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How to test three-phase electrical supply
This article covers basic testing of three-phase power and equipment. For general information on the
mathematics behind three-phase please see three-phase. For information on where how and why threephase is used see three-phase electric power.
Take GREAT CARE when testing anything at mains voltages (or higher) and if you are unsure what you
are doing and how to use your equipment safely then DON'T DO IT.
How to test three-phase electrical supply
A three-phase electrical supply consists of three active conductors and an earth ground.
A three-phase induction motor cannot function correctly if its electrical supply is not within certain
parameters.
Typical parameters are 208 or 415 volts between phases, 120 or 240 volts from any phase to earth or
ground, voltage within 12% of nominal values, and each phase within 5% of each other.
In a typical three-phase induction motor circuit, an appropriate place to test is at the line side of the directon-line motor starter.
Figure 1:
A B C
O O O
/ / /
/ / /
O O O
earth/
ground
Tests should be made between A and B, A and C, B and C, A and earth, B and earth, and C and earth.
Note that listed voltages are for countries which use 120 or 240 volts only!
How to test three-phase devices
Three-phase pumps, compressors, and the like, must be connected in proper phase sequence to avoid
damage. Typically such devices will draw less current when connected wrong, and can be easily checked
with an amprobe (clip on ammeter) for current draw. For example, testing an air conditioner that has a
scroll compressor, one will find that if it's hooked up in the wrong phase sequence, it will draw too little
current, and thus any two wires can be switched to change the phase sequence.
There are special small pocket sized motors that are used to check phase sequence by checking the
direction of rotation of the motor. These are expensive. A cheaper alternative is to use three neon lights,
and whip the head (or eyes) in a quick motion past them to see which way the phase sequence is going.
Topics including testing motor coil resistance and testing earth fault resistance are covered separately.