Megger questions

[joshfromoshoto]

I recently had to use a megger to test potentionally bad underground cables that power parking lot lights. On phase a I got an immediate 4000 meg ohm reading which is the highest the megger could read, and on phase b I got roughly 600 meg ohms. The circuit keeps tripping and it does not appear to be and of the wiring inside of the light. Am I reading the megger wrong? I had someone tell me that 1 meg ohm per 1kv of operating voltsge was acceptable, so according to him, 600 meg ohms should be more than enough. I guess I really don't understand the instrument itself and any help would be appreciated.

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[ActionDave]

This was already answered in your other thread. The one meg per kv is just a rule of thumb, it is not a standard, and in your case it is meaningless. You have a fault and if it's not in the pole it's underground.

 

[ammklq143]

If you haven't done so, make sure you isolate the conductors so you're only testing the them and not getting anything from the lights that interfere with the test. Also, disconnect the light and see if the breaker still trips when power is applied.

 

[Little Bill]

@ammklq143
Could you please update your profile to include your occupation and general location?
Thanks!

 

[hillbilly1]

Probably a bad ballast or driver if the circuits check good. Do not meg the fixtures unless you want to replace them. A simple ohmmeter test can usually find a bad ballast, a little harder for a driver. There should be zero continuity between the fixture wires and ground with the fixture disconnected. Another test is to clamp your ammeter around both fixture wires at the bottom of the pole with them on. You should get zero amps, if not, the fixture has a fault to ground.

 

[Little Bill]

Probably a bad ballast or driver if the circuits check good. Do not meg the fixtures unless you want to replace them. A simple ohmmeter test can usually find a bad ballast, a little harder for a driver. There should be zero continuity between the fixture wires and ground with the fixture disconnected. Another test is to clamp your ammeter around both fixture wires at the bottom of the pole with them on. You should get zero amps, if not, the fixture has a fault to ground.

What good would clamping the wires do if the breaker is tripped? OP said his breaker tripped.

 

[hillbilly1]

What good would clamping the wires do if the breaker is tripped? OP said his breaker tripped.

He said it keeps tripping, he did not say instantly or periodically. If periodically, the wire clamping would narrow down the problem.