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#122711 - 01/04/06 02:56 PM Toasty Fuse
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
From the thread originally posted by jkochan:
 Quote:
These fuses are for one of two heat strips in an roof top air handler. This emptied the building by tripping the smoke detector in the unit. The center fuse failed and burned,scorching the fuses above and below. I checked the disconnect, wires,fuse block,and contactor points for signs of overheating and found nothing. All connections were tight. There are no shorts to ground on the heat strips or anywhere else in the circuit. I cleaned up the soot on the block contacts, installed new fuses and everything fired up ok with normal amp draws and no voltage drops. I've never had a fuse fail without being able to find the cause. Any thoughts or is this going to be one of those unexplainables?





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#122712 - 01/04/06 03:07 PM Re: Toasty Fuse
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Better not forget Crowbar's comment:
 Quote:
It definitely looks like the center fuse had overheated to failure and fire by the looks of the copper in the second photo. If the fuse clips look good I would imagine the fuse itself may have developed a high resistance connection at the fusible element itself.
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#122713 - 01/04/06 03:41 PM Re: Toasty Fuse
Rewired Offline
Member

Registered: 01/01/06
Posts: 567
Loc: Hamilton, Ontario, Canada
Nice one!
Reminds me of once when I worked behind the counter someone brought in what was left of their stove block..... holder was all burnt, fuse clips, the caps off the fuse and the fuse ELEMENT were still intact!
everything else was gone.. makes you wonder how fuses are said to be safer because they are not "mechanical"ike breakers eh!

A.D

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#122714 - 01/04/06 04:00 PM Re: Toasty Fuse
Dave T Offline
Member

Registered: 01/22/05
Posts: 157
Loc: Waukesha, WI, USA
Yes, I agree that it doesn't look like a fuse clip failure like I've seen. In my opinion the clip doen't look too discolored from heating and the wire doen't appear to be damaged either.
Usually if there is a fuse clip fail there whould be greater fuse clip discoloration and wire insulation damage.

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#122715 - 01/05/06 07:35 AM Re: Toasty Fuse
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
That and arc marks & other visible damage. I'll side with Dave & Crowbar - the evidence doesn't look like an external fault, which would point back to a high resistance condition that developed internally in the center fuse.

Radar
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#122716 - 01/05/06 12:09 PM Re: Toasty Fuse
mxslick Offline
Member

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA
Maybe a really dumb question, but would it matter that the fuses are mounted upside down?

Since the fuses are filled with silica or some other substance, maybe lack of filler around what might have been the thin part of the fuse element?

My understanding of fuse construction is that the filler provides arc quenching and helps to dissipate heat from the element under load.
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#122717 - 01/05/06 12:32 PM Re: Toasty Fuse
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Thanks for the input. The unit ran all night with no "smoke test". Just for giggles I cleaned up the caps on all three fuses and measured the resistances. Phase 1 and 3 fuses were .1 ohm. The phase 2 fuse despite its outward appearance was still conductive. It however showed an internal resistance of 69.9 ohms. The heat strip is is a 10Kw unit. Wouldn't the fuse impedance/resistance need to be much higher to have the effect indicated? Suppply voltage is 480 3phase (277/Leg).

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#122718 - 01/05/06 04:33 PM Re: Toasty Fuse
Check Pilot Offline
Member

Registered: 11/25/05
Posts: 145
Loc: Edmonton Alberta Canada
I did a bit of figuring and ohms law tells me that the fuse alone became an approximately 1200 watt "heater" every time the strips were energized. 69.9 ohms in series with 277 volts and about 12 amps to the heater strips yields a lotta watts. It could be more or less since the fuse reading probably was different when it was hot than when it was cold. The initial inrush current would have given the fuse an awful jolt of current before the impedances went up when things were cooking.

Hope my thinking is right.

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#122719 - 01/05/06 05:54 PM Re: Toasty Fuse
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
I'm inclined to think that just a few ohms is more than enough to create a big heating problem. Look at heating power this way: P = I²R, voltage doesn't really matter.

These fuses are 15 amp fuses, so presume for the sake of figuring that the load current is 15 amps (I know it isn't). If the internal resistance of the fuse is, say, 5 ohms, then you have 15² x 5, or 1.125KW of heat being generated inside the fuse. More than enough to make it very hot.

Do the same figuring on a 100 amp fuse, a 400 amp fuse, etc. You get into really big numbers quickly.

That's also how loose, high resistance connections at wiring devices, CB's, etc start fires. The high resistance doesn't have to be very high, but still many times higher than a good connection or conductor. The resultant heat does the damage, not arching (at least initially).

Radar
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#122720 - 01/06/06 07:24 AM Re: Toasty Fuse
electech Offline
Member

Registered: 02/18/02
Posts: 113
Loc: Northern Il
The resistance is 69.9 ohms after it cools and with a very tiny current from the ohm meter. I bet it isn't the same at higher temps/currents. As a test, you could run some current through the fuse into a light load (literally a "light" load, as in a light bulb) and measure the current through the circuit and the voltage drop across the fuse. Doing this at a couple different currents (different wattage bulbs) might show the change in resistance with change in current/heating.

I'll bet 10 bucks the resistance will change substantially under different loads. If wrong the 10 bucks will be "donated" to my local tavern tonight after work. If right, one of you guys needs to make the "donation". Any takers? 'hope I'm wrong

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