8/20/2019 Insulated Earth Systems

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Insulated neutral system

AdvantagesThis system avoids the risk of loss of essential services e.g. steering gear i.

If the neutral was earthed and a short circuit on one phase causes the fuse in that phase to blow the

system would now be singled phasing and may burn out motors

ii.

In an insulated neutral, one earth fault does not interrupt the supply but an earth leakage detection

system will give warning.

iii.

Low earth fault currents in insulated systems gives a much less fire risk.iv.

DisadvantagesOn the insulated system the voltage to earth is 1.73 Vph e.g. 440v vs 250v i.

Tracing an earth fault is more difficult because although selective tripping may trace the earthed 

circuit, the actual position on the circuits may still be difficult to locate. Resonant or intermittent faults

in say a contactor solenoid or a transformer with an insulated neutral can cause voltages to be

magnified to say 4 times the normal voltage to earth (250v x 4 = 1000v)

ii.

Note: electrical shock is not reduced by using a non-earthed neutral as large voltages are

involved. Both systems are equally dangerous

Earthed neutral system

When an earthed neutral system of generation is used earthing is to be through a resistor. The

resistor is to be such that it limits the earth fault current to a value not greater than the full load

current of the largest generator on the switchboard section and not less than three times the

minimum current required to operate any device against

ted neutral system http://www.marineengineering.org.uk/elgen/elneu

7/17/2008