Western Electrical Contractors Association, Inc.

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Troubleshooting service panels and series resistance

From the desk of Instructor Jimmie Slemp 

(Just as with old police shows, names have been omitted to protect the innocent and not-so-innocent.)

There's a certain expectation that, as an electrician, you know a little about the theory of electricity and ohms law.  Now if you have been through WECA's program you certainly were taught it at one time or another-- but would you retain it and when might you ever need to use it?

I received a call from a journeyman electrician - he told me that he was troubleshooting a service panel and this is what he was reading at the main circuit breaker -

L1 to L2 = 240 volts

L1 to Neutral = 120 volts

L2 to Neutral = 10 volts (approx.)

I told him that he should call the utility (SMUD or PG&E ) as soon as possible - not that there was any immediate danger but that perhaps he could be there when they look at it.  (Side note - you would hate to have them troubleshoot it when you are not there and simply say "it's not our problem.")

Later he called me back, and here's the recap:  The utility service tech came out and proceeded to check out the panel.  He shut off all the branch circuits and tested voltage at the main circuit breaker.  These were his findings -

L1 to L2 = 240 volts

L1 to Neutral = 120 volts

L2 to Neutral = 120 volts

I believe he was about to leave and say those famous words - "It's not our problem," when the journeyman electrician, (a little puzzled), said "let's turn the branch circuits on and test it again."  So they did just that - turned all the C/B's back on and retested.  Well, if you had to guess, what do you think?  Yes, the results were back to the original readings, with L2 to neutral, only reading 8 volts.  What do you think could/would cause this?

The Answer: