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Ground pin toasted

Ground pin toasted

Ground pin toasted

(OP)
We have a three phase pad type transformer we are using to step 240 open delta up to 480 to provide shore power,(HEP), to rail cars.

It's set up with a big power cord feeding the primary. The transformer has a pigtail with a male plug on it.

The first time it was powered up the method was to plug in the 240V 3W w/gnd plug into the mating receptacle in the long power cord.

On plugging it in the ground pin vaporized.  It was hastily un-plugged! (shorts where changed - wasn't me)

Resistive measurements from all phases to the case showed no shorts no continuity.  The ground pin was ultimately connected to the transformer housing.

It was plugged in again, (with breaker OFF this time), and the breaker thrown.  Nothing unusual appeared.  The now, ungrounded case, has been checked for voltage between it and all local grounds.  There is no potential differences.

If no differences how did the pin fry?

What theories have you got to explain this?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Ground pin toasted

Taking the narrative on its face value:

May be the 'vaporization' of pin also got rid of the original short!

Did you check the voltage between the  ground receptacle (female connector) and its phase?

Check the continuity of the supply end ground pin to the source ground as well.

Other than that:
Is this your first hand account? Were you present there when it happened? I ask this because usually key facts are missing in seemingly unexplainable in second hand story lines.

What is the size of transformer/breaker? Sparking due to inrush current may have shaken up the person plugging it the first time causing jarring and accidental connection to live part?

Is it possible to shove the ground pin in phase slot of the receptacle?

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Ground pin toasted

Is it possible that when the pigtail was wired there were one or two strands of conductor from a hot conductor that missed the proper connector and splayed over to the ground connection?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ground pin toasted

(OP)
Please see ###


Rafiq;
Taking the narrative on its face value:

May be the 'vaporization' of pin also got rid of the original short!
### This gets my vote and was all I could think of.

Did you check the voltage between the  ground receptacle (female connector) and its phase?
### This has not been done.  Not sure why not.  I will request it.

Check the continuity of the supply end ground pin to the source ground as well.
###  There is a ground that comes from the utility and ties to a ground rod at the base of the pole and the breaker box is mounted to the pole and has a connection to the ground rod too.

Other than that:
Is this your first hand account?
### Nope.

Were you present there when it happened?
### Nope.

I ask this because usually key facts are missing in seemingly unexplainable in second hand story lines.
### Yep! As is probably the case here.

What is the size of transformer/breaker?
### Not sure, somewhere south of 100A ~50kW

Sparking due to inrush current may have shaken up the person plugging it the first time causing jarring and accidental connection to live part?
### I could see one of the phase pins toasting because of this but not the ground. (With this particular guy.)

Is it possible to shove the ground pin in phase slot of the receptacle?
### Not sure but again this guy just doesn't make those kinds of mistakes - ever.



Bill;
Is it possible that when the pigtail was wired there were one or two strands of conductor from a hot conductor that missed the proper connector and splayed over to the ground connection?
### That could be because these are large, heavy, very flexible cords, which likely means lots of fine stands.  I will check that.

Thanks gents.
 

Keith Cress
kcress - http://www.flaminsystems.com

RE: Ground pin toasted

I have seen stray strands that vaporized inside the plug but with much less damage than the burning on the plug pins themselves. Strange that it would be the ground pin, though. The ground pin usually makes connection first and most of the damage is on one of the hot pins. Was the ground pin dirty or loose? I mean a loose connection.
My theory: The stray pins act as a solid connection and most of the damage is done to the point of connection or poor connection. Then the stray strands act as a copper link fuse and clear the fault.
But, without seeing the damage first hand----
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

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