Replace Fuse with Circuit Breaker

[iamtheari]

Per my thread about my alternator issue[1], I’ve replaced the voltage regulators in the 310 with the fancy Plane Power substitutes. They seem to work fine. One thing I am a little concerned about is that the over-voltage protection in these regulators works by shorting the enable pin to ground, causing the airplane’s circuit protection for the alternator field switch to open.

The 310 has fuses for the alternator field switches. The failure mode I’m concerned about is an over-voltage from one alternator causing both regulators to blow their fuses, leaving me scrambling to dig out replacement fuses. Murphy’s Law dictates that this will happen when I’m trying to escape icing with the prop heat and every other electronic circuit in the plane turned on, at night, in turbulence, and with frozen-numb fingers due to the heater also deciding not to work that day.

To mitigate that scenario, I am interested in replacing the fuses with Klixon circuit breakers. The Plane Power voltage regulator installation manual calls for a maximum 7.5A breaker, but I’d probably stick with 5A so as not to go bigger than the fuses in the plane. This assumes that 5A breakers will physically fit in place of the fuse holders.

Can an A&P replace a fuse with a circuit breaker and return the plane to service, or does it require the whole IA and FSDO paperwork process?

[1] https://pilotsofamerica.com/community/threads/alternator-issues-suspected-over-voltage.146420/

 

[Bell206]

The Plane Power voltage regulator installation manual calls for a maximum 7.5A breaker, but I’d probably stick with 5A

FYI: CBs and fuses are sized on the wire gauge and not the component.

Can an A&P replace a fuse with a circuit breaker and return the plane to service, or does it require the whole IA and FSDO paperwork process?

Yes, an A&P can substitute a CB for a fuse as a minor alteration. The only time it would need involvement of an IA or FSDO would be if the substitution somehow elevated to a major alteration and you required a 337 and/or data field approved which would normally be the exception and not the norm.

 

[iamtheari]

FYI: CBs and fuses are sized on the wire gauge and not the component.

Yes, an A&P can substitute a CB for a fuse as a minor alteration. The only time it would need involvement of an IA or FSDO would be if the substitution somehow elevated to a major alteration and you required a 337 and/or data field approved which would normally be the exception and not the norm.

Thank you.

 

[asicer]

FYI: CBs and fuses are sized on the wire gauge and not the component.

Wouldn't it be a good idea to size it on both the wire and the component?

 

[Capt. Geoffrey Thorpe]

Wouldn't it be a good idea to size it on both the wire and the component?

If you read the instructions that come with at least some avionics, it will specify the fuse size independent of the size of the wire.

 

[iamtheari]

Wouldn't it be a good idea to size it on both the wire and the component?

Ideally, you would size the wire to the component and the breaker to the wire. Since this is a retrofit, the instructions call for a maximum 7.5A breaker (no minimum) but the old 5A fuse was probably a product of the wire for the old component. I’ll go with a 5A breaker if there is room for breakers there.

 

[Bell206]

Wouldn't it be a good idea to size it on both the wire and the component?

If you read the instructions that come with at least some avionics, it will specify the fuse size independent of the size of the wire.

The wire is sized for the component load/installation and the CB/fuse is sized to protect that wire. The sole purpose of the CB/fuse is to protect the wire and not the component. Plenty of guidance on this. Does it still protect the component? Sure, but that’s more a built in feature than the purpose of the CB/fuse. Ironically, a component can let all the smoke out on its own but never trip the CB/fuse its connected to.

In a case where the CB rating is mentioned separately by a vendor as mentioned above, the majority of time the vendor lists the minimum wire sizes in a different section of the instructions as the vendor can’t determine the correct final wire size in all installations. The installer does. Regardless, if the component installation requires a larger size wire, it is still permitted to use the lower rated recommended CB as it still protects the wire. But its usually not a problem on small Part 91 aircraft. Now if the installed wire is too small for the vendor recommended CB, then that wire must be replaced with a larger size wire or use a lower rated CB to be within guidance.

 

[AV8R_87]

I’ll go with a 5A breaker if there is room for breakers there.

One option, if you can't install a breaker (and if your A&P is comfortable with the idea) is to use a PTC self-resetting fuse.
If you have the fuse holders with the solder lugs, like the one in the pic below, you can solder the PTC fuse to the existing terminals and remove the fuse from the holder.



This one (https://www.digikey.com/en/products/detail/bel-fuse-inc/0ZRP0500FF2A/9468324) would be a suitable replacement. Your alternator switch is wired to interrupt the circuit between the fuse and the regulator/field circuit, correct? That's the only catch with these things, to have them reset the circuit needs to be completely interrupted.