A good explanation as I could never understand why sometimes the telly is working but lights and fans are not.Nereus wrote: ↑Sat Apr 07, 2018 11:14 amMost small consumers are only supplied with single phase. If you have a 3 phase supply consider yourself fortunate.Dannie Boy wrote: ↑Sat Apr 07, 2018 10:02 amAlthough I worked in the Power Generation business for over 40 years, I don’t understand the technical side of the distribution/domestic supply, but we have a 3 phase supply in our house and have some power and light circuits on and some off - maybe Nereus will provide the reasoning behind it all
The distribution system used in the street consists of 2 different types of electrical connections. The high voltage uses 3 wires and is connected in what is called "delta", or "mesh" if you are a Yank, and the triangle shape of the connection means that each of the 3 points of the triangle is 1 phase.
The voltage on each phase will be High Voltage, 6KV, 11KV, or whatever and of no use to a consumer. So a transformer has to be provided to "step down" the voltage to a useful level, in this country around 220 volts. The "primary" side winding of the transformer connects directly to the HV supply, one phase to each winding.
It is the secondary side of the transformer that is used for the consumer supply. Here the connection of the windings are connected in what is called "star", or "wye". This is a "Y" shaped connection with 1 phase available at each end of each leg of the "Y".
BUT, the connection also has a centre point, called the "star point". This point is where the "neutral" connection is made. If you look at the "star" you will see there are in fact 2 sets of windings in series across 2 phases. The voltage appearing here by transformer action is around 380 / 400 volts, which can be used to run 3 phase equipment. The voltage appearing across 1 phase and the centre, or star point, is 1,732 (root 3) of the phase to phase voltage.
In a power failure situation it will depend on just where the failure occurs. If on the primary side of the transformer and just 1 phase fails, then the other still active phases will still provide for some output on the secondary side, but it may be at a reduced voltage, or "brown out". If the failure is on the secondary side of the supply it depends on which phase that the consumer is connected to wether or not you will still have a supply. That is the reason why if you have a 3 phase supply you may still have some single phase power, and your next door neighbor, who may be connected to a different single phase, still has power while you do not.
As always, TIT, so that is only the theory!
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