Power Distribution - Low current breakers or fuses?

[diviney]

Don:

A FMEA... Interesting thought. Perhaps this thought could turn me around into thinking a 20 amp breaker can be "safe" even with a control circuit using "light gauge" wire.

The worrisome failure mode in this case is a wiring short. The PD specs indicate that the Wago connection to the VB3 breakers can accept AWG10-24. If we were to use AWG 22 wiring as suggested on the diagram on page 13 of section 3, then you would worry about the wire fusing. Although 20 Amps is well above where you should operate a 22 gauge wire, it's fusing current is roughly 40 amps (well, this depends on a lot of things, but hey, you sort of have a 100% safety margin above the breaker trip point). I suppose you could envision a partial short failure such that you drew 20 amps constantly (not enough to trip the breaker, or fuse the wire, but could cause dangerous heating rather quickly while something tries to dissipate the resulting 240 watts). This is much less likely than a full short however.

Maybe you wouldn't use AWG 24 however because it's fusing current is more like 29 amps (only a 50% margin). AWG22 or heavier sounds like it may be reasonable safe...

If you look at the VB3 performance curves, these breakers will take around a second to trip at 200% of the rated current. I would think that the wire would take significantly more than a second to fuse, although it would be interesting to perform some tests to see. Our students would probably love to do some experiments which resulted in sparks and smoke (in a very tightly monitored and safe way of course) to explore this !!!

Of course the self-resetting breaker will repeatedly stress the wire as it tries and tries to recover. The VB3 data sheet states "less than 15 seconds" to reset.

Let's see what the official rules say tomorrow. It is quite likely they will say the same thing as last year, but we don't have to wait much longer to find out.

-Tom