal conductor (i.e. the load metal casing) during the leakage, the ELCB circuit is designed to detect a leakage current as low as 5 to 30 mA, and trip the circuit breaker in less than 0.1 second from the starting time of the leakage (the point of time the person touches the energized metalwork).
This will ensure that the shock current flow is removed or stopped before it reaches 50 mA, the lethal limit ( or so the experts say) for human.
A test switch is always provided on the ELCB so its operation can be regualrly tested easily. The test button works by bypasing the return coil. This simulates an out-of-balance condition so it trips the circuit breaker.
A test resistor is provided to limit the magnitude of the bypass current, and that is also used to check the sensitivity of the ELCB.
However it should be remembered (as explained in details in the following section) that the test switch only confirm the health of the ELCB unit, it does not check or confirm the condition of the shock protection system.

(a) What does an ELCB need in order to operate properly?

What does an ELCB need in order to work properly? I apologize for playing with words, but I am actually trying to emphasize a very important point. The question should be what does the shock protection need in order to work properly. The electricity users need to remember that electricity that comes into their houses carries with it dangers.
So the electrical equipment that is installed is not just to provide them the electricity, but also to protect the users from the accompanying dangers. There is a HUGE difference there.
So when we think about the electrical equipment and other electrical things in our home, think not just the electric uses that the equipment is supposed to provide, but also the dangers that the equipment is designed to protect us from.
That way we can train ourselves to be more alert to the hazards from electricity supply.
This point is particularly important in the case of ELCBs. An ELCB may be functioning properly and you can check this easily. While the electricity is on (i.e. not during the mains blackout) just open the electric panel cover, locate the ELCB unit and the TEST pushbutton on it. The test pushbutton will test whether the ELCB unit is working properly or not.
Now gently push the TEST pushbutton on it. If it trips (i.e. the ON/OFFswitch will snap and drop to the lower position. You can see the OFF label or symbol), then the ELCB is working properly.
Since the ELCB is working properly, then you are safe, right?
WRONG. The test facility provided on the home ELCB will only confirm the health of the ELCB unit, but that test does not confirm that the ELCB will trip when an electric shock hazard occurs. This misunderstanding has left many homes totally unprotected from shock risks.
This brings us to the second requirement for the proper operation of our home electrical shock protection system, which is the electrical grounding.

(b) A Functioning Electrical Grounding System

You can think of the ELCB as the brain for the shock protection, and the grounding as the backbone. Therefore, without a functional grounding there is totally no protection against electrical shocks in your house.
A brief on the grounding has been given at the earlier section of this article and I will not go into the technical details of the grounding today. That will be a title for another post in the near future, but a few major points must be stressed here to complete the lesson on Earth Leakage Circuit Breakers.
(a) An improperly grounded home electrical system is a serious hazard. In fact it is a serious hazard in any electrical system.
Unfortunately that is the most common violations of the electricity by laws. That is how prone people and companies are to overlook how important it is. Visit Home electrical earth installation for a more detailed discussion of house electrical grounding.
(b) All exposed metal parts and casing of the home wiring must be connected to the grounding system and it must be at 0 volt. Otherwise,they can be energized. How do we know that they are at zero volt? By testing, of course.
This is also another reminder for the DIY hobbyists who like this stuff. After you extend wiring, install additional electrical equipment (maybe like a small compressor in the garage), or whatever, ALWAYS test the new installation PROPERLY.
I know instruments that can really test this is not very cheap (notlike the multi-meters that you use to play with your electronic toys), but electricity is dangerous and the risks are loss of human lives, house and properties.
So either you buy one, borrow one, or get a qualified electrician to do it for you.
c) Ensure that all appliances and equipment that are plugged in also have their ground wires properly plugged in. Plug into safety first before you plug them into the power sockets. The metal casing of these things may be energized at some point of time.
If not properly grounded, the leaked voltage cannot be channeled into the ground, and therefore it cannot trip the ELCB. So anybody who happen to touch the casing will get the electric shock.
(d) The previous point brings us to the point of extension cords. They are one of the most highly abused electrical components in a home. Theyare very useful and very flexible. However extension cords are meant to be a temporary measure.
Do not use them as a permanent wiring. Due to its fragility the ground wires of these cords shouldn’t be relied on for safety, especially one of human lives.
So if you still insist onusing the extension cords, regularly check them for damage and broken connections.
(e) In many areas the metal water pipes are used as the grounding conductor to the earth mass.
If this is the system practiced in your place, make sure that all your pipe works are made of metals. Pay a particular attention to renovation works that might have been done to house or the piping (maybe by the previous owner).
If part of the piping have been upgraded or changed to non-metal piping then the grounding system now no longer works. Many house fires and electrocutions actually happened because of this type of errors.
If you are not sure about this at your house, and you are not good enough to check this yourself, then a qualified electrical contractor must becalled in to check it. A new grounding conductor that run to the whole house may need to be done just to be sure you have a proper grounding system in place.
(f) The last point I wish to emphasize on the grounding issue is about abuse, theft and vandalism to the copper parts of the electrical grounding. The abuse and vandalism part have always been there, but they were never a big issue.
However the issue of theft to electrical grounding (which also cover the lightning grounding) has become important in many places.
Maybe part of the reason is the rising cost of copper materials in thelast few years. Whatever the true causes are, the trend is clearly rising and this factor is very important to the integrity of the shock protection system (and the building's lightning protection system).
This is due to the fact that the system operates in silence and it is generally maintenance-free. Any missing part along the main ground path may not be noticed until major damages have been done or serious injuries have occured.
This brings us to the very reason why the electrical ground system must be regularly inspected. This is the only way to ensure it is working and that the electrical shock protection system is providing an optimum protection.

E. Other variations of ELCB

Other names are also used to call the ELCBs, the most common beingResidual Current Circuit Breakers (RCBs), Residual Current Devices(RCDs) and Residual Current Circuit Breakers with Overcurrent (RCBOs).
The difference in names is meant to show some difference in the design used in their manufacture. The purpose and operation of the parts are all the same. So do not be confused when they are the ones installed atyour house electrical panel.

F. How to test an ELCB

As described above, the ELCBs can usually be simply tested by a Test Pushbutton on the unit itself. It is recommended that the ELCB be tested once a month and after every thunderstorm to make sure it is still working properly.
However the electrical grounding system must also be in good working order for the shock protection system to work. In between the routine inspections that should be done by the qualified electrician, this grounding can be inspected regularly by the homeowner.
Just like other parts of the electrical installation, visually checking your home electrical is quite simple and it does not require much electrical skills.
If you feel you have found something that is not right with the electrical installation in your home, then a qualified electrical contractor can be called in to do a more detailed inspection and carry out the repair works if necessary.
Proactive attitudes towards this aspects of maintenance can effectively reduce the risk of electrical accidents at home and at workplace.
© Copyright http://electricalinstallationblog.blogspot.com/ All rights reserved - ELCB - Home Electrical Shock Protection

Temporary Electrical Installations

This post will present you with pictures of temporary electrical installations, the temporary switchboards (at this post, Temporary electrical switchboard), temporary electrical DB, temporary cables and poles.

The post is open ended. I will keep adding more pictures and comments when I find something that I think the readers will be interested in.

Someone also commented that I have yet not uploaded any picture on temporary lighting.

Well, now you have a few of  the pictures at this post,  Temporary site floodlights. There is also more information on site lighting here, Temporary lighting installation.

I have also just sent a new anchor post,  Free electric installation pictures, to partially fulfill a few requests to have all the pics at one place.

So if construction site’s temporary electricity supply is a subject of your interest, keep checking this post every few weeks, you may find new information that you can use.

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RELATED POSTS: (a) Electrical DB installation; (b) Electrical injury pictures;

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For readers who have visited this site before, you can see that this post keeps changing. That’s because I need to adjust the layout and some contents here and there so the newly added information can fit in and also to make the various content more coherent.

One other thing, if you have the time, check out the following link. It is a link on how to convert your car into an electric car at minimal cost. It is a way we in the electrical industry can contribute in the global effort to save this planet. In any case, it can save some significant dollars of our daily transport costs. Check it out. It’s a good reading.

Content:

Section A. First case – 20 floors of temporary electricity supply without the electrical grounding.
Picture 1 – Temporary switchboard, temporary DB, temporary cables, temporary extension cords, temporary sockets.
Picture 2 – Another temporary DB (Picture quality not so good. Sorry…)

Section B. Another DB – Temporary electrical panel, temporary cables and extension cords.
Picture 3 – Temporary distribution board – Right view
Picture 4 – Temporary distribution board – Front view
Picture 5 – Temporary distribution board – Left view

Section C. Site temporary electric supply – The risk

Section D. Another case of the electric supply without the earth grounding.
Picture 6 - Electrical panel location (Click on the picture to see full size)
Picture 7- Electrical panel inside view (Click on the picture to see full size)
Picture 8- Earthing copper bar without connection to earth (Click on the picture to see full size)
Picture 9- Incoming 4-core twisted cable without earth (Click on the picture to see full size)


Section E. A simple example of temporary earth
Picture 10 – Temporary electric supply: The complete system
Picture 11 – Temporary cables, temporary meter, temporary distribution board, temporary earth wire and protective PVC conduit.
Picture 12 – Temporary earth grounding


Section F. Site power distribution and equipment

The supply equipment

The distribution system
Picture 13 – Temporary electric poles
Picture 14 – Accidents on electric poles

Section G: Is there a foolproof system for a site temporary power?
The weaknesses of RCD (or ELCB)
Diagram 15 – Three- phase reduced low voltage supply system
Diagram 16 – reduced low voltage supply system (single phase)


Section A. First case – 20 floors of temporary electricity supply without the electrical grounding.


Today I am going to share with you a picture of one of the most important parts of temporary electrical installations in construction works, that is the temporary supply sub-switchboard, distribution board and their cabling.

Below is the picture of a temporary electrical assembly. It was taken at one of my projects a few years ago. What you see in the picture is the floor temporary sub-switchboard, which sits on the left of the assembly. Next to it is the temporary DB with an on-board three phase isolator and a few 13A switched socket outlets.

Picture 1 – Temporary switchboard, temporary DB, temporary cables, temporary extension cords, temporary sockets.



The project was a multi-storey residential building and the temporary supply was taken from the temporary main switchboard at the ground floor. The authority's meter panel was installed at the main switchboard. So the twisted 4-core submain cable was run from the main switchboard to each sub-switchboard at individual floors (one sub-switchboard for every three floors actually). Since it was a 21-storey condominium, seven subswitchboards were installed. From each sub-switchboard, cables were run at high level to distribution boards at various locations at the three floors in its coverage.

Back to the picture… So what is so special about the electrical board assembly in the picture? This is one example of the usual practices in the installation of temporary electrical supply at construction sites in our country. Not all construction sites are like this example, of course. Many job sites practise very good safety standards and I can personally testify to that, but quite frankly the practise shown in the picture is quite common. And in this case I have seen one situation that I have never seen in my entire 18 years of involvement at construction sites. I apologize the picture may not be very clear, but I hope you can see that the incoming cables to the floor sub-switchboard is only four-core. It has NO EARTHING at all.

I joined the supervision team while the work was already quite advanced with only a few month left to the completion. The temporary electrical supply has been in operation for almost a year. The electrical sub-contractor has also been on board for many weeks. Yet the electrical supply distribution from first floor to top floor at Level 21 has no earthing. I was shocked when I notice this on my second day at site. It was unbelievable. Some unlucky workers could have died from this oversight.

The 25 by 3 mm earthing copper tape was finally installed later, after many days of my urging them to do it. The distribution boards and the temporary cabling were also much improved.

As you can see, apart from the earthing conductor, obviously there were a few more things that were wrong with the distribution boards, the cabling and the plug and sockets in the picture. Maybe we can talk again about these in my future posts.

For now I just wish to emphasize the importance of checking the temporary electrical supply installation at your construction sites. Do not assume that the main contractor of a RM230 million building project will have enough people to look at safety matters as important as this one.

More Temporary Electrical Installations

I have also attached below two more pictures of temporary distribution boards just for the fun of it.

Picture 2 – Another temporary DB (Picture quality not so good. Sorry…)







These temporary electrical DB are also located along the main access route inside the building under construction. Exposed, without barricade, without any protection at all. This is how you can get electric shocks.

That's it for now. When we meet again I will show you a few more pictures of electrical installations from my previous projects and maybe we can learn some things from them.
That's it for now. When we meet again I will show you a few more pictures of electrical installations from my previous projects and maybe we can learn some things from them.


Section B. Another DB – Temporary electrical panel, temporary cables and extension cords.

These pictures I have already posted on my other blog a few months. It’s a new blog I just set up but I don’t have much time to spare on it. So it’s not drawing much traffic. So I thought I might as well put it here so it can be of some use to someone.

Below you will find a few pictures on a temporary electrical panel, sockets, plugs and extension cords. I have added a few comments below the pictures on aspects that I think is important. Further comments will be added when I think of something readers may be interested to know.

Picture 3 – Temporary distribution board – Right view



Picture 4 – Temporary distribution board – Front view

Picture 5 – Temporary distribution board – Left view



Is there anything wrong with the electrical installation in these pictures? The temporary panel seems to be quite new and definitely in a good condition.

You may question about the mounting method for the panel. But this is a temporary panel for a construction site. At times, the subcontractor needs to move the panel from one place to another every few days. So fixed wall mounted method is not a practical choice this circumstances.

Yes, you may want to suggest that the mounting stand be made of a much better material and design. That I would agree.

The supply temporary cables to the panel seem to be four single core cables. I do not know what size just by looking at the cable from a distance, even though an electrician might be able to give a good guess. Four core means four phases plus one neutral.

You can also see two very good-looking wires with green insulation twisting along the four-cable bunch. These must be the earthing wires, or Circuit Protective Conductor to be precise. Is the total cross section sufficient of the earth wires sufficient? I am a bit rusty on this nowadays. But temporary electrical supply only needs residual current protection as the shock protection. The green wires seem like 4 square mm. Two of them will give 8 sq.mm. I think that should be enough. Of course the required cross section of the earth cables depend on the distance of the temporary panel to the earth electrodes.

That covers the electrical panel and the incoming supply cables. Now let us look at the outgoing circuits from the panel.

First the socket outlets, or the receptacles as the Americans say it. They seem to have the original SIRIM sticker on each of the sockets. In any case all the sockets are new, so there should not be much issue about them. However what has been plugged into each of the sockets is a different matter.

The appliance connected to the right socket of the lower row should be okay. I think it's the charger for a walkie-talkie. All factory-manufactured and all new.

However, the cords connected to the other four sockets may have some serious safety issues. I will add comments on these in the near future.

Have a nice day.



Update Jan. 2010: You can see pictures of 11kV switchboards at my new post Hospital LV Electrical Installation.



Section C. Site electric supply – The risk

It has always been a challenge to provide the electric supply to the site people safely. Construction sites are among the most challenging environments to the safe use of electricity.

A lot of works are done outdoors, in all sorts of weather conditions. Wet and damp conditions present very high risks of severe electric shocks.

The workflow of site works is constantly changing as the construction work progresses. Therefore, the temptations to improvise the electrical distribution system are often too great to resist.

Routine construction activities, the demolition and excavation works may all result in damages to both the temporary supply and the newly fixed permanent supply systems.

When the activities at a site are at its peak with hundreds or thousands of workers (not to mention construction vehicles and machineries), the site usually become very congested. This sort of situations makes the control of risks very difficult. Temporary cables and electric construction tools and equipment are very likely to be damaged by the movement of heavy machines and materials.

The people who use electricity at site have various needs, and sometimes conflicting interests and expectations. The workers and team leaders themselves work for different subcontractors and suppliers. In order to maintain any reasonable degree of safety and control of risks, a effective site management is an absolute must-have for the site main contractor or the client’s people who are in charge at the construction site.

Due to the nature of construction works, a risk-free environment is impossible to attain most of the time. However, some risks can be avoided by careful planning before the work commences at the site.
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Section D. Another case of the electric supply without the earth grounding.

This is another case of missing electrical grounding that I already published in my other blog.

The following pictures of a temporary electrical panel show another common type electrical hazard at construction sites. I took these photos at a building construction site about four months ago.

This panel was located under a shade at the concrete mixing station of the construction site. As you can see, the location of the electrical panel was full of water. No, it was not due to rainwater. The location was always that way, because the plant consumed a lot of water daily. The situation presented a very high risk of electrocution not just to the worker operating the concrete mixing plant, but also to other workers.

Someone at the site told me that the electrical panel was too high above the water. So there was not really much risk. What do you think?

Picture 6 - Electrical panel location (Click on the picture to see full size)





I opened the electrical panel door to check inside (See Picture 7). The panel was a good quality and relatively new. The internal wiring inside the panel was also done nicely and it was neat.

But wait.... Where is the earthing to the panel? There was good earth wiring inside the panel - the green wires. All neatly run and properly terminated to the copper earth bar. However, the connection just stopped there. There was no outgoing connection to earth (See Picture 8).

Picture 7- Electrical panel inside view (Click on the picture to see full size)






Picture 8- Earthing copper bar without connection to earth (Click on the picture to see full size)



I checked the incoming twisted cable. It was a four-core. No earth cable anywhere (See Picture 9). I checked around the panel to see if the person who installed it used a different conductor and run it direct to earth somehow. Nope, no other conductor installed.

The conclusion? The electrical panel was not earthed at all. So what happens when there is a considerable leakage current? What protects the worker at the concrete mixer machine there, especially with the wet condition there? Sadly to say... no protection at all.

Feeling exasperated, I checked inside the electrical panel again. Surprise... there was not even an earth leakage circuit breaker there, or any other type of residual current protective device.

Picture 9- Incoming 4-core twisted cable without earth (Click on the picture to see full size)




I have to go now. I will add more info to this post in the near future. I know there are questions in the minds of some of the readers relating to the electrical installation shown in these pictures. The electric shock protection issue can be tricky sometimes.

Until next time...

Section E. A simple example of temporary earth

The safety of an electric supply often depends on the existence of an effective earth. It is the responsibility of the person in charge of the construction site to ensure that the earthing of the electric supply system is effective. The site’s electricity supplier (the electric supply authority) has nothing to with this part of the electric supply.

Many electricity supply authorities use protective multiple earthing (PME) system. In this system the electrical system’s neutral and earth are combined. When this system is used, all metalwork including structural metalwork must be bonded together in such a way to make all metal parts electrically continuous.

However in most real life construction sites, this bonding is hard to do and almost impossible to maintain properly throughout the duration of the construction period. So usually, the public utility supplier will not connect the supply until an alternative electrical earth is provided and fully tested. They also insist that evidence of the earthing test results endorsed by a competent electrician is submitted together with the completed application forms for the temporary electricity supply to the site.

Several methods are used to provide the alternative electrical earth. The most commonly used method is to use an independent earth electrode installed at the location near the main intake temporary switchboard (this is usually the location where the temporary meter is installed). This will ensure that the protection fuses will operate and disconnect the site electrical installation from the incoming public supply in the event of a fault. This disconnection is an absolute must in order to minimize the damages to the site’s electrical system due to the fault and also to prevent the fault from spreading upstream to the electric supplier’s distribution network. The latter is the main reason the electricity supplier is worried about this aspect of the temporary installations.

Picture 10 – Temporary electric supply: The complete system


I will devote one whole post in the future for this subject of earthing. But for now I have a few picture of temporary electrical grounding for your viewing pleasures. I seek an apology from those readers who already have some electrical knowledge. These pictures are not for you guys. They are too simple. However for those managers and construction people who have always been intimidated by those colleagues on these sorts of issues, let me assure you that if you understand what these three lousy pictures say, then you already understand what an electrical grounding is.

Picture 11 – Temporary cables, temporary meter, temporary distribution board, temporary earth wire and protective PVC conduit.


Picture 12 – Temporary earth grounding


Now lets start with picture 10 (You may need to click on the picture to see it in full size). The black case on the wooden panel there the temporary electric meter supplied by the electric authority. Below it in white PVC casing is the temporary distribution board. You can also see coming out downward from the distribution board a piece of wire in green insulation and this wire goes straight into a two meter length white PVC conduit, and (if you watch it closely) it comes out of the vertical PVC conduit at the bottom and goes straight into the concrete floor just beside the plywood partition wall. All these you can see much more clearly in Pictures 11 and 12.

Back to the black energy meter. Above it you can see two lengths of black insulated cables coming down from high level (below the roof level of the temporary wooden structure). One cable terminates at a black piece of component just above the meter, while the other one terminates at a cheap white plastic terminal block. The two components each have a cable of the same size coming out at the bottom connection and terminate at the temporary kilo-watt-hour meter.

Now lets get a little bit more technical. This is what we call a single-phase electrical installation (a three-phase supply would have four black cables coming down from the roof level – three phase cables and one neutral cable). So the incoming supply from the authority is also a single phase supply (we apply single-phase supply, so they give us a single phase supply).

An electric supply from the authority can be one of a few types. The type as recorded in the pictures is the single-phase two-wire type (three-phase 4-wire if we apply three-phase supply). This type will have two incoming cables from the authority’s distribution network – one live or phase cable and one neutral cable. Other types may have three incoming cables from the distribution network for the same one-phase supply – the same live and neutral cables, plus a third cable, which is the earthing cable.

Now as I said earlier in this post, a construction site really needs an independent and reliable earthing, an electrical grounding that is not dependent on any third party’s grounding or even the electricity supplier’s grounding. Failure to provide this can result in fatal injuries from electric shocks, even multiple casualties in a single accident. Never mind my emotions on this (if you can sense them), but an electrical accident is that dangerous and electrical shocks can strike silently without warning.

Because of the need for that independent and reliable earth for the temporary site supply, the green electric cable is installed below the white PVC distribution board (Picture 11 shows this clearly). Without this cable, the electrical system can still work. The workers can still use their tools and do a good job for their employers so the construction venture can end up being very profitable for the shareholders of the company. But one fine day, an extension cord that carries current to the electric drill one of the workers has been using gets injured, exposing the live wire to an unintentional contact with any of the workers around the area. Being hard at work, the workers bodies and clothes are usually very damp or very wet with sweat. One accidental contact at any part of a workers body with the exposed live conductor of the damaged extension cord, then you will severe electric shock injury, even death is highly likely depending on where on the worker’s body the contact to the live wire happens. If the contact is at the hands, then the electric shock current will travel through the chest and the workers heart before going down to the legs and the ground. Then you may have a case of fatal injuries there.

Sorry for the diversion. This electrical earthing matter is so simple that I have to drag the stories into the injury aspects to make it a bit longer ;-)).

Not only it is simple, it is also plain cheap and low cost. Look at the picture again. You have a short length of the green wire and two meters of the white PVC pipe. Wait… I know what a few of you are thinking… Yes, there is copper earth electrode in the concrete, which goes straight down below the concrete about 1.5 to three meters into the ground.

The example that I use here is a very small installation, so it looks simple. This wooden structure only uses a few amperes. However even for a large installation, the grounding is relatively just as simple.

The point that I am trying to make in this section is that providing an independent and effective earth is not a challenge at all in most situations. So, do not risk human lives just to save some construction cost there.

Section F. Site power distribution and equipment

The supply equipment

Equipment selected for use in a site temporary electric supply distribution need to be designed for use in the environment it in is going to be operated in. During the selection process, the most important is to make sure that the manufacturer’s restrictions of use are considered. For example a sub-switchboard that need to be moved frequently, left exposed under sun, and rain during operation must necessarily be of adequate IP rating.

The construction of the equipment must also be robust enough to withstand the damage caused by the rough treatment at the construction site.

Most of the time, equipment purchased for use at a project site will be transported for use in another project or work locations. Therefore, it needs to have provisions for frequent loading and unloading, transport and storage while throughout all these it cannot be damaged to the point of becoming malfunction and unsafe for use. Under the pressure of worksite demands, even a good site electrician tends to improvise on the safety aspects site electricity supply. With damaged equipment, the risks are compounded even more.

The equipment selected also shall include the provision for isolation of supply and facilities for locking the switches. This is especially important main switchboards and sub-switchboards where the locations of the board and the local distribution boards are quite far away and not within the line of sight.

The distribution system

The distribution system is the system of cabling and equipment that are arranged to distribute the electric power to the various machines and current using locations throughout the construction site. This temporary distribution system will be removed after the construction work is completed and the permanent power supply system is commissioned and operational.

The location of the switchgear and metering apparatus must be secured in terms of security from unauthorized access and safety from damage and the effects of the environmental factors. Access to the switchgears and the main isolation switch must be accessible at all times in case of emergency.

Just because the supply system is temporary does not mean it can be assembled without all the proper engineering calculations and considerations required of a permanent system. Fuses, circuit breakers and cables all must properly selected and sized.

Many practices shown in the pictures of temporary installations above are bad practices. If contractors are allowed to provide these bad installations on site power supply, accidents whether in the form of damaged equipment, fires or electric shock or even electrocutions is bound to happen sooner or latter. Makeshift arrangements such as unprotected wiring, twisted or taped cable joints are often dangerous and cannot be accepted. All the temporary installations works must be done in accordance to the appropriate standards.

Site distribution cables must be properly located. They should not be installed where they are likely to be damaged by the construction activities or vehicles. Picture 13 below shows an example of a site distribution cabling running on temporary electric poles. This method of installation is fine but here they are run across one of the main road within the construction site. The next picture (Picture 14) shows the temporary cables on the ground. Only a few minutes earlier, an earth moving tipper truck accidentally hit and dragged the overhead cable while unloading the earth for a drainage work next to the temporary poles. There were workers nearby when it happen, including me. Luckily, no one was injured.

The above accident could have been prevented if the stretch of the cables crossing the construction have buried in a G.I. pipe just two or three feet below the road. With all the construction machineries at the site, these could have been done in a matter of one or two hours. However, here the main contractor chose the easy way and just installed the whole stretch on poles.
Picture 13 – Temporary electric poles

Picture 14 – Accidents on electric poles


Section G: Is there a foolproof system of site temporary electricity?

RCD (or ELCB)

RCD (residual current devices) or ELCB (earth leakage circuit breaker) is not an ideal device for use in the tough environment of construction sites. It may not even be possible to maintain the integrity of the housing where the RCD devises are housed in.

It is also not the kind of devices that can be fitted on portable equipment, which usually are subject to mechanical shocks and vibration.

Electric shock protection based on RCD alone has many weaknesses. It cannot guarantee safety.
a) When an RCD malfunctions or it fails to operate, this failure can go unnoticed so the affected workers are unaware of the dangers awaiting to turn into accidents.
b) The RCD only protect against faults to earth, and it required sufficient bonding to earth for operation. It cannot operate when there is no earth connection, for example, when current passing from live to neutral. This type of situation is still dangerous: the workers can still get electric shock, injuries or even electrocution even though the RCD is working properly.
c) The use of RCD will not make the system safe if it is not properly designed, properly installed and adequately maintained. The popularity of RCD may have been due to difficulties to obtain low earth resistance on many installation conditions. However, RCD is not a substitute for reliable and efficient earth grounding.
d) RCD designed for electric shock protection have a rated tripping current of 30 mA. At sites where temporary load requirements are higher, installing a 30 mA at the point of intake (like an ELCB installation in a house distribution board) will lead to frequent unnecessary tripping, shutting off the supply to the whole site.
e) Electrical loads at construction sites consist of tools and equipment that have high leakage currents. This factor has led to many ‘nuisance’ tripping of the supply. These are also unwanted tripping that frequently lead to the RCD devise being defeated in some by the workers involved, leaving the whole supply system with a shock protection.
f) RCD can be chosen to be very sensitive to small current leakage, and trip fast enough to avoid serious injuries. However, with this principle, it only limits the duration of time that the current flows in the event of shock, it does not limit the magnitude of the current flowing. If the devise fails to operate, the current will continue to flow continuously. Even low shock current if allowed to flow continuously through the body can cause serious injuries.

Reduced Low Voltage supply

Most of the time the electric shocks that occur are between the live parts and earth, which are the results of damaged cable insulations, faulty power plugs, or faulty or damaged equipment. Whatever is done to the electrical system, or the electric shock protection to reduce the risk of shock to people, the potential is always there. RCD itself is a active electromechanical system that can fail. It effectiveness requires an effective earth which can be hard to maintain in a real life construction sites. While the scope of protection covered by the device still have gaps, that is left without shock protection (between live and neutral).
Fortunately, there is one system that has been proven almost foolproof against electric shocks at construction sites. It is called the reduced low voltage system, where the supply voltage is stepped down to a level which is practically safe without depending too much on an active shock protection, or a sophisticated system that is hard to implement at construction sites.
This system reduced the 415/ 240 volts supply voltage to 110/64 volts for a 3-phase supply. If only 1-phase supply is needed, then a 240/ 110 volt center-tapped transformer is used to reduce the single phase voltage to 110 volts with the electrical grounding connected to the center of the secondary transformer coil. This will present a maximum possible risk of only 55 volts to the users of the electricity instead of 240 volt (or 415 volts for 3-phase) as I the original 415/240 volts supply.
Technical studies that have been carried out have shown that the maximum indirect contact touch voltage to earth is only 40 volts. This is below the danger voltage as specified by the IEE Regulation. Even the SELV (safety extra low voltage system) as specified in the regulation is 50 volts.
The following diagrams show the basic configuration of the reduced low voltage supply system.

Diagram 15 – 3-phase reduced low voltage supply system


Diagram 16 – single-phase reduced low voltage supply system


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