We had an "interesting" situation last weekend. I regularly fly in an aircraft where one of the passengers is seated backwards. We have a little 12V camera system installed so that the passenger has a bit of a forward view as well. The camera system is non-permanent, with all components velcro'ed to the interior, and power drawn from the cigarette lighter socket.
The cigarette lighter socket is on master bus 2 and is protected by an on-board CB. Furthermore, the cigarette lighter connector has an internal glass fuse as well. Unfortunately I don't have the amp rating of these fuses to hand right now.
Due to some carelessness, the cable from the cigarette lighter to the camera system was ripped without anyone noticing. Both leads (+ and ground) were exposed, and the full cable was severed. When we went to investigate we noticed the loose wire. We had the master on at that time and as soon as I touched the cable, I rubbed the ends against a metal (but painted) part of the cabin and it started sparking.
I don't know if the sparks were between the + and the fuselage, or between the + and ground leads. But what amazed me is that these sparks were not tripping either the on-board CB or the glass fuse.
In the end we did not investigate further obviously. We switched off the master and dismantled the whole system. I'm now looking for a replacement cable. But for the future we would like to know why both the CB and the fuse were not tripped. Possibly we can install a lower Amp rating glass fuse to prevent this in the future, as these sparks could easily have happened in-flight.
So, does anyone know how much current is required to produce sparks in a DC 12V system?
Fuses blow and CB's trip by means of a heating mechanism, to melt the fuse wire or operate the trip mechanism. Heat is produced as a function of current and time. Therefore a fuse can take a very high current for a very short time, but less current continuously. There are graphs to show time to blow vs current. This is a good thing, as tolerance of little spikes of current allows fuses to give robust protection from gross failures, without nuisance blowing.
They are designed to blow when there are fault conditions resulting in a massive increase in current.
It is possible that your camera wire was making only intermittent contact, and or was so thin that it limited the current to a level which the CB or fuse could carry.
Note that say a 10A fuse will carry 10A, and will blow at a higher figure.
To add to that - the reason that fuses behave that way is that their main purpose is to protect the cabling from overheating and the fire this could result in, so all it needs to do is blow before the cable gets too hot.
Cigarette lighters need a lot of current - looking online they often seem to need 15 amp fuses. Certainly you can make sparks with a lot less - e.g. you can get a spark from a button cell - 3 volts and about 1 amp from a CR2032 cell.
It's true that cigarette lighters do need a lot of current. I once had an inverter that would provide 220V AC at 150W from a cigarette lighter socket. Working back from that, 15A fuses or CBs in a 12V system are certainly in the right ballpark.
I also just checked and the glass fuse is rated at 5A. So I can well imagine that small sparks will not trigger a CB or fuse trip as they were not sustained long enough for the heat to build up. But I'm now convinced that a sustained short would still blow the CB or fuse and that's what I wanted to know. Thanks.
The problem with GA aircraft CBs is that they are thermal i.e. very slow acting - of the order of 100ms to 1 sec or even 10sec depending on the size of the overload.
Their purpose is to protect the downstream wire from melting in case of a short. For that the CB does not have to be fast acting.
This means that it takes a while to trip them. In fact a fuse (in the same wire) might blow well before the CB trips - or vice versa.... and there are many speed grades for fuses too.
Mains (220/240V) CBs are magnetic and very fast acting - of the order of 1ms even for a slight overload.
Those little sparks are from a very small bit of metal getting heated white hot. Since it's only a tiny bit of metal where the loose conductor is just scraping whatever it's sparking off, it doesn't require much energy. Think of the amps required to make the tungsten filament of a car sidelight glow white hot (a far larger piece of metal than the tiny piece in a spark) - it's not very much, only a few hundred milliamps.
