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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