As predicted, got called back today. Flipped breaker back on , and began waiting for it to trip again, so we could isolate it (hopefully)

After two more hours of head scratching, was lucky enough to be in the attic when the breaker tripped again. In a frightening manner , a poof of blown fiberglass and orange Sparks geysered out of the attic floor.

Pawed through fiberglass, found a piece of 1960s Romex coming up through top plate , blackened right where it exited the drill hole , above a room we never even did any work in.

We cut an access hole in room below to retrieve the bad segment of wire as proof. There was a clean gash across the jacket, as if it had been sliced back when it was installed originally . Inside the gash the ground and neutral had been melting .

Go figure .

 

 

Meant to say , hot and ground melting obviously

 

Just curious, do you have a megger?

 

I don't. Sad to say (forgive my ignorance) didn't know about them until I started googling this issue last night, before I had it figured out. I'm a JY on my 3rd employer and no boss of mine has ever mentioned such a tool before! Must not be common among resi contractors round my neck of the woods.

Id like one now though!

 

Good job (and luck) finding that

 

The QO breaker saved the building. ( Quick Off )

 

Got to love Square D QO ;-)

 

A circuit that's tripping a QO c/b will certainly be exposed by a megger.

The OP got very lucky that he was in the attic when it blew... looking in the right direction, no less.

I must recommend that the OP obtain a toner and wand, too.

The hot being shorted to ground would show up right off during routine toner procedure.

( One always tests for it, as such a short will ruin toning operations.)

A short able to trip a QO would absolutely show up with the toner's electronics.

 

I don't know about this...

The whole point here is that it wasn't shorting (until it did again). So the toner wouldn't show anything.

The OP used an amp probe and had zero current flowing. The short was only happening intermittently, probably when the temperature of the conductors changed enough to make them move into contact.

A megger would find that "almost" short by jump the gap at the times when it wasn't an actual short.

No?

 

Did some work at a house recently, after we left, HO reports tripping breaker. Non arc fault , Square D QO.

Upon return next day, I reset breaker. It trips about 30 seconds later. I reset it again , stays on for 5 min. Reset it a 3rd time, shuts off in 30 seconds.

I replace breaker, and it stays on for more than 5 minutes. So, I put on an amclamp. Reads 0.1A. I wait 1 hour , then decide that bad breaker was culprit.

Proceed to re-attach panel cover. As I'm tightening screw 6 of 6, breaker trips. No wires touching screws in gutter either. I check continuity hot to ground- nothing . I check bus for corrosion, it's pristine. I Notice there's a doorbell trans pigtailed onto circuit, I move it to another circuit.

You reset a circuit breaker multiple times?

THAT IS SUCH A NO NO, We are professional electricians we are suppose to trouble shoot and ascertain the issue why the CB tripped the first time.

This stuff ain't magic and is not self healing.

 

I had a bad experience, which could have been much worse, a few years
ago when I reset a breaker immediately after it tripped. Lesson learned.
P&L

 

Another thing ...

I wasn't entirely sure if non arc breakers would trip outside of overheating and overloads . A part of me wondered if some weird high frequency fluke or inductive load could set them off at low current. That was part of my confusion.

Henceforth, I'm going to always play it safe and assume I have a definite short to locate .

 

An easy way to test a breaker is to put it on a circuit you know is good.

 

My favorite little megger for general use. They run just over $100 right now, better for a real reading than the Supco.

 

But.... But... Where is the green and red lights!

 

Using the Supco as a selling tool reminds me of the lie detector they gave that young kid in The Wire.

Only the cool people who watched The Wire will know what I am talking about.

 

I don't know if it would be an advantage with a megger, but sometimes an actual needle on an analog meter is better because you can see very brief intermittent blips that a digital meter might miss. I know sometimes with controls work I'll break out the mighty Simpson.

 

I do like that feature on the little megger I use. If the moves you can figure it isn't a line to line short in a compressor as much as it could be moisture or acid in the oil. Then you can drain it and recheck and find it was the oil.

 

I had a similar situation a while back. It was a service call to an older rental house that had been remodeled many times over the years. The breaker would trip randomly. I braved the crawlspace on three separate occasions before I finally found it. I just happened to tug on a cable that went up in to a wall, it pulled right out. It had been cut and abandoned at some point in the past (handyman remodel?). They were damn lucky the place didn't go up.

 

Wow, crazy what happens in resi during remodels!