Electrician Talk banner
1 - 4 of 4 Posts

·
Hackenschmidt
Joined
·
13,463 Posts
I was troubleshooting a short. I put one DMM lead on the wire terminal of the breaker which was turned off. I put the other lead on the neutral / ground bus of the service panel. I measured 10K ohms. I turned on the breaker and it tripped immediately as is typical of a short.

I found a completely burned outlet, fixed it by moving it up about 5" to get fresh cable, and repeated the ohm measurement by shorting BLK and WHT wires at the outlet. The result was less than 1 ohm as expected.

My conclusion is that the charred remains of the outlet conducted current as if it were a dead short if 120V was applied but did not conduct much current at what ever voltage the DMM uses to push current down the wire.

I don't want to troubleshoot shorts by switching on the breaker. Does anyone have any insights into this?
This in a nutshell is why you megger rather than just test resistance, the megger generates the voltage necessary to get a result more consistent with actual utilization conditions.

IMO it's best to have a megger that you can start low, like 50V, see if you missed any loads on the circuit, then work up to normal voltage, then 2x normal system voltage if you really want to be thorough.
 

·
Hackenschmidt
Joined
·
13,463 Posts
The issue I see with the 1000v Megger is that you better know everything that is connected to that circuit before using it and that is almost never the case in these situations
I was shopping around online and saw some decent meggers at good prices but they didn't have the lower voltages, I don't think I'd buy one that didn't let you start at about 120V so you can start low.
 

·
Hackenschmidt
Joined
·
13,463 Posts
How do you properly test 300V wiring at 120V?
I meant that when shopping they all had the higher voltages, but fewer and more expensive had the lower voltages.

My impression is for this kind of troubleshooting I'd test at 120 or lower, then step up to twice the system voltage, for 240v systems, 500V, for 480V systems, 1000V.
 
1 - 4 of 4 Posts
Top