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In our home, we have two electrical panels. One main panel (in the basement) and a sub panel on the first floor. The lights to the kitchen on the first floor run to a breaker in this sub panel.

We installed a dimmer incorrectly and accidentally tripped the breaker in the sub panel. However, at the same time, the main panel in the basement shutoff and the entire electrical system shut down.

It was our understanding that when you cause a short, it would cause only the one breaker to trip, but not the entire system.

This has made us wonder if our main/sub panel setup is functioning correctly, or if there is an issue that requires further investigation. The installation of the sub/main panel was done by certified electricians.

Tester101
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user15817
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3 Answers3

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I don't think this indicates a problem.

Circuit breakers have at least 2 different triggers for cutting the power:

  1. Thermal mechanism for small over-current protection, e.g. trying to draw 20 amps from a 15 amp circuit. This may take several seconds or even more than a minute, depending on how much over the rated capacity you are.
  2. Magnetic mechanism for short-circuit protection, which should kick in around 20x the rated ampacity. This should trip the breaker virtually instantly.

You can look up the time-current curve for your breakers if you're curious how long it takes them to trip under different conditions.

(There may be additional features that cause the breaker to trip, such as arcing or ground fault detection.)

If your circuit had tripped because of a slight over current, such as too many appliances running at the same time, only the breaker for that individual circuit should have tripped.

However, for a short-circuit condition (which I assume you had with your dimmer), either one could go. It's a little surprising that both tripped, but there is a slight delay in response time so it's not inconceivable.

(Actually, you could have had 3 different breakers trip: the breaker for the circuit, the breaker for the sub panel feed, and the main breaker).

Hank
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  • @Tester101: I think we're in agreement that 2 breakers tripping is not worrisome or particularly surprising in this case. However I think the 10kA value you're thinking of is the UL Short Circuit rating, which is the maximum current the breaker can withstand without damage. The magnetic short-circuit trip current is substantially lower, typically around 20x the rated current (so ~300A for a 15A breaker). You can look up the time/current curves for any breaker, e.g. for the Square D QO series, check out page 20 in this Square D manual: http://tinyurl.com/kgq4c3j – Hank Oct 29 '13 at 01:00
  • You could possibly even cause the [cutout fuse](https://en.wikipedia.org/wiki/Fuse_cutout) on the [distribution transformer](https://en.wikipedia.org/wiki/Distribution_transformer) to trip. – Brad Gilbert Oct 29 '13 at 16:12
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In a short circuit condition, there are very high currents. As previously stated, the initial over drawing heats a bi-metallic strip that acts as a thermal cutout for excessive loads.

The second is a small coil inside the breaker that when exposed to high currents (short circuit) acts exactly like an electromagnet and pulls the breaker open immediately. Once the short circuit condition happens anywhere in your system, it can potentially travel through everything using the bonding connection at the neutral bus and grounding bus and travel back up the main neutral to the supply transformer.

What happened in your house is how you pray your electrical system works. You got a free test on your equipment and it operates exactly as designed!!

creepyrodney
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Whether it's a bulb blowing or an appliance failing in our house, the main 80A / 30mA RCD breaker always trips but the Type B MCBs for the individual circuits (e.g. lights, sockets, cooker, shower, immersion heater etc.) never do.

Surely that indicates that the panel is incorrectly wired or that the main breaker is too sensitive.

Felim Doyle
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