neutrals of different systems,same busbar

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benaround

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Arizona
An Electrician told me that on his job someone tied the grounded conductor of a 480/120vac control transformer to the neutral busbar of the 277/480vac lighting contactor,the 120vac was the coil voltage. Upon energizing these circuits he said "it took two seconds and all the coils were in flames". Could someone please explain to me just what did happen,so I can relay it to him. Thank You.

frank
 
Re: neutrals of different systems,same busbar

From waht you describe he was a lucky guy to be able to tell you about it without doing so from a hospital bed. First look at what he did, in effect he tied the primary and seconday conductors together. Thus creating a short circuit. Think about what a transformer is, 2 balls of wire wrapped around an iron core. totaly seperate circuits exist. he made them one, with a very low impedance from the primaty to the secodary. I wish I had the talent Ed does to make a graphic for this.
 
Re: neutrals of different systems,same busbar

Jeff,

How did they become one? I've got this drawn out,right in front of me,there are multiple choises for trouble,I would like to know exactly which path made it happen.

thanks
frank
 
Re: neutrals of different systems,same busbar

Maybe I'm not understanding this fully, but if the grounded conductor (one of the two 120 volt conductors) were tied to the neutral from a 277/480 system I fail to see why anything would happen based solely on that connection. The grounded conductors of both systems are just that, grounded, meaning that somewhere they are connected together, therefore nothing should have happened.
 
Re: neutrals of different systems,same busbar

Maybe the wire was identified incorrectly?

:( :eek:

[ April 27, 2005, 12:29 AM: Message edited by: davedottcom ]
 
Re: neutrals of different systems,same busbar

The connection of one conductor from the control transformer to the grounded conductor of the 480/277 wye grounded conductor was not the problem. These are separate systems and this connection will not cause current flow. In fact this is required, indirectly, every time you install a 208/120 transformer fed from a 480/277 system.
Don
 
Re: neutrals of different systems,same busbar

I don't completely follow exactly what this person did and why, just wanted to add that all transformers do not have isolated primaries and secondaries. If it was a control transformer, it may have been an auto-transformer in which they would be interconnected. Not that I'm implying this had anything to do with said problem.

[ April 28, 2005, 04:31 PM: Message edited by: BruceH ]
 
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