Small Appliance circuit tripping breaker problem

[newservice]

20A residential kitchen circuit with 3 receptacles trips breaker. Did find a fault at the back of a single gang metal box, the clamp had pinched the 12-2 romex. Fixed that two weeks ago, however customer now states it tripped breaker the other night when husband turned on microwave. Connected appliances are refrig and microwave. Current is 10.5A on micro, and frig was 1.6A, but I now realize the frige compressor was likely not running as that seems too low? Anyhow, the first part of the circuit is in 3/4 EMT with multiple other circuits and breaks out to romex under the kitchen.
I suppose a megger may be in order, but does anyone think the microwave and frige could be >20A, or, that the microwave could be spiking when they turn it on?

 

[Ponchik]

I don't think it is the current draw of the appliance.

How long does it take for the CB to open?
Did you install the circuit?
The appliance it self could have a fault in it.

Just some thoughts.

 

[newservice]

I don't think it is the current draw of the appliance.

How long does it take for the CB to open?
Did you install the circuit?
The appliance it self could have a fault in it.

Just some thoughts.

I also think it's unlikely the current of the appliances but idk. The CB opened immediately when the guy turned on the micro but this is the first time that has happened. I did not install, they called me originally for the same problem, and I did find the feed to outlet 1 to have burnt the romex where someone overtightened the clamp in the back of the box. To this end, I also suspect the other romex cable could also have the same problem as it was under the same clamp.
Until I can get there I think I will have them move the micro to the other counter and see if it happens there.
What would I expect to see on a good circuit with a megger? My brother has one but I will look and see if the other cable is shot first.

 

[Ponchik]

I also think it's unlikely the current of the appliances but idk. The CB opened immediately when the guy turned on the micro but this is the first time that has happened. I did not install, they called me originally for the same problem, and I did find the feed to outlet 1 to have burnt the romex where someone overtightened the clamp in the back of the box. To this end, I also suspect the other romex cable could also have the same problem as it was under the same clamp.
Until I can get there I think I will have them move the micro to the other counter and see if it happens there.
What would I expect to see on a good circuit with a megger? My brother has one but I will look and see if the other cable is shot first.

I don't have any experience with meggers.

 

[growler]

If you have a short that's tripping a breaker you often will not need a megger. It will be enough to show up with a continuity tester. Use your meter and see what resistance you get from hot to neutral and from hot to ground.

When troubleshooting the problems it can be anything, it can be a bad microwave or a bad refrigerator. The idea is to either eliminate some of these things or to find a problem that can't be eliminated.

If you found one circuit shorted to the connector of a metal box there is always the possibility this happened more than once. Some people just have no idea of how to install romex in metal boxes. If you have a short why does the microwave need to be on for it to show up? If you have a short why is the GFCI protection not tripping?

One more thing, is this an arc fault breaker. Had a bad one just the other day and it was just the breaker.

 

[ptonsparky]

One overtight clamp = multiple overtight clamps. Count on it.

Wires move ever so lightly when loads start/stop. That may be enough to make/break your faulted conductors.

Fridges don't draw much these days so that low current could be all it needs to operate.

AFCIs are another issue.

 

[newservice]

If you have a short that's tripping a breaker you often will not need a megger. It will be enough to show up with a continuity tester. Use your meter and see what resistance you get from hot to neutral and from hot to ground.

When troubleshooting the problems it can be anything, it can be a bad microwave or a bad refrigerator. The idea is to either eliminate some of these things or to find a problem that can't be eliminated.

If you found one circuit shorted to the connector of a metal box there is always the possibility this happened more than once. Some people just have no idea of how to install romex in metal boxes. If you have a short why does the microwave need to be on for it to show up? If you have a short why is the GFCI protection not tripping?

One more thing, is this an arc fault breaker. Had a bad one just the other day and it was just the breaker.

From previous thread it is established well that the CB will trip on overcurrent short long before the GFCI will. The CB here is not arc fault and was just replaced by me a few weeks ago.
My brother has a nice megger, the idea is the insulation of the conductors is good to 600v, the megger puts 500v (at low current so as not to damage anything) on the line, and if it breaks down and shorts out it will show as a fault by moving the needle.
I agree, the microwave being turned on should not trigger a fault in the premises wiring per say.

 

[growler]

From previous thread it is established well that the CB will trip on overcurrent short long before the GFCI will. The CB here is not arc fault and was just replaced by me a few weeks ago.
My brother has a nice megger, the idea is the insulation of the conductors is good to 600v, the megger puts 500v (at low current so as not to damage anything) on the line, and if it breaks down and shorts out it will show as a fault by moving the needle.

Yes a breaker will trip before a GFCI on a dead short. You don't really need a megger to find a dead short.

If you have a megger handy that's great but if you don't there is no reason you can't troubleshoot a problem like this. You normally wouldn't use a microscope to look for elephants.

 

[newservice]

Yes a breaker will trip before a GFCI on a dead short. You don't really need a megger to find a dead short.

If you have a megger handy that's great but if you don't there is no reason you can't troubleshoot a problem like this. You normally wouldn't use a microscope to look for elephants.

I rather disagree. An intermittent problem like this can be tough, esp when there are 2 different wiring methods (emt with multiple circuits behind drop ceiling, and romex) involved.

 

[newservice]

Update. 20A circuit likely was overloaded for connected appliances. Instead of visiting the site again I spoke to the homeowner and she told me the nameplate ratings on the frige and microwave, which totaled to 19.1A. Go figure. So I surmise the circuit was fine and the breaker had tripped because of the start up current to either one. She is going to put the 1kw toaster oven (resistive load) on the plug where the microwave was. It only tripped once in a month and that was when they had just turned on the microwave oven, likely when the frige was running. Frige says 7.90A, Micro was 1350W.