[kutsar]

What do the terms 'first fault' ('single-pole fault') and 'second fault' denote (in the context of an electrical system)?

[sathe1963]

Dear kutsar, first at today's opportunity; I would like to explain you about first and second fault for ungrounded system. There are ungrounded system used / impled in food processing industry especially.
Secondary of the transformer neutral is not gorounded or there is no neutral. In such systems, when first fault occurs, means any of the three phases goes to ground nothing happens as a short circuit only thing is potential across the cable insulation is increased by sq.rt.3. Thus insulation of cable is stressed than the normal supply voltage. (but, in this type of systems, ground fault signalling arrangement is incorporated to detect the first ground fault. Once the ground fault is annunciated; the production / raw material in pipeline is taken out first and then system is shutdown to investigate the reason and area of first ground fault.
Before detection; suppose first fault is perisisted and again 2nd phase goes to ground; then this is called as second fault. Which caused actual short circuit to flow through the system... feeder... source. And this has sever impact on the system.