Safety issue on Fuse Blocks - Check your connections

Walt · #1 05-14-2014, 06:57 AM

While in the process of a panel upgrade/re-wiring the fuse blocks I found all the bus bar stud nuts loose on the fuse blocks

The nut that attaches the wires to this stud was still tight so apparently the fuse block material itself had deformed/compressed enough to allow the bus bar stud nut to loose its tension.

This is the nut you see in the picture below, with this nut loose a high resistance bus bar connection may exist which could result in overheating, a melted fuse block or other hard to find gremlins. I never would have thought to look here and would not have noticed it unless I had the main feed wires off the stud.

If you use these style fuse blocks, I strongly suggest you check these block for any signs of overheating and tighten the connections.

rhill · #2 05-14-2014, 07:36 AM

Good Catch Walt! You should write a book,I'll buy a copy if you sign it for me.
RHill

rocketbob · #3 05-14-2014, 08:11 AM

Its always a good idea to put something non-conductive behind these fuse blocks if mounted on the airframe. Several years ago I had a an aluminum shaving work in behind one of these fuse blocks which caused a moment of excitement in-flight with a little bit of smoke as the shaving vaporized.

Low Pass · #4 05-14-2014, 08:50 AM

Quote:

Originally Posted by rocketbob

Its always a good idea to put something non-conductive behind these fuse blocks if mounted on the airframe. Several years ago I had a an aluminum shaving work in behind one of these fuse blocks which caused a moment of excitement in-flight with a little bit of smoke as the shaving vaporized.

Have released the smoke once on the ground. In the air must really get the attention level elevated!

Thanks for the heads up, Walt. I have one similar to that under the panel I will inspect.

RickWoodall · #5 05-14-2014, 08:58 AM

Thanks for the post Walt. Oddly enough at annual I saw some weird discolouration there and crawled up underneath with a flashlight. On one of my blocks, the star washer was made of some odd metal and was so badly rusted (reaction) that it looked like it was 20 years old. When I replaced it I could not believe the RUST that had formed. Odd. Metal reaction I guess. But finding it caked it rust there was a very odd finding. But....just a note to do a good thorough annual.
Thanks.

rapid_ascent · #6 05-14-2014, 09:44 AM

Hard to say if the original torque for this nut was correct though. I assume that the stud is probably pressed into a bus bar of some type on the back. It might be more of something to be aware of when doing the install. Can you flip it over and take another pic of the back I'm curious about this since I was considering using this type of fuse block.

Walt · #7 05-14-2014, 10:00 AM

I removed the block to inspect the assy, the stud is encapsulated in the block so it cannot migrate out the back, nor can any of the other hardware, so it appears the only issue is if the connection loosens it could overheat and melt.

Tom Martin · #8 05-14-2014, 10:31 AM

Another way to handle this situation is to use a second nut and capture the terminals between the two nuts, and washers. The second nut can be torqued properly against the original nut. This would reduce the tendency for the original nut to be tightened too much, and squeezing the plastic.
I will add checking this bolt to my annual checklist, good catch Walt.

Walt · #9 05-14-2014, 11:42 AM

Quote:

Originally Posted by Tom Martin

Another way to handle this situation is to use a second nut and capture the terminals between the two nuts, and washers. The second nut can be torqued properly against the original nut. This would reduce the tendency for the original nut to be tightened too much, and squeezing the plastic.
I will add checking this bolt to my annual checklist, good catch Walt.

There was a second nut on top of the terminals, and it was tight so the leads were captured tightly between them, but the base nut is what keeps the "buss bar" in contact with the stud.

rapid_ascent · #10 05-14-2014, 05:53 PM

Here is the thing I don't understand. These fuse blocks I think are rated for 100A on the input. This is a lot of current for sure. The electrical connection I would suspect is the cause for the melting of the insulator. If there is melting, I would assume it is the result of the electrical connection being a poor connection (high resistance) which created heat and any melting that may be present.

Walt how do you feel about talking your dremel cutting wheel to the fuse block insulator so we could see what happened to the bus bar?