Perplexing breaker trip

[Dave Mills]

In my shop (ex - garage) I have two 120V circuits fed from our house main 400A panel. One is 15A and powers the lights (3.5 amps) and garage door opener (1 amp). The other 120V circuit (20A) feeds all the wall outlets, and only a radio was on. I think this is pretty traditional house construction setup. I also have a small sub-panel in the shop that uses a 50A breaker from the main, and creates three 20A 240V circuits, for tools (inc dust collector and bandsaw).

This afternoon, as I've done many times, I had all the overhead lights on, as well as the dust collector. I turned on the bandsaw (which I'd been using fairly heavily), and the lights went out, breaker blown. The bandsaw and dust collector were still running, as was the radio on the 120V 20A circuit.

My basic understanding of how this stuff works is, that's not supposed to happen. But the lights went out absolutely the instant I turned on the bandsaw.

Any input as to what happened would be welcome! I just flipped the breaker back on and continued on my way, but didn't actually need to use the bandsaw anymore. I did use the jointer which probably sucks about the same amps.

[Bruce King]

If it was just a one time event I would think that the bandsaw was a coincidence. I have had one or two of those before. The other thing is the power grid in CA doing strange things. Was the breaker that tripped a GFI? Also could be a problem in the door opener, or a bad breaker. Was it hot when you reset it?

[Dave Mills]

Thanks for the input, Bruce. We (Calif) are not in the middle of one of our many power problems at the moment, but you're right, it can't be ruled out! The breaker is AFCI, and there are no GFI outlets on that circuit. The door opener works, but wasn't being used at that particular moment. As to whether the breaker was hot when I reset it, I didn't notice anything in particular but wasn't really checking for it being hot.

[Jim Becker]

AFCI can be pretty sensitive in my experience...which is a good feature when you consider their purpose, but occasionally can result in "weirdness". If it happens again, I'd investigate further, but it may very well just be a fluke.

[Rod Sheridan]

The AFCI may have tripped due to the opening of the starting switch in the motor.........Rod.

[Prashun Patel]

Backing up a bit, dumb question: Aren't AFCI's designed to detect arcing conditions? A bad switch on the bandsaw could cause that. If your bandsaw has one of those round, pushbutton rotating lock switches like on Grizzly's (read mine) they can wiggle loose over time, loosening the connection. That has happened to me.

I don't know about AFCI's but maybe they can detect these conditions on adjacent lines not just on the same line? Is it possible your two lines are not truly isolated but share the white or ground?

[Bruce King]

The AFCI is not even in the same panel as the bandsaw. It has to be a one time issue or a problem with the AFCI device, lights, wiring or connections. Is it a new house? It’s not common to have AFCI on an older garage light circuit. It is common to have electrical problems in new houses. Grounds rubbing on neutrals. Loose connections.

[Dave Mills]

The AFCI is not even in the same panel as the bandsaw. It has to be a one time issue or a problem with the AFCI device, lights, wiring or connections. Is it a new house? It’s not common to have AFCI on an older garage light circuit. It is common to have electrical problems in new houses. Grounds rubbing on neutrals. Loose connections.

The house was built in 2013, and I'm pretty sure all the 120V breakers in the panel are AFCI. Certainly the one in question is. But since you mention problems in new houses, grounds, etc, I should say I did have an electrician add a couple new ceiling light connections (the circuit in question) about 6 months ago. Perhaps if this issue crops up again, I ought to have him climb back up in the ceiling and check his connections.

[Bruce King]

Electricians hate AFCIs because often nothing is wrong except incompatible vacuum cleaners and drill chargers etc. On houses under warranty they are required by contract with the builder to show up at the houses of customers that call. I would call the manufacturer of those lights and Google to see if they are problematic.

[Dave Mills]

I will do that, Bruce, thanks for the suggestion.

Dave

[Timothy Orr]

The AFCI breakers in my house have a LED that flash a code to tell you why it tripped. Might look to see if yours have that feature. I had to search for the manufacturer’s spec to find out how to decode. There was no indication on the breaker itself of what the flashing LED meant.

Might help troubleshoot.

Tim

[Dave Mills]

The AFCI breakers in my house have a LED that flash a code to tell you why it tripped. Might look to see if yours have that feature. I had to search for the manufacturer’s spec to find out how to decode. There was no indication on the breaker itself of what the flashing LED meant.

Might help troubleshoot.

Tim
Interesting, thanks. I may have reset that code when I flipped the breaker back on, but if it happens again I'll take note.

[Tom Bender]

While it has no effect in this case, I think you have an error in your load estimate. A garage door opener probably draws more like 4 to 8 amps.


If this happens a few more times you might want to install some kind of backup lighting, since the saw continued to run in the dark.