[dford143]

Okay so I've been racking my brain on this random issue for a couple weeks now. I've got the main panel in the house feeding my garage subpanel 240VAC service that has been installed and working for years now. Recently it seems during inclimate weather (rain/thunderstorms although not every time it rains making it more frustrating) the main breaker in the house feeding the garage will trip. When I go to reset the breaker, it will instantly trip again. I've found that usually when I go to the garage and turn off all the breakers that I can flip the breaker in the house back on. Then I can go to the garage and individually flip the breakers back on and all seems well.

Since its the line from the house to the garage tripping I'm thinking it might have been the wire in the ground however if I turn the breakers off, I seem to be able to bring the circuit back up. I also swapped out the breaker itself thinking maybe that breaker may be worn however after doing so it has since tripped.

Normal load when this seems to occur would be a dusk to dawn light on the exterior, a DVR/camera system, the garage door opener, a network switch/wifi extender, a stereo receiver and possibly an exhaust fan although since its been cool, the thermostat likely has it turned off.

I'm really at a loss here at exactly even how to approach this problem. I'm pretty good with electricity and wiring it but this is simply a bizarre problem that I've simply never seen.

 

[joed]

Disconnect the hots in the main panel and turn off the main breaker in the garage. Measure from each hot to ground for resistance. There should be infinite resistance. If the needle moves at all you have an issue.

 

[dford143]

This is a fantastic idea.* Thank you!* More to come!

 

[J. V.]

Next time this happens, go to the garage and turn off the main breaker. If you do not a have a main breaker in the garage, turn all breakers off.
Then return to the house and turn the garage breaker back on. If it holds, its most likely not the wires in the ground. Likely, but not 100%. But a pretty good start to the elimination process.
Then return to the garage and turn on one breaker at a time. Do this until you find the offending circuit in the garage. The garage circuit that trips the main in the house.
This is how I would look for the issue. But I would also megger the feeder. You do not have a megger? And a megger can cause other issues like destroying equipment if not used properly.

Using a meter to measure resistance to ground can be misleading and may not help you as you need to narrow this down. Down to the buried wires or circuit in the garage.

 

[dford143]

Next time this happens, go to the garage and turn off the main breaker. If you do not a have a main breaker in the garage, turn all breakers off.
Then return to the house and turn the garage breaker back on. If it holds, its most likely not the wires in the ground. Likely, but not 100%. But a pretty good start to the elimination process.
Then return to the garage and turn on one breaker at a time. Do this until you find the offending circuit in the garage. The garage circuit that trips the main in the house.
This is how I would look for the issue. But I would also megger the feeder. You do not have a megger? And a megger can cause other issues like destroying equipment if not used properly.

Using a meter to measure resistance to ground can be misleading and may not help you as you need to narrow this down. Down to the buried wires or circuit in the garage.

I do not have a breaker to shut the panel down in the garage, only the house side. I've done exactly what you've described as far as shutting them all down and bringing them back up individually and interestingly they all come back up and the main in the house holds. That's the part that really has me confused. To me, if I shut down all the breakers in the garage one of them when flipped should take the entire panel back down by tripping the main in the house, however it does not. The other bizarre thing here is that when I find the breaker tripped inside the house, I can reset it several times only to have it make a bit of a buzz and trip again immediately.

I unfortunately do not have a megger.

 

[seharper]

Does the breaker in the house have a TEST button on it?

 

[J. V.]

I do not have a breaker to shut the panel down in the garage, only the house side. I've done exactly what you've described as far as shutting them all down and bringing them back up individually and interestingly they all come back up and the main in the house holds. That's the part that really has me confused. To me, if I shut down all the breakers in the garage one of them when flipped should take the entire panel back down by tripping the main in the house, however it does not. The other bizarre thing here is that when I find the breaker tripped inside the house, I can reset it several times only to have it make a bit of a buzz and trip again immediately.

I unfortunately do not have a megger.

How often does this happen? You could start turning off all the breakers each time you are done in the garage.
This way if the main does not trip anymore you know its not the feeders underground. You must eliminate one thing at a time.
You get where I am going?

 

[Missouri Bound]

A buzz and trip is usually a dead short.
A few questions:
How long a run is this to the garage?
What is the amperage feeding the sub-panel?
Is the wire literally in the ground or in conduit?

Check the voltage at the mains in the garage.
Then turn on as many loads as you can and check that voltage again.

From what you have said so far, tying the problem to the weather is a good indicator that it is a fault created in the buried wire when it gets wet.
One nick in a cable is all it takes for these to develop. I put everything in conduit unless it is physically impossible to do so to avoid those issues.

A couple more questions:
How many circuits in the garage?
The garage is not attached....right?
And one more question. Do you know if when this occurs there is ever any load at all on the circuit?

 

[dford143]

A buzz and trip is usually a dead short.
A few questions:
How long a run is this to the garage?
What is the amperage feeding the sub-panel?
Is the wire literally in the ground or in conduit?

Check the voltage at the mains in the garage.
Then turn on as many loads as you can and check that voltage again.

From what you have said so far, tying the problem to the weather is a good indicator that it is a fault created in the buried wire when it gets wet.
One nick in a cable is all it takes for these to develop. I put everything in conduit unless it is physically impossible to do so to avoid those issues.

A couple more questions:
How many circuits in the garage?
The garage is not attached....right?
And one more question. Do you know if when this occurs there is ever any load at all on the circuit?

There is no 'Test' button on the breaker.

To J.V.'s point, I could shut down breakers when not in use except for the circuit that runs my cameras and the garage door opener. That would begin the elimination process.

MissouriBound, from house to garage you're looking at ~60ft underground and no, the garage is NOT attached to the house. 60 amp (I believe but don't quote me on it and I'm not there to verify) breaker feeding the sub-panel. I believe the wire is direct burial and in the ground as such. I believe it comes through the concrete slab and into the wall via conduit however the garage was built 20 years ago and is insulated and covered so memory isn't great and not easy to verify. The wire however has been in the ground likely longer than I've been alive.

I'll run the voltage check before and after dropping circuits to measure the changes. I believe weather is a factor here since every time its tripped it has rained.

6-8 circuits in the garage. When it occurs, as far as I'm aware there is nothing on the circuit other than a dorm fridge, exhaust fan, stereo receiver, Amazon Alexa, network wifi hub/switch that could possibly actually be on. I recently shut down the circuit to the remote shed in case something was abnormal there however even with that circuit off, the issue persists.

 

[Missouri Bound]

What I would do:

Next time it happens, leave the breaker off and disconnect the wires from the breaker. Then go to the garage and disconnect the wires from the sub panel.
Put an ohmmeter on the two wires you have just disconnected. You can determine if there is a short by doing this. If you can, do it right after the breaker trips. That way the condition that caused it may still be present.
After this test, if nothing is found, leave the wires off of the breaker at the panel and reconnect the sub panel. With all of the breakers off, keep checking by turning the breakers back on with no load on the circuits. Unplug all devices and keep the lights off. With everything off and/or unplugged you should see no resistance on the circuit. You may have a combination of issues.
But I still lean towards an underground fault.
Are you in a climate that freezes and puts frost in the ground? Heaving ground does a number on pipes, wires, fence posts etc.

 

[seharper]

A buzz and trip is usually a dead short.

In my experience, a dead short (more than 10x aka 1000% of the breaker trip current) results in an *instant* trip, no buzzing, because the magnetic part of the mechanism operates at 10x amps. +/- 10%.

Buzz *then* trip, that means you're still in the thermal trip zone so <900%. Then you need to look at the thermal trip charts, and it'll show you probably for a 1-second trip, maybe 300-500% of rated current. So either an overload (locked rotor on a stalled saw that never got turned off?) or indeed a dead short but up a long long wire run.