wire hit broken conduit

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ohmhead

ohmhead

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ORLANDO FLA
Well 400 amp 3 phase 480 volts dead short or phase to phase or phase to ground we have all seen or heard of these as electricians at one time or another .

This happened at work on friday we have two temp services on our job site each are 1600 amps 480 volts three phase just for construction power .

During construction theres lots of digging by other trades including us someone hit one of the 250 mcm alu runs in one of the 2 1/2 in conduits of the two runs to a 400 amp disconnect meaning two paralleled feeders to a disconnect the bang was loud it sounded like a bomb went off in the building .

When we checked it out the one conduit hit underground was just pull out of the coupling by the track hoe bucket looking at the disconnect ahead at the service the cover was blown out or pushed out by the blast when we opened up the cover all three fuses were blown .

Each phase A B C came out of the lugs but only one run of the two runs came out of the lugs meaning just one run of the two paralleled they all hit the encloser case at the same time blowing fuses at the same time lots of metal welded .

Meaning the circuit path went from the service thur the conduit and returned back to the service on the good run that wasnt hit it was 300 feet long one way .

So would this be a ground fault ? or a phase to phase ? If they all hit together at the same time to ground which they did this really happened on friday were having a debate on what happen your comments please ?
 
LarryFine said:
What? :confused:
Click to expand...

Larry wire was pulled out of a three phase disconnect all three at one time and all shorted together at one instant in time .

This was a paralleled run of two runs of 250 mcm alu 400 amps 480 volts .

They all hit the case at the same time all phases ABC the other run was still connected to the lugs and service .

So current flow was from conduit one and back thur conduit two to fused disconnect it was a paralleled feeder of two runs . Got it now ?
 
It was a phase-phase-phase-ground fault.

It most likely started as a phase-phase or a phase to ground fault and then quickly escalated to involve all conductive paths.
 
A lay mans answer:

When you let the smoke out of the wire, (reminder that AC voltage runs to the outside of its diameter), now dirt, grim, other matter will be used by the power aa a path run on the outside of the jack as the race way and wants to travel on this new path. Thus it used all the conductors as a raceway. The blow up and melting was because the other phases touched via the outer jacket as they were out of sequence to each other.

You might have noted a leading split of induced burns or clear burns or outer jack rip or even inner insulation, as to the direction the power went.

Directionally does it matter? The power was still under load, but still lightly loaded, IE a weaker resistance, and a better travel path. IE the pressure was still coming down the line. :)
 
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