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All...

My garage had only one outlet so I extended it using surface raceways. Like a dummy, I did not turn off the breaker when I started the installation of the raceway. "I know what I'm doing and I wrapped electrical tape around the existing outlet, and I'll be careful". (Stupid and lazy!) Of course, I created a short. No injuries - just my pride.

When I went down to the basement to check things out I discovered something odd. I'm not sure if this is something I should be concerned about...

The 15A garage circuit is in a subpanel. Our automatic backup generator (Generac) is not large enough to power everything in the house. The subpanel contains a transfer switch. Commercial power comes into the transfer switch via a 50A breaker in the main panel.

The 15A garage circuit breaker tripped, as expected. What I did not expect was to find that the 50A breaker in the main panel had also tripped. The generator and transfer switch did their thing, firing up the generator and throwing the transfer switch. I reset the 50A breaker and everything went back to normal, again as expected.

Is the tripping of the 50A breaker in the main panel something I should be concerned about? I would have expected that tripping the 15A breaker would have "protected" the 50A breaker. Is there a test I should do on the two breakers to make sure they are functioning correctly? They seems to be but...

TIA for your help...

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    Yeah, even the pros don't work live when they don't absolutely have to... and they're professionals at being careful and have all the PPE gear. The novice's impulse to work live is due to insufficient information about the hazards.
    – Harper - Reinstate Monica
    Commented Mar 8, 2021 at 21:29
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    Lesson learned...
    – Clint
    Commented Mar 9, 2021 at 17:13
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    @Harper-ReinstateMonica The pro's have several options to work safely, but will in this case choose for turning off power in order to work faster. We have a joke about peeling shrimp with boxing gloves. Here, cutting cable, strip insulation, screw in terminals, everything goes faster with power off. For the pro, while working safely, time is money.
    – Roland
    Commented Mar 10, 2021 at 15:49

5 Answers 5

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Grats on your successful generator test :)

Whenever you have a bolted fault, you flow hundreds of amps. The only thing that impedes flow is the resistance of the wires themselves, which you can look up on the internet. "Hundreds of amps" is well within the range of the "instant trip" functionality of every breaker in the panel except perhaps the main.

Due to manufacturing tolerances and environment changes, "instant trip" has an acceptable range. The acceptable range runs between 4x and 10x, where it becomes a guaranteed trip.

So for instance, a 500A bolted fault is a guaranteed trip for a 15A and 50A breaker, and a possible trip for a 100A main breaker. Probably will not trip a 200A main.

If your short had flowed only 90 amps, both breakers would operate in thermal-trip mode (most likely). The 15A breaker would see a 6x overload, and the 50A breaker would see a 1.8x overload. The 15A breaker would trip in a few seconds at most, and the 50A breaker would trip in a few minutes, based on the usual trip curves for breakers. In that case, the "selective coordination" you were hoping for would work as you'd expect.

However, all bets are off in a bolted-fault/instant-trip mode.

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    So, if I understand correctly (which I don't think I do), the fault I created momentarily pulled more than 50A, enough to trip the 50A breaker in the main panel, and more than enough to trip the 15A breaker in the subpanel. What I don't understand is the sequence of events that resulted in both breakers tripping. Was it so fast that both breakers were in the act of tripping simultaneously?
    – Clint
    Commented Mar 9, 2021 at 17:11
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    Exactly: Fast and BIG so they tripped simultaneously.
    – manassehkatz-Moving 2 Codidact
    Commented Mar 9, 2021 at 17:39
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    @Clint The instant-trip function is generally implemented with a magnetic coil that physically unlatches the switch. There's a tiny delay while the mechanical parts move, and electricity is fast, so both coils can get energized to the trip point before the actual disengage and following drop in current happen.
    – chrylis -cautiouslyoptimistic-
    Commented Mar 9, 2021 at 22:03
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    @Clint -- for currents in the instantaneous range of both breakers, that's basically the case -- both breakers "see" the current at the same time and start tripping on it; once that happens, it's a race condition to see if one breaker can disconnect the fault completely (including arcing time) before the other breaker gets to the point of unlatching (because after the trip mechanism unlatches, the bell can't be unrung)
    – ThreePhaseEel
    Commented Mar 9, 2021 at 23:59
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In general, breakers are guaranteed (required) to trip when their rated current is exceeded for specific time (Different types of breaker - [A,B,C,D in EU] - have different curves). For this reason breakers are arranged from highest to lowest rating, so the smallest one reacts first.
But
In case of short, breakers may trip in any order, including higher rated breakers up the chain. Short is basically operating outside specs, so no order is guaranteed.
[Personal experience: After "finding" a cable with angle grinder, the light breaker popped, then after couple seconds of buzzing noise, a main fuse popped as well. Had to break utility company seal to replace fuse and call them to retag the fuse.]

Answering your concerns:
Yes, this is normal when dead short happens.
You can reset both breakers after ensuring the short is fixed.

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  • Thanks, everyone. As always, learn something every day. Just, hopefully, not the hard way like in this case! :-)
    – Clint
    Commented Mar 10, 2021 at 16:07
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    Keep in mind that the trip curves specify a minimum and maximum trip time for each level of overload, and that acceptable range probably overlaps between breakers that are close in value (and varies somewhat between manufacturers). So it's possible to have situations where larger breakers trip faster than smaller ones without violating their specs. But yes, in general, smaller breakers are likely to trip before larger ones on the same overloads.
    – Nate S.
    Commented Mar 10, 2021 at 16:58
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Apart from your 15A sub panel breaker and 50A main panel breaker, there is the main fuse in the house entry panel, which is sealed by the utility company, and a fuse somewhere in the street. Your complete question would be if you should be concerned about tripping any of these FOUR devices.

The answer is that these protective devices should be coordinated, so that only one of those, closest to the fault, should trip. Remember the commercial where a hobbyist plugs in his new Christmas tree lights and the entire city goes dark? That is of course what nobody wants.

How high is the short circuit current? Ohms law says voltage divided by resistance. There is resistance in all wiring between the transformer in the street up to your short circuit location. Also, add in the impedance of the secondary winding of the transformer, and the resistance of the protective earth conductor. The value of the short circuit current is by design pretty high in order to trip the fuse or breaker fast in case of a short, mainly because the earth connections should be low resistance. But not too high to avoid the fuse or breaker to explode.

Does coordination work? Most of the time. In your case, the 50A breaker trips just as fast on that short circuit as the 15A breaker. Circuit breakers can trip really fast, and the ratings are 'only' a factor of 3 apart. Fuses may be a bit slower, so you did not have to call your utility to replace that one. But if you toy around too much, you may be out of luck here.

Should you do a test on your two breakers? If you already called yourself a dummy, messed up with electrical tape, and also need to ask the question, I'd say you would better leave this to a specialist :-)

I know something about electric but am not an installation specialist, but I doubt that new house installations are routinely tested with short circuits. Coordination is designed with short circuit calculations and by carefully evaluating the specs on the circuit breakers. Measurements are done in high tech labs by circuit breaker manufacturers.

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    Generally in residential settings in the US, breaker trips are not coordinated, and the breakers don't have any provision to coordinate them. Certain industrial grade breakers have some adjustments possible to allow for this, but they're expensive, and not typically seen in residential panels. There's no safety risk to having them uncoordinated; it's just for a more convenient reset, so it's not required. The utility fuse should always blow slower than a breaker though.
    – Nate S.
    Commented Mar 9, 2021 at 17:09
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Due to there being two sources of power it's difficult to say exactly what happened. Here are a couple of possible scenarios:

  1. 50A breaker tripped, cutting power to panel. Generator started, transfer switch activated, short-circuit continued flowing current. Now the 15A breaker trips.
  2. 50A breaker and 15A breaker were both heated concurrently by the short circuit. Due to manufacturing tolerance that particular 50A breaker might respond a little quicker and the 15A breaker a little slower than nominal. If so then for this particular fault it could be that both activated their magnetic trip trigger at approximately the same time and therefore they both opened.
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  • Generator has a 15 second delay delay before starting so it would seem that #2 (essentially simultaneous open) is the more likely scenario.
    – Clint
    Commented Mar 9, 2021 at 19:55
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    This would be a magnetic trip, not a thermal trip. For this kind of trip, the high current through a coil creates a small electromagnet, which pushes an actuator that disconnects the circuit. They should both start tripping (i.e. the magnet starts pushing) instantly, and there's probably a small delay as the actuator physically moves before the circuit is fully disconnected, which gives enough time for both of them to actuate.
    – Nate S.
    Commented Mar 9, 2021 at 20:36
  • @NateS. ok - fixed!
    – Greg Hill
    Commented Mar 9, 2021 at 22:27
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This in general indicates poor breaker coordination. It's a fairly involved topic, with books written about it. In general, with a properly designed electrical system, it shouldn't be happening. But you may end up having to buy parts that don't come from Home Depot.

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  • [citation needed], because I don't think that's true that it's necessarily a poor breaker connection. Read through the other answers; there's other ways for this scenario to occur, which are probably more likely in OP's case. And a properly designed electrical system doesn't need to guarantee this doesn't happen, since there's no safety concern with tripping multiple breakers.
    – Nate S.
    Commented Mar 10, 2021 at 18:30

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