The questions are this:
Have any of you seen a set of fuses functioning normally at 110C?

If so, under what conditions?

I have a set of 100amp Fuses for a motor each of which is at 110C, with ambient at 25C. They look normal, with the exception that the temperature is high.
I don't yet know the loads, but I suspect they are approaching the limit of the fuses.
I have never seen normally functioning fuses at these temperatures, but I do not preclude it.

I have never seen fuses anywhere near that hot.

fwiw, I took a quick look at some standards and found:

Table 1 of Std C37.40-2003 for HV power fuses (above 600 volt) indicates that at the continuous current rating when in an anbient oif 40C, the spring clip contacts shall not exceed a total temperatue of 75C for bare copper contacts 95C for tin coated contacts 105C for silver or nickel contacts. There are other temperature limits for other parts of the fuse which can get up to 220C depending on the materials used.

Table 6-1 and 6-2 of FU-1 2002 for LV power fuses (less than or equal 600 volts) gives limits when at the continuous current rating. The limits apply to the entire fuse exterior and contacts. These are given as temperature rises. They range form 55C-95C, depending on the rating and "class" (H, J, K, R, T, CC and GG).

I think I remember hearing that dual element or slow blow fuses can have an internal heater where the meltable link is a distance away from the heater to cause the delay. (Can anyone comment whether that is true?)

I guess there is a small comfort gained if the high temperature appears uniform accross the cartridge, no evidence of extra heating on the contacts, no visible discoloartion, and similar on all three phases. But still worth investigating (perhaps measure load and discuss with fuse manufacturer)

quote:

I guess there is a small comfort gained if the high temperature appears uniform accross the cartridge, no evidence of extra heating on the contacts, no visible discoloartion, and similar on all three phases.

Hmmm. That was a misleading comment. Even if the clips are not heating abnormally and the fuses are uniform and similar, there is of course a chance that the fuses themselves are on the verge of blowing, either through normal action of high sustained current or through some abnormal thermal effects (fuse is a thermal device after all). Blowing of a fuse is probably a much more common cause of equipment disruptions than opening of a circuit due to a deteriorating contact. If this were critical equipment, I would be a little concerned, not because I know it is a problem, but because I can't yet prove it's not a problem.

To me the next logical steps would be measure the current, get the fuse part number, check how close you are to the normal setpoint, and call the fuse OEM to discuss it.

This message has been edited. Last edited by: electricpete,

I thought I would send the Thermograph....

This is about 100 amps, so it is not nearly the kind of services you were describing in your first post.

Take a look and let me know what you think.

Fuse_Block.doc (82 Kb, 34 downloads)

You really need exact loads for this to make a proper call.

I have seen fuses operating at this level for sustained periods. You have to remember that the whole basis of fuse design is that the fuse wire melts, this means high temperature. As your load approaches the fuse rating it will heat up, it has to. I agree with your initial conclusion that it may be approaching the upper load limit for the fuse. It could operate at or even above this rating for some time before finally failing.

My advice is firstly to try to get laod readings, if this is not possible, then monitor it and make sure there is spare fuses available in case it does blow. If this does fail, and there are no spares available, and you knew there was potential for it to blow then you may come out looking bad.

This message has been edited. Last edited by: Bob Berry,

Bob: A little more info:

First, this unit was in a VFD cabinet with no vfd. It appears that the VFD was removed from the cabinet. I also found previously blown fuses in the cabinet, although no one in Maintenance could remeber when or why it blew.
It might be that this installation was significantly changed, and therefore the load has changed..
My recommendation was that the installation be reviewed by the Electrical Engineer.