Three phase ground fault
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.

More information.