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

[dtengineering]

Originally Posted by usbcd36

Still don't understand what they have against small fuses for low-powered devices. If it's all about safety, it's worse to use a large breaker for small wire.

While I agree that it makes sense, from a stricktly engineering perspective, to use a small fuse rather than a large breaker, and I doubt FIRST has anything "against small fuses", there are a couple of reasons why they might choose to limit what can be used:

1) Inspection: Standarizing the power distribution means that there are fewer chances that an inspector might miss an inappropriate value or type of breaker/fuse. While many inspectors are capable of determining whether a particular breaker or fuse would be acceptable based on the robot design, may inspectors are volunteers inspecting a robot for the first time. Limiting the options speeds inspection and prevents disagreements over what size or type of circuit protection is appropriate.

2) Experience: FIRST has been doing this robot design stuff for a while. They may have found that teams that used low value fuses would have them pop too often and that would negatively affect their experience of the game. FIRST may have also observed that using high value breakers does not present a significant safety risk compared to using low value fuses. In this case, why not specifiy the higher current, self-resetting breaker to ensure teams have fewer equipment failures and get a better experience out of the game.

No, I don't know the WHY behind the rules... this is just my speculation... but I do believe that most of the rules have some kind of reasoning behind them and while we may not always agree with the rule or see the reasoning behind it, there is usually (although not always!) something there. After all, the rules were made up by a fairly rational group of people.


Jason