# Three phase ground fault

Does a fault in ground of three-phase do more damage to home appliance than a fault in ground of single phase? and why?

In a home with three phase input and single phase loads, fault in the ground damages all connected appliances (circuit breaker didn't move!). While in home with single phase input and single phase loads, a fault in ground was protected by the home circuit breaker.

Three phase is exactly the same voltage to earth as single phase. In fact, what comes to our homes is usually one phase out of the three. There are various methods of distributing and sharing all 3 phases in a road, street or small area to give a semblance of balance to the grid but the earth remains the same and there is no single earth fault on single phase that is somehow different on three phases.

In a home with three phase input and single phase loads, fault in the ground damages all connected appliances (circuit breaker didn't move!).

If my house developed a ground fault (of some type) I wouldn't expect my neighbours house to have their appliances damaged - is this any different to a house with all three phases distributed amongst the rooms? I might expect that if there were two neutral faults this could happen; (a) neutral becomes disconnected at source and (b) a single line shorts to neutral - this would apply line voltage ($\sqrt3$ x phase voltage) to be applied to appliances on the other two phases - sure this would cause damage BUT it's a long way different from some form of ground/earth fault.

• The fault was outside the houses, in the distribution point which supplies 3-phase electericity to tens of buildings. Commented Apr 3, 2014 at 22:32
• was it a ground fault or a loss of phase fault ora neutral fault? Commented Apr 4, 2014 at 7:48
• Don't know exactly, the engineers just said "fault". What I saw was the ground (black cable) was cut. Commented Apr 4, 2014 at 11:40
• That sounds like a neutral fault to me - earth wires are not usually black but maybe they are where you live? Commented Apr 4, 2014 at 11:58
• Well, may be it is neutral. I don't know the difference! Commented Apr 4, 2014 at 12:04

A single line to ground fault on a three phase supply causes the phase-to-neutral voltage on the un-faulted phases to rise. The voltage rise depends on how your three-phase system is earthed. Unearthed systems will have high voltage rise under fault; effectively earthed systems have lower voltage rise under earth fault.

Let's say that you live in Australia, where our normal phase-neutral voltage is 240VAC and our phase-phase voltage is 415VAC. You have a three-phase supply consisting of red, white, and blue phases.

If red phase faults to ground, then (in the worst case, assuming an un-earthed system) then white phase and blue phase voltages could rise from 240VAC - the normal voltage - to 415VAC. Appliances connected to white phase and blue phase probably wouldn't like this.

Fault in ground is dangerous in three phase supply.

Normally voltage across the device is voltage difference between a phase and ground. For improper ground, voltage across the device is almost doubled. That is voltage difference between two phases. An electronic device cannot handle this high voltage and may burn off.

3-phase voltage

• Most homes that don't use three-phase use split-phase to feed the panel box. Some even run split-phase to duplex outlets (the two outlets powered by opposite phases). Is there any reason a ground fault would be more dangerous with three-phase current than with split-phase, given comparable failure scenarios? Commented Apr 3, 2014 at 17:41
• That's a very US-centric comment. In Europe split-phase is almost unheard of.
– qdot
Commented Apr 3, 2014 at 18:06
• Here in India too.. My reference is my country. Sorry for my limited knowledge. Commented Apr 3, 2014 at 18:22