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renosteinke #166324 07/17/07 10:29 PM
Joined: Jun 2007
Posts: 50
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So are we talking like a panel in a commercial kitchen where there is a panel right above or beside a hot oven and the ambient temp at the panel is around 150*F?

If so, that would explain the large breaker in the panel and the much smaller fuse on the oven. I saw this done at the pizza place where a couple of my friends worked. The area where the panel was was probably too hot to touch.

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hardwareguy #166334 07/18/07 09:48 AM
Joined: Jan 2005
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I suppose such a situation might arise where a panel was behind a pizza oven .... but that's not on my list of situations I've actually seen.

#1 was in a hot dog factory, where the panel controlled the sundry motors that moved the sausages through the 'oven' ... a 100 ft long tunnel with conveyor drives, steam injectors, smoke generators, water chillers, etc.

#2 was in a factory that made plastic boxes, and the heat was from the forming process .... it takes quite a while for plastic to cool. The panel was heated by finished product, as it was transported past the panel.

#3 was a place that made diving boards for the Olympics. There, the panel in question was between the powdercoat oven and the anodizing tank ... if they both were operating, there was a lot of heat in that corner.

#4 was the motor controllers at the top of a tower in a dog food factory. While the equipment was on the roof, between the summer sun and the heat from the tower (metal hard hats were needed, as plastic ones got soft) ... the motor starter heaters were already the largest made for that size of starter.

Another variation is when a fan is directed at a single-phase motor that has internal overload protection.

Trumpy #166407 07/19/07 10:56 PM
Joined: Feb 2007
Posts: 20
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Junior Member
Quote
Look at the photo, 3rd & 4th fuses have 'tensioners' on the fuseholders



Damn! I wondered what those things were.

Somehow I ended up with some of those in my warehouse an nobody knew what they were.

Thanks

Nice pic BTW smile The fuse test light is a classic!

joncon #167202 08/08/07 12:41 AM
Joined: Jun 2004
Posts: 56
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I have today had a most interesting lesson that relates to this discussion.
I work for a company that manufactures and distributes switchboards and componentry, including fuselinks. We have just received a shipment of fuselinks from a new supplier, and my job was to quality check them. One of the things that I got to check was how close to the published time/current curves these fuses were.
15A fuses were tested, firstly at 40A (near-instant blow), then at 30A (22Secs), then at 25A (11m 40s), then finally at 20A (> 4 hrs). These were very much according to expectations. The interesting thing was just how hot the fuse got at 20A. The whole office smelt strongly of frying fuse!The fuse was way too hot to touch, I would estimate it at around 120 deg C. Yet even after 4 hours of this torture, it was still hanging in there. I am told that this is entirely normal behaviour.
The point I am making here is that if the fuse board at the kick-off of this thread needed fan-cooling to stop the fuses from popping, they were not just passing rated current, but more probably a good 25% overload! Methinks that this cannot possibly be a good thing, no matter how you look at it.


Mark aka Paulus
Paulusgnome #167239 08/08/07 12:50 PM
Joined: Oct 2004
Posts: 806
Member
Originally Posted by Paulusgnome
<snip> The interesting thing was just how hot the fuse got at 20A. The whole office smelt strongly of frying fuse!The fuse was way too hot to touch, I would estimate it at around 120 deg C. Yet even after 4 hours of this torture, it was still hanging in there. I am told that this is entirely normal behaviour.
The point I am making here is that if the fuse board at the kick-off of this thread needed fan-cooling to stop the fuses from popping, they were not just passing rated current, but more probably a good 25% overload! Methinks that this cannot possibly be a good thing, no matter how you look at it.


Exactly. And since wiring as old as what is most likely in that panel is good for 60 deg. C anyways, it's only a matter of time before a serious fire results. If the fuses, are getting cooling from the fan, the wiring behind the front is not getting any cooling at all.


Stupid should be painful.
Paulusgnome #167244 08/08/07 03:00 PM
Joined: Jan 2005
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Cat Servant
Member
Interesting; I have never done the sort of testing you describe.

In the situations I have encountered, momentary amp readings showed current loads well under the rating of the fuse / breaker, yet there was that tripping problem, that was attributed to the ambient temp.

"Attributed" is the key here; perhaps I need to hook up a recording type of instrument to monitor the load at the time it trips. That is, if I ever encounter this again laugh

renosteinke #167271 08/09/07 02:22 AM
Joined: Jun 2004
Posts: 56
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Member
I did some more fuse testing today, and I have given the graphs to Trumpy to post for me.
I tested the 15A fuselink (a 22mm dia x 58mm barrel fuse) for temperature rise, first at 15A, and then at 20A. I had a temperature probe snuggled up against the barrel of the fuse, this being inside a fuseholder of the correct type for the fuse. This completely shrouds the fuse in a plastic case.
At 15A, the temperature rose to 55 deg C and held there, from a starting temp of 20deg C. This is within the allowable limit of no more than 40 Deg C rise at rated current.
The eye-opener was the 20A test. The temperature quickly climbed past 100 deg C and levelled out at 135 deg C! No wonder it made the office smell of frying fuse. One can only wonder just how close the fan-cooled fuseboard was to starting an electrical fire.
For anyone wondering about the test set used for these tests, here is a quick description : we have a 10A variac which plugs into a 240V mains socket. This feeds a transformer-type 140A arc welder which acts as the current source. The fuse (or breaker - we test them too) is connected as a short-circuit across the output of the welder with some nice fat 25mm2 cables we made up. A clamp-type multimeter provides ameans to measure the current. This setup lets us generate test currents of a little over 100A for short periods, and 50-60A all day long. A test current of 20A for today's test required the variac to be turned up to about 55V into the primary of the welder.


Mark aka Paulus
Paulusgnome #167354 08/10/07 06:44 PM
Joined: Jul 2002
Posts: 8,430
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Here we go Mark, sorry about the delay:

[Linked Image]


[Linked Image]


Admin #167395 08/11/07 06:29 PM
Joined: Mar 2004
Posts: 391
B
Member
This is interesting because I never really imagined that a there was that much of a temperature rise on a fuse or circuit breaker. I've heard numerous stories of "hot CBs" but just how hot has never been put into numbers.

A little anecdote: Based on what I've seen here I did an improv test where I found a small 2A fast-acting cartridge fuse. I ran it at about 2.05 amps AC for ten minutes, and sitting in free air it got up to 122 degrees as measured with a little wire thermocouple. I could see the fuse-link elongate and bow with the heat, but it never blew.

-John

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