DC Earth fault need to identify and remove as early as possible to avoid tripping of protection circuits. DC Earth faults are frequently occurring on FCBC Charger, they should be removed to maintain the healthiness of Battery bank and Charger.
How to know the presence of DC Earth fault?
The FCBC charger consists of DC Earth Fault Relay, which senses the leakage current from either bus or ground. The center point of the BATTERY bus is connected thru a resistance voltage divider circuit to the earth fault relay control circuit, which is also earthed. When any leakage path develops in either DC bus, the same leakage current will flow thru this circuit and if it exceeds preset value the earth leakage relay actuates and is interlocked with audio/visual annunciation circuit.
The DC Earth Fault present on the circuit is detected by DC Earth Fault relay and it gives DC Earth Fault annunciation.
Method to Detect DC Earth Fault:
First of all it is make sure that the earth fault is on which side whether it is load side or source side. Here source side means on the Battery Bank which provides DC supply to the Charger.
1. Fault present on Battery bank:
To find out Earth fault is due to Battery bank disconnect the outgoing MCB of Charger which allows the supply to DCDB. Now try to reset the Earth fault relay, if it is reset then it is clear that no earth fault on battery bank. Otherwise there is earth fault due to battery bank. Now try to find out the possible reason for earth fault on battery bank itself.
The general cause is that there is electrolyte leakage from cells. Try to identify which cell it is and if possible replace with the new one. To identify faulty cell measure the voltage across each cell. The cell which is earthed will give less voltage compared to other cells.
2. Fault present on outgoing circuit from DCDB:
If there is no earth fault present on the Battery Bank it is confirm that earth fault exists anywhere on the control circuit.
Suppose consider we are having 20 number of feeders on a H.T panel. Let us assume there is a positive DC earth fault present on the 9th number of feeder control circuit which is on BUS-B.
So to trace out the fault first we need to check the values of positive and negative voltages with respect to earth using Multimeter.
Let us assume the system DC voltage is 120V. If there is no fault the voltage with respect to earth is equally divided between positive to ground and negative to ground that is 60V each.
So if there is positive earth fault the positive voltage with respect to earth shows less than 60V and at the same time negative to earth voltage shows greater than 60V. This can be checked at the main DC MCB.
Hence, we can assure there is a positive earth fault present on the system.
How to trace out?
Now consider our system dived into two buses Bus A and Bus B which are having different MCBs to provide the D.C supply. Now turn on all the MCBs of all feeders. Since there is an earth fault an indication alarm shown on the FCBC charger, for positive earth fault.
Now turn off the BUS-A MCB and try to reset alarm, it won’t reset as fault is still present. By this we can ensure the fault is present on BUS-B.
Now turn off each individual feeder MCBs of BUS-B and try to reset the alarm. Do this process repeatedly until the alarm is being reset. The alarm resets when we turn off the MCB of feeder-09.
By this we ensure the fault is present on Feeder-09. Now we need to find out where the fault exactly present on the Feeder-09. So to identify it turn on all the D.C MCBs of feeder-09 and turnoff each individual MCB until the alarm resets. For a particular MCB the fault is associated. Assume it is F-3.
Now to find the rest of the circuit turn off the F-3 and remove its outgoing positive polarity wire and check the continuity with respect to earth, if fault is present at other point we get the continuity with respect to earth. So from that point remove the wires one by one and check the continuity with respect to earth. After the faulty wire is isolated from the circuit, we won’t get continuity with respect to earth on any part of the circuit. Now replace the wire with the new one.
Thus we can trace out and remove the D.C earth fault from the system.