Weird Stuff Happening With My Circuit Breakers

[mg]

I recently obtained an old 220-V, 28000 BTU air conditioner that was said to work fine. This morning I decided to power it up and test it before going to all the work of installing it and an inexplicable thing happened. The 30-Amp breaker in my shed doesn't trip. The 60-Amp breaker next to my service entrance, going to the shed, doesn't trip. But the 100-Amp breaker in my service entrance does!

Is their something really mysterious going on here? Or, does it simply mean that my service entrance breaker is faster than the other two breakers?

[RBM]

Without hanging an ammeter on each, it's impossible to guess, however, it's possible that the 30 amp was pulling less than 30 amps, and the 60 amp was pulling less than 60 amps, but all the breakers on the load of the main,combined, were pulling more than 100 amps, at least momentarily, which caused the main to trip

[Toller]

What RBM said... What do you have on when it blows? If you have a water heater and oven on, it is certainly reasonable. If it happens with nothing else of substance on, your 100a breaker might be bad.

[Meat Plow]

I don't think you have a breaker problem especially if nothing strange happened before you tested the a/c unit.

[Mr.E]

If the load (A/C) is shorted the main may trip faster than the branch breakers. A fully magnetic-trip main and thermal/magnetic branches have been seen to produce this condition with none of the breakers faulty.

[Jeff Wisnia]

This may be seen as an obviousity....

But how about temporarily turning off all the other brekers in the service entrance and see if the 100 amp breaker still trips when you turn on tha AC.

If it doesn't, then as several other have said, your total consumption may be exceeding 100 amps at times.

HTH,

Jeff

[mg]

Thanks, Mr. E.

I'm guessing you probably solved the mystery. After reading your comment, I did some Google searching and found that magnetic breakers trip almost instantly but it looks like those using thermal technology technology have a trip delay of about 10 or 15 ms. I'm not sure what type the main, service-entrance breaker is, but it does look entirely different than the other two breakers in that it's very large, while the other two are the small, snap-in type of circuit breakers.

I think it's definitely a problem with a dead short and not an overload. The only things I had running in the house at the time were a TV and a computer and some lights and a 1/2 horsepower swamp cooler and the AC unit under test was turned off. When I measure the DC resistance between the two input poles on the air conditioner, I get 2 ohms which probably tells me something, I guess. Usually, when you get a dead short with residential appliances the connections probably melt before a lot of current can be drawn and the local breaker trips. The air-conditioner, must have some heavy-duty connections that don't. That was kind of a scary experience. I've never had anything like that happen before.

[mm]

Well I've probably never tripped a magnetic breaker, but otoh, you might have melted somehing inside but it cooled in a way that left it shorted.

Regardless, if it did this when it was turned off, it should be pretty easy to fix. Either the on-off switch is broken (shorted to the other hot somehow, or unlikely: it said off but was on and there's a second problem also), or the problem is in the cord before the switch. That's a very limited area to search, and you'll probably find sooty or melted stuff to help you out. And even if not, I see you know how to use a meter.

Maybe that is the reason they stopped using it, even if the person who gave it to you honestly thought that it still worked, or was told so. When you're past 50, that stuff is real easy to forget.

[mg]

Well, now I have a very difficult confession to make. I tested the unit using one of my generator suicide cords and grabbed the wrong one which just happens to present a dead short to 220V power. Being that I'm pushing 70, maybe I can use age for an excuse, but that certainly was a stupid thing to do.

In any case, I have it hooked up correctly now and found that the blower doesn't come on at all and a loud noise that I assume to be the compressor only comes on sometimes. So, I decided not to fool around with it anymore and I ordered a portable, Amcor, 12000 BTU unit from Circuit City for $335.96 (model #ALD 12000E). They have a sale going on right now, by the way, and shipping is cheap also at only $3.99.