3 ph 240v delta questions

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kileke

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today i ran across a situation that defies all that i have learned and been taught. the strange thing is a fellow electrician talked about it like he knew what was going on.

anyway heres the deal:

complaint a piece of equipment was not working. a 3 phase disconnect was there. checked. a phase 240 to ground b phase 240 to ground c phase 0 to ground and continunity to ground. phase to phase had 240v any combination.
so i am thinking we are missing a leg. he starts talking about a "ghost" leg.

so we trace back to panel its marked 3phase 240v. we open it and get same results as we had at the disconnect, across the buses.

i feel stumped. i think there is a distribution problem, he thinks its a equipment faliure.

from the very basics i have learned electicity ALWAYS want to go to ground. i do not under stand why there would be no voltage between c phase and ground.
even more confusing is it look like a dead short to ground.

if anybody has run ascross a "ghost" leg please enlighten me.
thanks. :shock: :?: :?: :?:
 
I believe you either have an corner grounded delta system or you are dealing with an ungrounded system with a fault.

Electricity does not 'want to go to ground' it wants to go (it must go) back to the source.

On a grounded system that path to the source can be through ground.

On an ungrounded system the ground will not be a path back to the source.
 
thanks for the reply one other piece of info the panel was marked 3 wire. there was no netural.

i will lookinto the corner grounded delta to see how it should test out.
thanks again.
 
I agree with Bob. His first sentence spells out two possibilities. In a 3 wire, corner grounded Delta system one phase is intentionally grounded and becomes a grounded conductor, this is why you would have no voltage between one phase leg and ground. Although there is an intentionally grounded conductor, the system still operates as a three phase system.
 
i will lookinto the corner grounded delta to see how it should test out.
Click to expand...
It should test out exactly as you found. All phase to phase voltages should be 240. Two of the phases to ground should be 240, the third phase to ground should be 0. Note that the grounded phase should be white in color, but it often in not.
Don
 
240 v 3phase grounded B is the system you have.
This is a commom 3 phase system in my area. As Don said, it should test out exactly like you found.
 
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