Question

With reference to high voltage electrical distribution system

Sketch and describe a panel mounted earth fault monitor

Explain how HV system may be earthed.

The current transformer (CT) measures the phasor sum of the 3 line currents supplied to the motor. If the motor is healthy (no earth faults) the phasor sum of the currents measured by the CT is zero.

If an earth fault occurs in the motor, an earth fault current flows and the phasor sum of the currents is now not zero. The current now monitored by the E/F relay is used to trip the contactor in the starter to isolate the faulty motor circuit.

High voltage systems (3.3 kV and above) on board ship are normally ‘earthed’. Such systems are normally earthed via resistor connecting the generator neutrals to earth as shown below.

The ohmic value of each earthing resistor is usually chosen so as to limit the maximum earth fault current to not more than the generator full load current. Such a Neutral Earthing Resistor (NER) is often assembled with metallic plates in air but liquid (brine) resistors have also been used. The use of such an earthed system means that a single earth fault will cause that circuit to be disconnected by its protection device.

Earth indication lamps are arranged as shown above. If the system is healthy (no earth faults) then the lamps glow with equal half brilliance. If an earth fault occurs on one line, the lamp connected in that the line is dim or extinguished and the other lamps glow brighter.