Why did it arc?

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BMS-Electrician

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Warehouse in SoCal, (industrial complex with about 12 units in a building). The main switch gear (meter board) is 277/480 800A and is protected by main fuses. There is a meter and a 200A fuse block for each warehouse unit.

The customer (a tenant of one of the units) had a fuse blow on their unit and replaced it. the next day it blew again (it was the middle fuse both times). This time, when pushing the fuse block in, there was a very significant arc, the guy's hand has minor burns on it. That's when he called me.

When I arrived, the two outer fuses were blown (with very clear signs of arcing on the contact points of the fuse block) but the middle one was NOT blown, however, the Middle fuse on the MAIN SERVICE was blown.

Long story short, we found a direct short on the feeders (middle phase) in a junction box where a splice had been made and they did not use insulating tape, just electrical tape, and it was arcing against the cover of the junction box. We fixed the short, replaced the blown fuses (including the 800A main), cleaned the damaged contacts and they were up and running. 2 hours total.

Close examination proved that the fuse block contacts could not contact anything they weren't supposed to. The customer assured me that all breakers were off when he was installing his fuses (they were certainly off when I arrived). Why did it arc like that?
 
I wonder if when pushing the fuse block in against a ground fault it flashed over from the middle phase to the outside ones and blew the fuses. Is there not a disconnect switch for the tenant fuse section? Or is the fuse pullout supposed to be the disconnect? Or are you supposed to open the main switch before changing fusses. Either way the owner is VERY lucky this didn't turn into a bigger deal with equipment damage (although the carbon residue from the arc has be worried about future arc flash incident). Tenants should not be touching that switchboard.
 
BMS-Electrician said:
Long story short, we found a direct short on the feeders (middle phase) in a junction box where a splice had been made and they did not use insulating tape, just electrical tape, and it was arcing against the cover of the junction box. We fixed the short, replaced the blown fuses (including the 800A main), cleaned the damaged contacts and they were up and running. 2 hours total.
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Since when is electrical tape not an insulator?
 
The thicker insulating tape wont wear through as fast, or likely won't ever. It may be temperature change not movement that melted through the tape or caused it to unravel. Of course it's best if boxes are properly sized so that the splices don't have to end up pressed against the side.
 
You're being fed false information. I'm sure you realize this. A properly locked out/turned off switch - meaning switch is in the 'off" position AND the absence of voltage verified - will not arc: period. The fact you found poorly insulated/terminated connections speaks volumes of the maintenance/installation practices.
 
BMS-Electrician said:
Why did it arc like that?
Click to expand...

In a logical analysis, all of the possible causes are listed along with guesses, assumption, premises, opinions, and everything else. In the next step, each of those are tested for validity and invalidated guesses are discarded. In the final step, the cause is indicated from the tested validated guesses and assumptions. If only one valid guess remains at the end, that is indicated as the cause.

Guesses and premises. The fault existed downstream on B phase. The owner attempts to repower with good fuses or closes the circuit into the fault.

An arc develops on B phase eventually blowing the street fuse, but before that happens, in the fuse holder pullout there is a conductive plasma circulating, that potentially could flash over from A to C, blowing those fuses. When all three fuses blow, power is removed and there is no longer combustible fuel or sustaining energy, the fire goes out.

Another variation on that theme, A flashes to B and C flashes to B blowing A and C independently while B waits for the street fuse to blow

Another variation, it's 2022, the owner decides he can make the lights come back on, but there's no way he knows what the fuse does or where to get one. He replaces the B fuse with the first piece of metal he finds that fits in the fuse holder and closes he circuit with that (the Darwinian solution). That's why B waits to blow, A and C go first, then the street B fuse.

To look for evidence of this, he should be able to show you either two blown B fuses or the good B fuse that held while it blew at the street.

Repowering with good fuses into the cleared circuit, no plasma again develops, and the fuses hold.

The problem here is that the flashover deposits conductive carbonization onto the insulator, the fuse holder pullout. At 480 / 277, I would guess the carbonization would be enough to again take the fuses out, indicating a secondary problem remains. But at 208, this potential problem could go unnoticed if the fuses hold. If it's carbonized and conducting but not enough to blow the fuses again, it should be producing heat and show up on a thermal scan.

Testing these premises and guesses, thermal scanning the remaining equipment is indicated. Or you could advise changing the entire pullout and the block it plugs into, if you can get the part. Thermal scanning as a next step would be cheap () compared to the alternatives.

I would guess or assume the fuse block is carbonized, and toast at this point, or at least not warranteeable or insurable, until proven otherwise. I would advise thermal scanning it to see if there is any evidence of carbonization damage to it, and probably advise changing the block anyway just to give the owner the option of gambling further and not me saying "you're all set".
 
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