UNGROUNDED SYSTEM BASIC INFORMATION

In this type of system, a phase-to-earth fault only produces a weak current through the

phase-to-earth capacity of the fault-free phases.

It can be shown that Id = 3 CwV

V being the simple voltage,

C the phase-to-earth capacity of a phase,

wthe frequency of the system (w = 2* pi *f).

The Id current can remain for a long time, in principle, without causing any damage since

it does not exceed a few amperes (approximately 2 A per km for a 6 kV singlepole

cable, with a 150 mm2 cross-section, PRC insulated, with a capacity of 0.63 mF/km).

Action does not need to be taken to clear this 1st fault, making this solution advantageous in

terms of maintaining service continuity.

However, this brings about the following consquences:

c if not cleared, the insulation fault must be signalled by a permanent insulation monitor, c subsequent fault tracking requires device made all the more complex by the fact that it is automatic, for quick identification of the faulty feeder, and also maintenance personnel qualified to operate it, c if the 1st fault is not cleared, a second fault occurring on another phase will cause a real two-phase short circuit through the earth, which will be cleared by the phase protections.

Advantage

The basic advantage is service continuity since the very weak fault current prevents automatic tripping.

Drawbacks

The failure to eliminate overvoltage through the earth can be a major handicap if overvoltage is high. Also, when one phase is earthed, the others are at delta voltage (U = V* sqrt 3) in relation to the earth increasing the probability of a 2nd fault.

Insulation costs are therefore higher since the delta voltage may remain between the phase and earth for

a long period as there is no automatic tripping. A maintenance department with the equipment

to quickly track the 1st insulation fault is also required.

Applications

This solution is often used for industrial systems (< or = 15 kV) requiring service continuity.