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What are the rules on which avionics need their own circuit breaker?

10 Posts

Normally you have a CB per item, but sometimes this gets a bit ridiculous when you are installing several items which draw a small fraction of an amp and are thus protected with a 1A CB. It seems logical to do them all with a single CB (of course taking into account the risk in the simple point of failure).

I wonder if there are any rules or regs on this.

Administrator
Shoreham EGKA, United Kingdom

My understanding is that the CB is mainly for wire (shortage) protection than for protecting the unit itself.
Therefore the failure modes will drive the strategy who gets its own breaker.
If you for instance compare the recent Garmin G5 and G3X Touch manuals you will see different breaker
values for the exact same units and not all of the supporting boxes are on the same breaker.

Klixon do have a 2TC2 breaker rated 0.5A, but they are kind of hard to get hold of (never seen one in stock anywhere).

Andreas IOM

I have been studying this lately. The ULPower engine requires a completely different electrical setup than the non ECU type it replaced. For light GA there doesn’t seem to be any logic or regulations regarding the more “architectural” stuff. Circuit breakers are for wire protection, I gather that much, but why use a circuit breaker instead of a fuse for instance? Redundancy should be built into the architecture, not by manually resetting breakers, but …

I use ECB for the non essential stuff and (automotive) fuses for the engine related essential stuff. (if the fuse on one of the fuel pumps burns, than there is something seriously wrong with the pump, and just resetting a CB simply isn’t a good idea).

The Aeroelectric Connection, freely downloadable here, is a good source of info, as well as giving lots of alternative to the usual electrical setup used in certified GA (just look at the obvious solution of preventing voltage spikes from solenoids and contactors). By looking around on the main website, you can find lots of cool stuff, diagrams and so on. It’s more or less a bible for anyone building their own aircraft, at least for people not too used to electrical stuff. For some odd reason, the electrical stuff is always left up to the builder. IME, not being an electrical engineer, this is really difficult stuff to dive into, and with a FADEC engine, you simply cannot do it the way Cessna and Piper do it, because the result becomes a very poor one.

My only real gripe with the book is it doesn’t cover modern engines in enough detail. For instance the use of permanent magnet 3 phase generator, rectified and regulated with a MC type regulator is not mentioned. This is far superior to anything else, but it requires proper cooling, proper operational battery/capacitor and proper placement of contactors if you want to switch off the generator/regulator. Get this wrong, and it won’t last long, which is the reason people have trouble with them (in aircraft).

Also this site (both experimental and certified), has lots of stuff to sell, and kind of cooperates with the Aeroelectric Connection.

But, to the topic. I think the usual way is to protect each component – at the component. A fuse or diode or whatever protecting mechanisms should be mounted on/in the device. The in-cockpit fuses/CBs is for wire protection exclusively. (but don’t take my word for it and IMO it often is just sides of a coin, what exactly is protected by what)

The elephant is the circulation
ENVA ENOP ENMO, Norway

CBs have two purposes that I know of:

  • wire protection (which includes protecting from the instrument at the end of the wire drawing too much current), and
  • providing the pilot with the ability to isolate a particular item.

For example the on/off function on a GTX330 is implemented in software so if the unit crashes you can’t turn it off (or on).

Robert L. Nuckolls is well known. I had lots of correspondence with him regarding the B&C standby alternator installation. He’s the guy who designed the B&C alternator range including their voltage regulators. This was many years ago…

Socata use fuses too. They seem to use them for circuits which the pilot would never need to isolate deliberately and then I guess it is purely for wire protection. The problem is that these fuses are a bastard to get to. And fuses do fail too, after years, due to thermal cycling.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

providing the pilot with the ability to isolate a particular item.

That’s a good point. Do you think that is needed also when you have a mechanical switch in series with the fuse?

The elephant is the circulation
ENVA ENOP ENMO, Norway

There are switches that are also circuit breakers, Robin use a lot of these on the DR400 and could be quite useful it situations when panel space is an issue.

For me the biggest issue is the ability to turn off equipment without loss of the whole bus bar , with avionic manufacturers producing equipment without on/off switches ( GTN650 ) having some means of isolation is important.

Do you think that is needed also when you have a mechanical switch in series with the fuse?

No.

But a switch takes up panel space, so having the CB accessible is a useful solution. Also in a plane you want to use good quality switches, not €1 junk ones. APEM milspec switches are closer to €100 although often one can find unused ones on Ebay for much less.

Another thing is the debate on whether a CB can be reset e.g. here. I would say resetting it once is generally ok, but depending on the circuit in question. As I think A&C implies, if a bus CB pops, you can turn off the avionics on that bus, reset the CB, and then turn them on one at a time to isolate the faulty item. Whereas a fuse would prevent that.

I also think that if you are going to have an item accessible, it is better (if not cheaper) for it to be a CB rather than a fuse. Another factor might be that CBs have rugged terminals; can you get fuseholders with similar terminals?

There are switches that are also circuit breakers,

Indeed; Socata use them too. They are not particularly cheap though.

On the original topic, I have not yet seen much input. Presumably there is some risk-based approach? Is anything written down anywhere?

Administrator
Shoreham EGKA, United Kingdom

For certified aircraft the rules are quite easy, for a certain device if it is set to use a unique CB in any Install, ICA, AFMS or STC document. All others there are the requirements to keep a power load balance sheet, which circuit draws what and is what for. Sometimes it gets a bit more complicated, if the emergency procedures of the aircraft require certain separation of devices on CB, but all no rocket science. In the end you have to study your documents and try to figure out your setup.

Germany

Peter wrote:

can you get fuseholders with similar terminals?

Indeed, but not automotive ones (I have not seen any). Marine fuse holders on the other hand, usually always have screw terminals, but they are also a bit larger, sturdier. CBs are also used a lot in the marine world though. Lots of good solutions in the marine industry, and the stuff is made for harsh environment and rough use. Marine battery selectors are used for many experimental planes. They have IP rating (of some kind) and are spark proof. Excellent when using two batteries, and it eliminates contactors. Typical arrangement is OFF, battery 1, battery 2, both, and switching can be done at any time for the best ones.

Anyway, the Aeroelectric Connection has a separate chapter on approaching this from a risk point of view.

The elephant is the circulation
ENVA ENOP ENMO, Norway
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