Tuesday, April 19, 2011

Simple house electrical schematic

Many visitors came to this blog looking for a schematic diagram for a simple house wiring. The original post, House electrical wiring, symbols and checking, is too long. Many readers clicked away. My guess is that they cannot find the schematic drawing. So I decided to separate it into a small article, here.

Diagram 1 – Schematic diagram for a simple 2-bedroom house (Click to see full size)

In the original post, I gave a lengthy description of how to read this schematic so beginners understand how to read the electrical single line drawing.

I will not do that here. Many people don’t like long posts so I will try to make it really short.

However, there will be a number of small posts on house electrical installations and wiring. Therefore, all the materials will still be covered.

Using separate articles, I will try to explain them from different angles so all readers from different backgrounds will be able to understand them. That will be the way I will present these things to you.

The electrical schematic in the above diagram is a “single-line diagram”, or a “one-line diagram”. That is how to say it more precisely.

The word “schematic” gives an impression of a “wiring diagram”. If you have a wiring diagram, you can do a wiring work if you follow what is shown in the diagram precisely.

Unless you are an electrician, or you already have electrician’s skills, you cannot do a house wiring using the above electrical schematic drawing.

If you are a beginner and you try to do so without learning enough on the shock protection side of the wiring circuit, your wiring work may still seem to work perfectly.

However, the wiring work you have just completed may present electric shock hazards that you may not understand until too late.

I am not trying to turn any reader “off” by saying these things. Skills in electrical wiring is a kind of “survival skill” so everybody should know a little bit about it.

But the voltage of house electricity is a “lethal voltage”. It is a high-power voltage. If not handled properly, this voltage can kill people. It happens all the time.

So while you are in a high spirit to learn the house wiring, remember that you also need to learn the how to prevent the electric voltage inside your wiring from causing electric shock to people including yourself.

With that said, let us now go to the short description of the house single line diagram.

A. Incoming electricity supply and the electric meter

Let’s just start at the bottom of the single line drawing. Some people draw the single line from left to right. This one starts from the bottom. Both approaches are common for an electrical drawing of this type.

Image 2 – The electric meter panel symbol

The rectangular box here is represents the electric meter panel. All components inside the rectangle are actually on the meter panel in actual installation. This you can see in the picture below.

Photo 3 – A house meter panel

Remember that a single-line drawing is not really a wiring diagram. When one line is shown on the drawing, it does not necessarily mean one wire or one cable.

The wiring only shows the flow of electricity and the electrical connections in general between the electrical parts or components. You need to see closer to see if there are more details indicated on the drawing.

Image 4 – The supply cable symbol

Here I just extract out the symbol of the electric cable on the electric meter panel.

As you can see the drawing gives a label to describe some details of the cable from the KWH METER to the 60A SPN SWITCH FUSE.

It says 2-25 SQmm PVC/PVC IN CONCEALED CONDUIT. That means we have two lengths of cables with the size of 25 millimeter square size.

The “SQmm” is a short hand for “millimeter square “. It is the net cross-sectional area of the cable conductor, the part of the cable that carries the electric current.

Photo 5 – The 25 millimeter square electrical cable with PVC/PVC insulation

The word PVC/PVC in the label means the type of cable insulation. All electrical cables must be insulated. Otherwise the conductor part (the copper part of the cable, or the aluminum part if it is an aluminum cable) can be exposed to touch and cause danger of electric shocks.

Without insulation, a short circuit between electrical parts or components can easily occur which will damage the components and also cause fire to the house. That is one of the ways how house electrical fires happen.

PVC is a popular type of materials used to insulate electrical things including cables, socket outlets, etc. It is actually a similar material (but not exactly the same) to the house PVC pipes that are used in house water piping.

B. The consumer disconnection fuse and main circuit breaker

From the meter panel, the 25 mm square cables are run in a concealed conduit into the “Consumer Panel”. People in different countries call this panel by different names: electrical DB, house DB or simply DB, electrical panel, consumer unit, supply DB, etc etc.

Basically it is a box in a house that controls the distribution of electricity to the whole house.

Photo 6 – A house electrical panel

Remember that the electricity meter panel belongs to the supply authority. Well, not really the whole panel but that is generally the idea.

The consumer panel, on the other hand, belongs to the consumer. You can literally do whatever you wish inside the electrical panel. That is an advantage but it is also a source of so many problems and electrical accidents and fires.

The 25 sq.mm cables are connected to the consumer disconnection fuse or the main circuit breaker. Both methods of disconnections are commonly practiced but for this house electrical design, a disconnection fuse is used.

Image 7 – The switch fuse symbol

The label “60A SPN SWITCH FUSE” is used to describe the component used in this electrical design.

The main purpose of this part is to stop the flow of electric current from the meter panel to the house wiring when too much current flow through the fuse. A number of problems can cause too much current flow that will “blow” the 60A fuse.

I am not going to go into details here. That will be in separate posts.

Photo 8 – The house switch-fuse - refer Photo 10 (The house ELCB) below.

C. The electric shock protection section or “ELCB”

From the 60A fuse, the electric power flows to the shock protection section of the electrical panel. We call this component “ELCB” or earth leakage circuit breaker.

The label “100 mA” below “63A DP” says that the ELCB will trip when it detects a leakage cureent of more than 100 milliamperes. It is the level of sensitivity of the ELCB unit. There are ELCB unit sold on the market with other sensitivities such as 30mA or 300 mA.

The name (i.e. ELCB) of the shock protection component is really just a conventional name from the old days of electrical engineering. It is still widely used today for the electrical drawings of house electricity.

Image 9 – The earth leakage circuit breaker symbol

Nowadays the technology has changed and the right name of the shock protection component used in home electrical panels is “residual current circuit breaker”. The short form is RCCB.

However, other names are also used to say the same thing: residual current device (RCD), residual current circuit breaker with overcurrent (RCBO), etc.

Whatever name is used at your locality, this is the second component in your house electrical panel.

The picture below shows an example of house ELCB. The ELCB is the unit on the right with the word CLIPSAL on it. The unit on the right, with the word NEM is the switchfuse.

Photo 10 – A house ELCB

After the electric voltage passes the earth leakage circuit breaker, then it is ready to be distributed throughout the house.

The next section provides a brief explanation for that.

Before that, however, I wish to bring your attention to one important point of the house single line diagram.

Image 11 – Split-busbar electrical panel

This is important because many common electric users get confused and take actions that expose themselves to a very high possibility of electric shocks.

The above part-drawing shows what is sometimes called “split-busbar”.

I guess that name came about because the long busbar in the electric panel has been split. If you do not know yet what a “busbar” is, it is just a piece of long metal inside an electrical equipment that is used to more conveniently distribute current to many branch circuits.

A cable can also be used to do this, but it is often much easier, economical and more reliable to just use a long piece of metal for the purpose. It is called a “busbar” and is one of most common electrical terms in the electricians’ world.

In a house electrical panel, often a busbar is used. Often, but not all the time. Many lower-cost electrical panels just use cables.

In the single line drawing of a house electrical panel, however, the busbar symbol is almost always used regardless of whether the actual part used is a busbar or cables.

That means after the ELCB symbol (or RCD), you will always see the busbar symbol in the diagram.

On the image, I labeled the lines that represent the busbars.

The labels “5A SPN MCB 6kA” and “20A SPN MCB 6kA” are the symbols for the circuit breakers that protect the branch circuit. These branch circuits are the wiring that go to the house lighting fixtures, light switches, wall sockets, etc.

Why has the busbar been split?

The busbar has been split (one length of metal busbar “splitted” into two pieces) because we need to install two units of electric shock protectors each with a different level of sensitivity.

One is 100 mA and the other 30mA.

Observe that the branch circuits that are connected to the busbar with 100mA ELCB are the lighting circuits.

While those connected to the busbar with 30mA protection are the switched socket outlets.

I have sent a long post on the details of ELCB at this post, ELCB – Home electric shock protection. So I am not going to go through the details again here.

The important point that beginners need to remember is that 30mA is the absolute maximum current that the human body can take before serious injuries including death.

Therefore all 13A house socket outlets must be connected to a busbar protected by a 30mA ELCB.

Can we connect the house lighting to the 30mA busbar?

Yes, we can. However, over time a lighting fixture can leak a significant number of miliamperes of current.

If the house has a large numbers of lighting fixtures then the 30mA limit that the ELCB can handle can easily be exceeded. The result is frequent tripping of the ELCB.

When this sort of happening occurs, many users just disable the ELCB in any way they can, or bypass the ELCB altogether. Then they no longer have any protection from the possibility of the electric shocks.

Some users may be smarter, they go to the hardware shop and purchase a 100mA ELCB. They install this in the house electrical panel and keep the origibal 30mA ELCB just for spare.

Now with the socket outlets protected by a 100mA ELCB, the protection provided by this arrangement against electric shocks is very limited if there really any at all.

So this is the reason why the busbar in the house single line diagram has been split and two ELCBs of different sensitivities have been used.

D. The distribution busbars and the branch circuits

I think I have said enough about busbars for a short article. If there is a need I will use a separate post in the future just to give more detail and examples.

Now let’s just go to the distribution of the electric power as shown in the single-line diagram.

Image 12 – The distribution section of the electric panel

As said before, the busbar is used so that the connections for distribution of the electricity to the branch circuits can be done more conveniently.

This is one of the most important purposes of a house electrical diagram. With the diagram, we can see clearly how the current is distributed.

Symbol 13 – Miniature circuit breakers (MCB)

The above is the symbol of the miniature circuit breakers or simply called MCB in daily use.

Observe that for each outgoing circuit one miniature circuit breaker is installed. The MCB’s main purpose is to protect the cable that is supplying current for the electrical appliances or other electrical loads connected to that circuit.

Photo 14 – 20A Miniature Circuit Breakers (MCB)

Take note also the small circles with the numbers 1 to 6 at the end of each line of the branch circuits.

Symbol 15 - The circuit number labels

Each circle with the number is the circuit number for that branch circuit. It is the circuit label. Since there are a number of branch circuits in a house wiring, each circuit need to be uniquely labeled for safety and also for convenience during installation and during maintenance.

There are many ways to label a circuit number. This particular method is most suitable for wiring of small houses.

You can see how convenient these circuit labels are when reading the electrical layout drawing of the house.

You can see the layout drawing for this design is post, Simple house electrical layout (Coming soon..).

If you cannot wait, you can see it now in the original post, Home electrical wiring, symbols and checking.

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1 comment:

venugopal said...

Excellent dear... I look forward to your feedback /thanks for this man it was very help.

Electrical Wiring