Three Phase Electricity

3 wires

Our electricity infrastructure is all around us and often remains unnoticed blurring into the backgrounds of our lives. One interesting fact that remains unnoticed to all of us is the rule of three! When looking you may begin to notice that the transmission cables, pylons and posts all have a multiple of 3 wires carrying electricity. Most people when seeing this will naturally assume this would be expected given that plugs have three pins. This however is actually completely wrong. To understand why, we need to think back to school lessons on electricity when building simple circuits. We are all aware that a circuit consists of a closed loop with the electrons flowing from negative to positive terminals in a DC (direct current) system. This is the same for AC (alternating current) system which is what your house plugs most likely use. In AC the electrons vibrate at the frequency of your grid (50Hz in the case of the UK) as opposed to flow from positive to negative. The three pins on your plug represent a live and a neutral, just like in a circuit. The third wire is the yellow and green safety feature or “earth” wire. This protects you and any other critical kit from danger in the case of a short circuit.


3 phases

So what are the three wires or sets of thee wires we see everywhere? They are all actually live (and therefore extremely dangerous). Why are they all live and therefore how is the loop closed?

At the power station the generators produce three AC currents from three coils around the generator. Each is oscillating at the same frequency (50Hz) but 120 degrees apart which represents a third of a phase. They vibrate between positive and negative and the sum of the three lines always equals to 0 and the return path for the current from a branch is the other two loads. Three phase generation produces a more consistent load than a possible 2 phase load. It also allows for creation of a rotating magnetic field to produce a rotating electric motor.
A key benefit is that opposed to the standard single phase generation, three phase can transmit three times the power with only 1.5 times the amount of contactor material. These are split at the distribution level and within our houses (unless they are very big!) house we get the standard single phase power.

Do you have a novel way to distribute electricity?

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