Shunt Fuse

[outermarker]

I have to replace my shunt fuses. The part number is 31101Q, 10A. What does the Q stand for? What is the voltage? The retainer part number is 155020A. Does this number determine the length of fuse? Littlefuse isn’t any help.

thanks!
albert

[PT20J]

I think the 31101Q is obsolete.

The fuse holder is for a standard 3AG type fuse. If it were mine, I'd take one of the old fuses to an auto parts store and get a 3AG type fuse of the same size and amperage and slo-blo if the original was slo-blo. Voltage doesn't matter since this is a low voltage application

[outermarker]

The fuse was a 250v@10A slo-blo question the 250v value. 

[N201MKTurbo]

54 minutes ago, outermarker said:

The fuse was a 250v@10A slo-blo question the 250v value. 

That is a maximum value. I don’t think you will exceed it.

[MikeOH]

A fuse's voltage rating is only important when the fuse is being 'blown open' in an over current fault condition.  Under normal operation there is very little voltage across the fuse even in a 250V circuit.  When the fuse 'melts' during over current the interruption of current doesn't occur instantly.  During the 'fusing' process (vaporizing the metal) the current attempts to continue to flow and creates an arc across the opening where the fuse metal was. This arc generates a lot of heat (potentially current in excess of the rating times the now applied circuit voltage of 250) which could physically compromise the fuse (explode, in other words).  Thus the reason for different voltage ratings. And why the AC rating (peak voltage is what counts) is different than DC.

Point being, if your circuit is only 12V, a 125V, or even 32V fuse rating would be okay.  It's the current rating that protects the circuit.

[Yetti]

Is this the black fuse holder?   Just find a same amperage buss fuse.   Why did it blow is the real question?

[outermarker]

To further explain what happened.  I didn't blow a fuse. I was going to check the fuse because I didn't know how old it was and wanted some spare fuses to have in the airplane.  When I opened the fuse holder, I didn't realize it was under enough spring tension that the fuse launched itself never to be seen again. I then opened the other fuse holder and read the 250v @ 10A, noting it was also the slo-blo type. I went to the parts manual to look up the part number. That is where the Q on the part number came into question. This was simply a matter of checking something done before I owned the airplane and ensuring it was correct and having a few spare in the airplane.  For all I knew, the fuse could have been 50+ years old. So, onto this great forum for answers for all to benefit from. 

[A64Pilot]

The way a shunt works is it’s a very little resistance like maybe 1 milliohm, so there is a voltage drop across the shunt proportional to the current flow.

So what is measured is the voltage difference between the two ends of the shunt. I think we are talking something like 50MV to 100MV at max current the shunt can handle voltage drop.

The voltage cannot of course exceed system voltage and while I don’t know what current is drawn off of the shunt to drive the Ampmeter I think it’s miniscule.

So pretty much any fuse would work. Pretty sure the fuses exist to protect the wiring if one of them were to short to ground to prevent fire of course, not to protect the ampmeter etc

Dont put too much in my numbers, they are from memory, only point is the current flow through the ampmeter wires must be very small, pretty much any small amp fuse that fits should be fine.

[outermarker]

I reached out to Mooney and received this reply:

Hello Albert,

This is from my Electrical “These are all 3AG fuses. These are "fast acting" fuses, but are neither fast or slow blow. They're considered medium or normal blow. The differences between the 3 have to do with environmental and other factors that are not pertinent to our usage, therefore we do not specify a particular one. Any of them are acceptable.”

Littelfuse 311 Series 10 A Cartridge Fuses – Mouser

I checked Littelfuse and found this out while talking with Mooney. Neither Mooney nor I couldn’t determine what the Q meant.

I did find out that 312/318 series is fast-acting. The 313/315 series is the Slo-Blo type. The 31101(0)Q might specify which series.

Albert

Littelfuse 311 Series 10 A Cartridge Fuses – Mouser

 

[cliffy]

Remember we're talking 1950s designs here. Not really high tech electronics of today. Farmall tractor technology!