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by forgotusername6 1094 days ago
In our office we were so over the rated current for the building that after the breaker tripped (which it inevitably did) it wasn't possible to just switch it back on. The moment you did all the servers and PCs went back on and it tripped again. You'd have to go around and pull out plugs all over the office then switch the breaker, then switch them all back on one by one. We also had regular electrical fires. Those were the good old days.
2 comments
rootusrootus 1094 days ago
Having enough inrush current to trip a breaker doesn't seem so terrible or entirely unexpected, but electrical fires? That's certainly bad news, the breakers are supposed to be sufficient to protect the wiring.
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cesarb 1094 days ago
> Having enough inrush current to trip a breaker doesn't seem so terrible or entirely unexpected

It should still not happen, since breakers are supposed to deal with inrush currents. A quick look at a random circuit breaker manufacturer page tells me that this particular breaker model is meant to instantly trip once the current is 3 to 5 times larger than the nominal current; less than that, it should take several seconds to trip, giving enough time for the inrush current to cease. So either the breaker (and the wiring) is underdimensioned, or the device is using too much power.

(IIRC, the trick is that most breakers have two independent trip mechanisms: a thermal one which has a built-in heat-dependent delay, and a magnetic one which is instant.)

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rootusrootus 1093 days ago
Right, I don't think inrush should pop a breaker, and I'd replace that breaker to see if it's just got an over-sensitive magnetic trip (IME they get more and more sensitive the more they pop). But it's a safe failure, at least. If the thermal breaker isn't working, that's dangerous.
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weaksauce 1094 days ago
> (IIRC, the trick is that most breakers have two independent trip mechanisms: a thermal one which has a built-in heat-dependent delay, and a magnetic one which is instant.)

precisely it. the magnetic one uses the fact that ac makes an electromagnet and they tune it so it snaps away quickly if there's a short.

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calvinmorrison 1094 days ago
Right isn't that a time delay fuse is used for?
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Nextgrid 1094 days ago
Most server BIOSes have an option for a random delay on power loss recovery to prevent this exact scenario.
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brk 1094 days ago
True, but most PC BIOSes don't, and PCs can outnumber servers 100 to 1, unless you're talking about a datacenter.
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