Reading this article and with my own experience, it makes me wonder why CBs aren’t lifed items.
If anything on a plane should be life-limited, it should be the breakers. The reason is that they are thermal – not magnetic like the CBs in your house. Thermal breakers work by the current heating up a bimetallic strip. The reason they work at all usefully is because the heating is proportional to the square of the current, so as the current goes up you fairly rapidly reach a point where it gets “warm enough” to trip. But there is a huge problem: at say half the rated trip current the temp rise is still 25% of what is needed to trip it, so the CBs in a typical installation are often getting pretty warm.
They are made of plastic and the heat degrades all the materials. The trip point will shift as the plastic parts degrade, and old CBs tend to trip at a lower current i.e. nuisance tripping.
The plastic also cracks. The whole CB can just fall apart, due to cracks in the housing.
Magnetic breakers don’t get warm – they don’t rely on temperature to trip. They are much better but they have two problems: (a) they trip very fast which is good generally but not desirable with transient-high loads and not needed if you are using the CB for wiring fire protection, and (b) they cost more money.
There are other options (for experimental aircraft)
In that approach you need to do the design really really cleverly, because you are concentrating everything in one small volume. I take it they are using MOSFETs to switch the current, which avoids the tricky choice of relay contacts capable of carrying the full battery output current for x milliseconds without fusing, but there probably are avionics out there which rely on a thermal breaker to deal with some large switch-on transients.
Also there is the fairly basic issue of what exactly protects the downstream wire(s). The MOSFETs can fail and do fail short-circuit with a totally melted junction. Relays can fail the same way.
The latest Smart Fets don’t fail very easily. We use them in automotive applications and they are short circuit proof.
Well, a MOSFET should “never” fail if you protect for (a) safe operating area and (b) junction temperature 
But that doesn’t really deal with the basic issue of fire protection. I would still want to see a CB feeding the supply to that box, and be sure that everything downstream of that CB is big enough to withstand the current. That isn’t too hard to do, with modern avionics drawing so little current.
it makes me wonder why CBs aren’t lifed items.
I have had all kinds of problems on aircraft so far from burnt lightbulbs via blown tires to (lately) turbine failure, but never a failed CB. And I have flown on some really old and worn planes like 30 year old trainers with 13000+ landings flying on their sixth engine… But maybe the owner simply had superglued the CBs into place and this is why they never failed
Peter wrote:
The plastic also cracks. The whole CB can just fall apart, due to cracks in the housing.
Random squawks exhibit no 12345. The intermittently popping circuit breaker is not always what it seems.
Circuit breaker OUT. Not conducting:
Circuit breaker IN. Surprise. Not conducting as well:
Close up:
Indeed; CBs used in GA are a crappy solution. They run hot constantly, relying on the “sudden but not so sudden” I^2 * R dissipation in a heater to trip them. They are for wire protection only. The heat degrades the plastic…
I think – like so much in GA – they came in in the 1950s/60s when planes were not expected to last longer than cars.
what next wrote:
I have had all kinds of problems on aircraft so far from burnt lightbulbs via blown tires to (lately) turbine failure, but never a failed CB. And I have flown on some really old and worn planes like 30 year old trainers with 13000+ landings flying on their sixth engine…
We have the same experience. Premature tripping must be a very unusual failure mode. I’d be more worried about them not tripping.
IME, CBs tend to go open-circuit, but often “not quite open circuit”, and are likely to be intermittent.
It’s a poor design – quite unlike the magnetic ones we have in our houses.
