breaker and fuse in series...

prkaye · #1 08-09-2010, 07:15 AM

My planepower alternator is currently wired thruogh a 5A fuse. I want to change this to run through a 5A panel breaker instead. By far the most convenient way for me to implement this change will be to keep the existing wiring through the fuse block and just add the breaker in series with the alt switch. But I need a fuse in the fuseblock for continuity. My idea is to plug a 10A (or larger) fuse in (just as a "blank", to provide an electrical connection in the fuse block). The circuit protection will be provided by the breaker, but I want to be sure that teh breaker will always trip before the fuse (so that I never have to replace that fuse).
Can I be sure that the 5A breaker will open before the 10A fuse will blow? This would depend on how fast the current might rise during a fault, and also on the relative speeds at which breakers open and fuses blow. Will the 5A breaker in series with a 10A fuse prevent that fuse from ever blowing?

cfiidon · #2 08-09-2010, 07:34 AM

Quote:

Originally Posted by prkaye

Will the 5A breaker in series with a 10A fuse prevent that fuse from ever blowing?

Ever is a long time. If you are adamant about doing it this way, put a very large slo-blo fuse in the slot. But I think the better thing to do would be to disconnect the wires from the fuse block and tie them together (solder, good crimp, etc.) Not quite as easy, but from a troubleshooting point of view, I envision someone in the future beating their brains out trying to figure out this issue. If you should go the large fuse route, update the documentation.

I really have no idea what AC 43.13 says about this, if anything.

Don

apkp777 · #3 08-09-2010, 07:51 AM

Phil,

I don't see a problem with your plan. That 10amp fuse will hold. I am sure there are lots of "well, what if....". I'd probably do the same thing.

777drvr · #4 08-09-2010, 08:50 AM

Will the wire from your fuse block to the CB be protected appropriately ?

Marc

Norman CYYJ · #5 08-09-2010, 10:14 AM

Not a great idea. Another failure point in the future when you least expect it.

prkaye · #6 08-09-2010, 10:39 AM

Quote:

Will the wire from your fuse block to the CB be protected appropriately ?

Same as it would if the wire were going directly to the switch. As I said, the CB is providing the protection... the larger fuse is basically serving the purpose of a wire splice.

Quote:

Another failure point in the future when you least expect it.

A fuse or CB is an intentional point of failure. If a high current was going to cause that fuse to break, I would certainly WANT either it or the CB to open up. Preferable if just the breaker trips, so that it can be easily reset at the panel. If both the CB and the fuse open, it's no worse than if i just had a single 5A fuse in there... just means I'd have to replace the fuse.

Unless you're suggesting that a large fuse might fail spontaneously, for a reason other than a high electrical current? But in that case, it's no worse than just having the 5A fuse I have in there now (with no CB). Lots of people use fuses for circuit protection, and so I would suspect that spontaneous breakage of an installed fuse (at normal current levels) is pretty rare.

777drvr · #7 08-09-2010, 10:59 AM

Really splitting hairs but if the wire shorted between the fuse and the CB the upsized fuse may not blow at the appropriate load . You may have to upsize that wire to be suitable for the 10 A fuse.

Marc

erich weaver · #8 08-09-2010, 11:48 AM

Marc makes a good point here, and I dont think its splitting hairs at all. Fuse blocks serve as a bus and also provide protection for the wires between the bus and the various devices. Changing the location of the built in weak link will change what is being protected - you are defeating the purpose that the fuse is there for.

erich

Norman CYYJ · #9 08-09-2010, 01:29 PM

I am not suggesting that the larger fuse would blow. But a fuse in a fuse holder is not as good as a properly made splice. Fuses have been known to fail with time and corrosion in the connections to the fuse. Why install multiple failure points when doing it properly should be the prime objective?

prkaye · #10 08-09-2010, 01:44 PM

Quote:

if the wire shorted between the fuse and the CB the upsized fuse may not blow at the appropriate load . You may have to upsize that wire to be suitable for the 10 A fuse.

Ahhh... now i see. So suppose I bypass the fuse entirely, and go straight from the battery to the CB... then I would still have to use big wire for this (battery to CB), in case this wire from the battery shorts before the CB. Is it normal to have the alternator field on an "always-hot" circuit? What is the typical way of wiring this (alternator field through a breaker)?