When Does a Fuse Catch on Fire?

[ataslaki]

We had an electrical fire in a heater controller enclosure today, where a 5 Amp fuse was disintegrated!

The 5 Amp fuse was in a block of 4 fuses, from left to right >>20A>>20A>>5A>>5A. The 3rd fuse from the left.

No breaker was tripped. The fuse just isn't there any more; it's all carbon charred dust, with black soot all inside the enclosure. No other wires melted; all insulation appears to be intact.

What could cause this?

And, here are the details, if it helps. 5A fuse is protecting a transformer from which drops a 480VAC down to 110VAC. The 20A fuses are for the heater elements. The heaters are 480VAC 8KW. The heater controller is a 110V PID controller from Omega. Very simple operation.

The main question again; what could cause a fuse to burst into flame? Underrated or overrated?

 

[itsmoked]

Fuses typically do that because the spring clips that hold them are "sprung" this causes the clip to run hot. This oxidizes the clip material. Which leads to increased resistance of the clip to fuse connection. Which causes further heat increase. etc, etc, read vicious circle. Finally an arc will begin. 480VAC is pretty good at sustaning an arc. The arc may reach 10,000F. 10,000F = some major charing!! The first question would be are the fuses rated at 480?

 

[ataslaki]

Fuses are rated for 480 VAC. Your comment seems reasonable, even though the type of fuse block used does not have a spring. So, I would suspect that the fuse may not have been the correct size and may have been squeezed hard in its place, causing the heat, etc.

Can anyone else confirm?

Thanks

 

[analogkid2digitalman]

Actually loose connections are what generates heat. Ohmic drop dissipates power = heat. Also check the connections at the fuse block for tightness. Of course if you had access to a thermal imager, that would be the cat's meow to find components and connections that are heating up.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[OperaHouse]

If there was nothing left of the fuse, how do you know it was the right voltage?

If I only had a dollar for every time I was told they didn't do anything wrong!

 

[itsmoked]

ataslaki You never specified what kind of fuse holder... Was it the kind where you shove the fuse into some form of clip? If the answer is yes then springs are involved even though they aren't in the classical spring shape. The clips are springs.

analogkid2digitalman is correct in that this is usually caused by looseness which means it could be the actual connection of the wires to the fuse block too, not just the fuse.

That said... A 250VAC fuse jammed into 480VAC service is sure to toast during a fuse blowing fault event.

 

[DanEE]

I would be concerned about fuse and fuse holder ratings also.. Sounds like it sustained a prolonged plasma arc due to exceeding instantaneous interrupt ratings..

What is the current condition of the transformer?

e.g. if it developed a hard short that drew a high (peak) short circuit current, then a sustained plasma arc inside the fuse could have developed.

Have seen a 120V AC fuse (and fuse holder) blown completely out of a radio that took a direct antenna lightning strike.

Antenna lightning arrestors were installed (but old arc gap technology and, NO bonding from antenna ground to power service entrance ground, so the subsequent ground shift from the lightning hit just ate it up..

 

[ataslaki]

After cleanup, CSI revealed that the ends of the high voltage stranded wires going into the fuse blocks were dipped in solder. Could that be a cause for the arcing?

 

[analogkid2digitalman]

IF a screw was used to secure the wires in the fuseholder, the solder could be an issue by preventing enough force to hold the conductors. You usually rely on the screw force to 'mash' the conductors down to secure them. The solder could have prevented this and the screw worked loose over time. Leads to ohmic loss = heat

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[felixc]

I have had car fuses catching fire instead of fusing. Made-in-China fuses found in dollar stores. I wonder how can they be allowed to cross the borders.

 

[ScottyUK]

Felixc,

Somewhere I posted a link to the BEAMA website, but the search function won't find it. Here it is again:

http://www.beama.org.uk/Newsletter/Issue2/articles/beamainstallation.htm

The following excerpt is really concerning:

Our group, now upbeat following the finds in Wenzhou, then flew down to Guangzhou for the Canton Trade Fair (Chinese Export Commodities Fair). Another eye opener.

This fair is a sad indictment on the Chinese manufacturing industry. It's a huge exhibition with 8,000 stands. All have one thing in common - they export from China.

About 300 stands are showing electrical installation products. Almost every stand has dead copies or look-a-likes of every famous brand and design. Some blatantly display brand names. Most have no brand name or show their own name, but will easily supply you with any brand you want. If you ask for a particular name - you'll most likely get it.

Worse still, you can ask for any BS or IEC number. They will print anything on the product - whether it's applicable or not. Any fault level you want printed can be provided, true or false. Any product, any brand, any spec, to any market - nothing is sacred, no market rejected.

Even more worrying is the price. The lower the price, the lower the quality. They tell you this quite openly. The price governs the quality. If the price means some of the components have to be omitted, then so be it.

MCBs without overloads, MCCBs without arc chutes. Contactors without plated contacts. Wiring accessories without shutters. Fuses without elements. The characteristics mentioned on these copies are worryingly poor. But, if you want a test certificate, you'll get one - they'll do anything you want.
Out here, it's as if you cannot trust anything you are told or anything you see.

Beware of products that are too good to be true. They are probably from China!




----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[felixc]

Killing our jobs, and killing people.

 

[itsmoked]

Hmmmm, Do I hear a marketing jingle coming?

 

[zeitghost]

Solder is subject to tin creep.

So that the screw terminal may well be nice & tight to start with, but the tin tends to flow away from the pressure and the whole thing loosens up.

Same thing applies to crimp terminals.

And rewireable mains plugs.

For all these things it's best to cut off any soldered ends and crimp /screw down onto the bare copper conductor.

rgds
Zeit.

 

[felixc]

It looks like you have the reason why UL does not want to see tin solder on line cables. (even though I have seen crimps fusing after a while) I was wondering.

 

[itsmoked]

WOW.. I have never heard ANYTHING about this tin creep before. I have heard of tin whiskers but not flowing tin.

 

[drpappy]

I've never used them, but there's those ceramic fuses filled with sand to dampen the burning.

 

[zeitghost]

HRC fuses are filled with sand to quench the arc.

They won't stop overheating due to loose connections.