Had a case this morning where I had a 50 amp FRS-R fuse in a GE 60amp disconnect fail. I was not around when it occured however by all indications it really "blew" . The top was burned out of the fuse and the line conductors above it had the insulation charred. the Red arc sheild was melted and sagging. These fuses are on a air compressor motor and normal amp draw is 29amps free running and 31amps under full load. I replaced the burnt conductors and checked out everything else. There were no line to ground faults and the resistance on the motor windings all check out normal. I replaced the fuse and fired up the machine everything ran normal.
What are the chances of a fuse failing like that for no apparent reason except for being a defective fuse? This compressor with these fuses has been in service for 3-1/2 years with no problems.
This incident also just reinforced in my mind the importance of have all open knockouts plugged and using the appropriate enclosure for the enviroment (dust etc). With the fire that blown fuse must have created, It would have been an ideal source of fire had it not been contained inside a proper enclosure.

Sounds like high reistance in the fuse clip or the wire terminal caused the overheating.

There isn't much that can "malfunction" in a fuse that has a sufficiently high interrupting rating.

Ed

It's a judgment call, but overheated fuse clips can lose spring tension or 'bite'.

The books say one shouldn't be able to easily rotate [by hand] the fuse barrel.

There is a product to help when the fuse clips have lost their spring. Look at the TRON Clip-Clamps near the bottom of this page.
Don

I've come across fuses that were broken in half a few times. I figured that they burst or broke when they blew. No damage just alittle cleanup.

I suspect your burned wires and melted arc shield didn't happen in this instantaneous flash but that these things were hot for a long time from loose lugs or corrosion. Thats what I've seen melt those plastic arc shields and the wire insulation. Sometimes those arc shields look like a dali painting

[img] http://members.bellatlantic.net/~cruzan/forums/dali-time-icon.jpg [/img]

Nevin...actually these fuses are under size per code.your FRS-R fuses (time delay)are calculated at 175% with your full load current of 31 amps x 175% = 54.25 amps you can go up to the next size 60 amps fuses.the 50 are under size.and maybe a loose connection on the wiring or the clip did not help either.

Nevin, What voltage is your compressor running at? FRS-R's are rated for 250 Volts. If someone accidentally grabbed the wrong fuses out of the drawer, (I've seen it happen) and installed them into a 480 Volt circuit. The results from the insulation breakdown would be consistent with the damage you describe.

Bussmann FRS-R fuses are 600V rated.
It is their 250V family that is FRN-R.

Pearlfish...there is no way you can put 250 volt fuses in a 600 volt fused disconnect switch or 600 volts fuses in a 240 volt fused disconnect switch.(they just will not fit)base on the fuses(FRS-R fuse) info.they are used on 480 volts circuits.rating 600 volts.in this case a motor load.

if it was 240 fuses the number would have been FRN-R

[This message has been edited by lighthouse (edited 01-20-2003).]

Is the photo I see in the this post on the violations section? https://www.electrical-contractor.net/ubb/Forum4/HTML/000215.html
It sure looks like what your decribe.

Could just be bad fuse

[This message has been edited by Len_B (edited 01-22-2003).]

No that is not the photo. The fuses are all three FRS-R fuses. The connections were all sound. The replies in the first three posts seem to me to be the most likely reasons. Although I'm not quite sure why the spring clips would have lost their tension unless it was progressing to that point over the last few years or months and finally simply became to weak. I am putting those clip clamps on as resqcapt19 suggested.
I guess here's a prime example of where thermographic imaging could have caught the problem early on before it failed.

Nevin...i have a question

what size horsepower motor is on the air compressor.is it a 25 hp ? we know by the fuses it"s 460 or 480 volts.right?

I apologise guys.

My eyes crossed columns when I looked in the catalog. Thanks for catching my error.

On "...why the spring clips would have lost their tension..."

This usually happens over a long time interval. The metal loses spring tension by softening through the annealing process.

Yes it is a 25 hp motor. it is an older one. Not a newer motor with the better energy efficency. I will have to look again on the motor for all the Specs.

Nevin. you need to put in the 60 amp fuses per code.sizing motors,wiring, overloads, breakers or fuses is a gray area for a lot of electricians,even me years ago.but it not that hard.take the hp of the motor and check the code book 430-150 3 phase go down to 25 hp.now over to the voltage 460 it says 34 amps.now go to 430-152 time delay fuses it says 175% so now 34 x 175 = 59.5 amps.code 430-52 allows you to go up to the next standard size fuse or breaker look in 240-6. put in the 60 amp fuses

an here one to think about. if you were to use one time fuses 34x300%=102 amps,you would go up to a 110 fuse and all on that # 8 wire.