An unusual short

[Steve from Socal]

I used the drillpress yesterday no issues, today I try to turn it on no go. It has an old mechanical starter switch, thought that may be it, nope. Plug in other drillpress no joy. Go to the breaker panel, yup breaker open. Go to panel that feeds the sub-panel with blown breaker switch the sub-panel off.
Reset the open breaker and walk over to re-energize the sub. 200 amp breaker snaps as soon as I flip it? The plug for the drillpresses is disconnected so the fault is somewhere in the circuit TO the drillpress.

Walk back to drillpress and smell a bit of burned wire at the recepticle, open it up and find this. There is no abrasion or marks on the wire and as you can see the fault has been sparking for a while. Very odd failure mode?

 

[mm08822]

Seems like one conductor was pinched between the receptacle and screw head, possibly the conductor laying in/across the screw slot. Every time the receptacle was used, the insulation was further pinched and sheared due to the corner of the slot.
Eventually, the insulation fully gave way reaching the conductor.
Probably like this since installation.........just a time bomb waiting.

I've found where bare solid ground wires made contact with hot terminals "for no apparent reason" years after install. Probably just due to carelessly stuffing wires/device back into box.

Replace the 1.5" deep 4"sq box with a 2.125" deep, cut off damaged portion of conductor.
Make sure the box/conduit has a solid ground path.

Next time, if mutiple cb's trip, close the smaller cb (most down-stream) last.

 

[Norcal]

Seeing the BOY color coded wire, assume 480V?

 

[isb cornbinder]

My friend is an electrical contractor. He sent this along.

 

[Steve from Socal]

Seeing the BOY color coded wire, assume 480V?

Yes Sir,

 

[wyliesdiesels]

Yes Sir,

wow im surprised you didnt get bigger fire works...

 

[MBfreak]

Hi.
It may have been a "latent" fault, which is possible in areas with lots of lightning. The overvoltage thus present will find a weak spot and make a small puncture wound. Low level discharges will occur at normal voltage, and a carbon track will be established, that eventually results in a lowohm path to earth. Usually to a pointed metal piece.

My $0,02 guess

Ola

 

[Steve from Socal]

Hi.
It may have been a "latent" fault, which is possible in areas with lots of lightning. The overvoltage thus present will find a weak spot and make a small puncture wound. Low level discharges will occur at normal voltage, and a carbon track will be established, that eventually results in a lowohm path to earth. Usually to a pointed metal piece.

My $0,02 guess

Ola

That is possible, I had another circuit damaged by lightining. The damage to the other circuit changed the way I prep for thunder storms but , it may not be that effective? My shop has several panels both 480 and 240/120. When storms are in the area I shut off the power to the 480 sub-panels. I can't shut off the primary as it also powers my transformers for 240/120. I have not had any issues on the 240/120 side.

In thinking about this and the other hit, I do recall maybe 8-9 years ago having to replace a 600 amp fuse after a storm.

 

[wyliesdiesels]

Sounds like your GES and bonding is not up to snuff

 

[MBfreak]

The induced overvoltage will travel thruout the 480 V system and discharge where there is a weak point AND a change in wave impedance.
Such as an open 480 V circuit.
Hard to protect.
If you are brave install surge arrestors where the 480 enters your place, both between phases and from each phase to ground.
Be sure to install them in a fire proof enclosure

Ola