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Three Phase Electrical Power

3 Phase Power

What is three phase electrical power? In this article we describe three phase electrical power, how it should be wired, the importance of balancing the loads on each phase and the importance of good earthing or grounding of your electrical circuits.

So what is three phase power and is it any better than single phase power, what is the difference? More importantly, do we understand important factors when we install it in our homes?

Electrons have lots of potential, if we send lots of electrons into one end of a piece or wire and there are no electrons at the other, we have a difference in potential and the electrons will get excited and run along the wire from the high point (positive potential) to the low point (negative potential) to balance things up. The potential difference is measured in volts.

What is three phase power?

In a single phase supply there are two wires to each house with a difference of 220 volts between the two wires, a "live" wire and a "neutral" wire. It would follow that to transmit 3 times the amount of power we will need three pairs of wires - six wires. People don't want to use six wires, electricity wires have traditionally been made of copper which is very expensive (aluminium is used these days which is much cheaper).

Back in the 1880's some pretty smart people who wanted to save a bit of copper realised that you can transmit 3 times the power only using three wires instead of six by making sure that at any one time the potential difference between any one of the three wires with any other is 220 volts. These three different power differences are called three "phases" and each wire acts both as a live and a neutral wire (explaining how this happens is a little too complex for us to look at today but is based on the fact that the power is alternating current - the electrons go backwards and forwards along the wire).

All around the world you will find that electricity is transmitted by high voltage power cables crossing the countryside and always in threes. To carry very large amounts of power to supply cities and towns the cables are on tall pylons and the voltages are very high (up to 550,000 volts - that is rather a lot of electrons).

Once in the towns and cities the power is reduced feeding into medium voltage cables which distribute the power along our streets. At this stage it is still three phase power in three cables with a voltage of 400 volts.

Somewhere in your neighbourhood these cable go into a transformer which will convert it yet again to a low voltage (220 volts in Indonesia, 110 volts in America) but here there is a difference. This local transformer changes the power to what is effectively 3 single phases with three live wires and one common neutral wire. Three wires go into this transformer and four wires come out. At this stage we still call it three phase power.

If we have single phase power to our house we have two wires coming in - a live and a neutral.

If we have three phase power to our house we have four wires - three live wires and one shared neutral.

The four wires are colour coded so we can tell which is which and here in Indonesia the three live wires are Red, Yellow and Black. Red is known as the "R" phase, Yellow is known as the "S" phase and the black is known as the T phase. In other parts of the world the phases are L1, L2 and L3 and the colours used vary from country to country.

Now we come to the important parts in all this:

  1. The electrical loads on each of the three phases should be around the same.
  2. We must take particular care with the earthing of our electrical circuits.

1. Balancing the loads

When electricity is generated the same amount of power is generated in each of the three phases so to keep the system stable we need to use about the same amount of power equally from each of the three phases to keep it balanced. Of course this is not always possible and so the power company will have to balance things out. If each of us makes sure that our electrical systems are as balanced as possible then the whole system benefits and is more stable.

Three phase electrical installations are more complex than single phase systems, they should be correctly designed to balance the use of power from each of the three phases. Just bear in mind that your average tukang listrik has no formal training in these matters and is perhaps better skilled at riding a camel. You need an MEP (Mechanical, Electrical and Plumbing) design engineer or a very capable electrician to do this for you.

2. Careful Earthing of the circuits

Our earth connection is the single most important feature in our electrical installations. The earth carries any "out of control" power safely to earth. If a wire shorts out (it happens) and the steel cabinet of your washing machine or microwave becomes live the earth connection will, if the system is designed and installed correctly, save your bacon. Do you get shocks or tingles from your computer? - then it is not properly earthed.

In a healthy three phase system the power is flowing between the three phases and so the neutral wire is effectively zero (the same as earth) and unfortunately, to save time and money in installations, it has become common practice for earth connections to be made to the neutral wire. This arrangement is used very often but it is dangerous.

The Indonesian electrical standard says that the earth and neutral wires in a three phase system should not be connected together on end circuits. An end circuit is the electrical wiring that reaches your lights, light switches, plug sockets or whatever. Generally speaking contractors agree that the blue neutral wire should not be connected to earth (green yellow striped) after the first distribution box in your house.

Safety Precautions

Having come across many electrical installations with problems such as voltage drops, power surges and, due to system imbalances, neutral cables that are not zero (they are partly live) I suggest that three important steps be taken.

  1. Make sure that each distribution box has a very good earth connection (maximum resistance of 0.3 ohms).
  2. Make sure that each and every plug socket and ceiling fan has an earth connection (yellow green striped wire). Hard wired equipment such as electric ovens and water heaters should all have earth connections.
  3. Make sure that the earth (yellow green wire) is not connected to the neutral (blue wire) anywhere in your system.

As a final additional precaution install earth leakage detectors and make sure they are working correctly, these will cut out and save your life, normal contact breakers will not.

Copyright © Phil Wilson August 2015
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