Earth Fault in DC System

with a 500 volt megger against earth or chassis

Megger Corp makes a test unit to search for and isolate ground faults on ungrounded DC systems. See:

http://www.megger.com/common/documents/BGFT_DS_en_V12.pdf

I assume you do not want to power the system down and do simple resistance checks to determine the faulty branch circuit. Or you have already tried that and it did not work.

First find out which is grounded + or -. If - is grounded, hook a 100W lamp from + to ground. This will put a 1.1A load on the ground fault(s) so that it may be easily located. If this blows the fuse or trips the breaker on the faulty branch circuit, you have found the problem! If it does not, proceed as instructed below.

Take your trusty DC Amp clamp meter and put it around both the + and - conductors for each branch circuit. By metering the + and - at the same time, the amp clamp will only indicate 1.1A current draw, or some fraction of it, on the branch circuit(s) which short to earth.

On those branch circuits which have no ground fault, the clamp meter will read 0 current as the same but opposite polarity current is flowing in the + as in the - conductors.

Once you have located the branch circuit with the ground fault, check each device on the branch circuit the same way. Devices with no ground fault will show zero current differential on the power leads. Devices with leakage to earth will show some current differential.

When you have cleared the last ground fault, the lamp on the + lead will go out as it no longer has a return path through ground.

If you find the + conductor shorted to earth, do the same as above except all of the polarities are reversed.

p.s. Remember to remove the 100W lamp when you are done as it will now the a ground fault on the system if you don't!