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CBs and Switches for PMags?

rockitdoc

Well Known Member
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My PMags are on three-position Honeywell switches with down (off), middle (on), up (mom test). Originally, I was fusing the PMags, but I have seen panels with circuit breakers, instead.

What is the rationale for CBs instead of fuses? What circumstance would warrant pulling a breaker instead of switching the PMag off? I am not planning to use the CBs to test the Pmags during runup, but instead, will use the up switch position.

I'm trying to minimize the number of items on the panel for simplicity, but not at the expense of safety.
 
My PMags are on three-position Honeywell switches with down (off), middle (on), up (mom test). Originally, I was fusing the PMags, but I have seen panels with circuit breakers, instead.

What is the rationale for CBs instead of fuses? What circumstance would warrant pulling a breaker instead of switching the PMag off? I am not planning to use the CBs to test the Pmags during runup, but instead, will use the up switch position.

I'm trying to minimize the number of items on the panel for simplicity, but not at the expense of safety.
One of the best homebuilt aircraft electrical system designers, Bob Nuckolls, recommends 1 circuit breaker, and that's for the alternator so it can be tripped by the crowbar over-voltage protection. For everything else, he recommends fuses. That's what I did for my RV-8 with pmags. So far, the smoke is staying in the wires.
 
Like which primer to use - builder’s choice.

I built my first RV, an 8A, with fuses as that was the fad at the time. On the third panel redo the fuses went away as I found them a pain to mount in a way where there was easy access. I mounted the fuses on a removable panel on the baggage compartment side, again as was the trend to provide access to the back of the panel. That was a mistake as leaving enough slack in the wires to angle the access panel out proved not all that great. I found it far simpler to design the panel so it comes out of the plane. This one thing goes a long way to making maintenance, upgrades and such simple.

Even on an RV-8 there is plenty of room for the needed breakers.

Carl
2-E6595-EA-C757-4-F80-96-DA-1-C7144-EC9-D6-F.jpg
 
I used breakers on my Pmags that doubled as the "test" switch. Used magneto key switch and Toggles for On/Off. Simple. I never died the whole time I was flying Pmags.
 
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Other threads have discussed this ad infinitum but I'll throw in what I did and my rationale.

I have the 2 traditional mag switches that I use to enable / disable the EI and check RPM drop. (I don't have a key, just master switch, starter button and the "mag" switches that are now EI enable/disable switches).

I also have 2 separate pullable circuit breakers that provide circuit protection and can occasionally be used to test the respective ignitions' internal generator by verifying that RPM does not drop when pulled.

Lots of ways to do this and yes it's four items but that's just how I did it, works for me.

EDIT: I should add, most of my electronics are protected by spade fuses, except for a small subset on pullable CBs for things I might want to pull, or see when popped, or even try resetting in a pinch.
 
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I have the 2 traditional mag switches that I use to enable / disable the EI and check RPM drop. (I don't have a key, just master switch, starter button and the "mag" switches that are now EI enable/disable switches).

I also have 2 separate pullable circuit breakers that provide circuit protection and can occasionally be used to test the respective ignitions' internal generator by verifying that RPM does not drop when pulled.

Randall,

Are your mag switches set up to check the internal generators on the Pmags, or do you use the CBs, instead?

Scott
 
The way I like it as well: (same as mentioned above by RV6n6r)
 

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Breakers

The only reason for the circuit protection in our case is to protect the wires from switch to P-mag. Mine is like yours.
One fuse would be adequate, but what if only one wire was shorted? It would pop the one fuse and both curcuits would be dead. Two fuses or breakers gives a tiny bit of redundancy. You also need protection on the feed side for the main power supplying the fuses or breakers. That wire or wires goes through the panel too. Probably from the battery side of the contactor. Just my $.02 but I don't stay in Holiday Inn. :D
 
Like the OP, I'm using a 3 way switch for each P-mag that incorporates the test function, so I suppose fuses would work fine. I just used breakers because I had room in my breaker layout for them.

The only practical benefit I can think of is that since I don't have a key switch, being able to pull the breakers during maintenance or show and tell might be handy from a safety standpoint.
 
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