Peter wrote:
It’s a poor design – quite unlike the magnetic ones we have in our houses.
I’m not arguing that point. I’m just saying that with the experience of being involved in a running a club for some 12 years now during which we have produced some 15,000 flight hours, most in aircraft from the previous century and several from the 1970’s, I can with some confidence say that premature tripping is a non-issue.
Different types of circuit breakers may have different failure modes, some of them insidious. For example, the mechanism may still trip, but the contacts may be shunted through some part that is conductive, but its resistance is high enough to heat up under heavy load and eventually burn up the breaker. On the other hand, as @Airborne_Again rightfully said, premature tripping does not belong among typical failure modes. However, spurious tripping may otherwise still occur because of transient overcurrents while the average load is well within limits. This can happen if you have large capacitances or inductances in your circuitry.
Many types of pullable circuit breakers are not switch-rated – that is, you are not supposed to use them routinely as ON-OFF switches because they wear out too fast. When installing a circuit breaker in the field, always check if you need that function. Remember also that AC and DC ratings may be different, and some breakers (and switches, too) are AC-only.
Well I think CB’s are a menace – here’s why,
Half way across Sweden, the low volt light came on. No charging. OK, check everything, cycle the master, etc., still no charge. I’m about 30 mins from Bromma and I reckon the 430 on it’s own will last that long. Advise Sweden who allowed me to turn off the xpndr and gave vectors. On final to Bromma “Bing!” all the lights came back on. Situation normal.
On the ground engineers couldn’t find anything wrong. So I arranged for an alternator to be flown out and everything was hunky dory – for about 20 Hrs. And then ‘Bam!’ it happened again. This time I was in range of home base and so it’s ‘change the regulator’ then, ‘low voltage switch’, etc.. All OK for another 20 Hrs until….
On the ground in Elvington, running up as normal, suddenly ‘Low voltage’. It’s happened again. This time, on the ground, there was time to think. About the only thing left was the alternator field CB. I’d thought about this in advance and just gently flicked the button with a fingernail. Wham! Alternator back on line.
I still have that CB, beside me as i write. PA-28-161 by the way, and why do we need CB’s? Most are not even pull type. And if one trips in flight due to overcurrent, are you really going to fix anything by resetting it? With modern radios?
They are for wire (fire) protection but they are a poor design which is affected by the inevitable thermal cycling.
If say a 3A field connection was made with a 30A rated wire, protected with a 30A CB, there would be negligible heating in the CB. But nobody does that.
The dud ones tend not to trip. They just go kind of open circuit.
