Three-phase Electric Power

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Three phases, each 120° out of sync
Red Ceeform Socket

Three-phase refers to an electricity supply that has 3 live connections, in addition to neutral & earth connections. It is often abbreviated to 3P+N+E, or 3P+E if phase-phase connections (i.e. no neutral) are used. This is different to the electricity that you are used to having at home, where you have just a single live connection - 'single-phase', or P+N+E.

Each phase of a three-phase supply is 240V (RMS), just like the supply you have at home; however we will see later that connecting a load between two phases gives rise to the 415v rating that is oft quoted for three-phase. A three-phase supply is capable of providing substantially higher current than a domestic supply; lighting equipment in the StAge is powered from the three-phase supply in the Dimmer Plant Room.

Three-phase, just like your normal home electrical supply, is AC (alternating current) which means that the voltage of each phase changes from +240v to -240v & back again at a regular rate (50Hz in the UK). However the waveforms of each of the three live connections are 120° out of phase, shown in the picture beneath.

This means that you get constant power transfer over each cycle of the current, as when one phase drops towards 0v the subsequent phase is climbing towards peak line voltage. The second phase is 1/3 of a wavelength behind the first phase & the third phase is 1/3 of a wavelength behind the second phase (or 2/3 of a wavelength behind the first phase, if you prefer to think about it that way).

Some very large pieces of equipment use all 3 phases of a three-phase supply. The Rig motors, for example, exploit the revolving magnetic field characteristics that a three-phase supply provides. However all other equipment that we use in the Union (with the exception of the Distro and the Jands Dimmers) only use a single phase each, simply exploiting the much higher current provision of the three-phase supply over a domestic single-phase supply.

A single-phase load can be connected to a three-phase supply in two ways;

  • across 2 of the live phases - this means the single-phase load will receive 415v
  • between 1 live phase & neutral - this means the single-phase load will receive 240v

Single-phase loads should be distributed evenly amongst the 3 phases; you don't want to be drawing 50A from one phase & only 2A from each of the other two phases.

Three-phase power is only delivered through red Ceeform, or through Powerlock which is coloured to define which phase is which.

The Jands dimmer racks take Powerlock (converted from 125A ceeform). Each of the 4 racks of 12 within the unit gets 3 phase power. On each rack, switches 1-4 are for the first phase, 5-8 the 2nd, 9-12 the third.