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Three phase power systems are made up of three distinct sets of power waveforms set 120 degrees (one third of a cycle) apart from each other. In the UK, the frequency is 50Hz, and standard low voltage distribution is 400V nominal (was 415V). Note that the term low voltage here does not mean that it is safe to touch in any way!!
Single Phase systems require two wires for one phase. Three phase balanced systems (no neutral, Delta) require only three wires for three phases, three phase un-balanced systems (3P+Neutral, Star) require only four wires for three phases. In each case distribution of three phase power is more efficient than single phase (as it requires fewer wires per phase).
For a given power a three phase motor can be a lot smaller, have a greater starting torque and can apply a lot more force before stalling than a single phase one. As a result a lot of motors such as those found in hoists will be three phase.
Star (a.k.a. Wye)
The pictures above show the old UK colour coding system for fixed installations. Between 2004 and 2006, the IEE led a transitional period to new harmonised colours for fixed installations throughout Europe:
|Earth||Green & Yellow||Green & Yellow|