sparking when circuit is off

[Rich Elec.]

I went to a job for a few days last week to help wire light fixtures. This is a new building that has it's permanent power on. The service is 480Y. I noticed that the grounding is has not been completed yet. While wiring light fixtures when I tap the nuetral wire to the ceiling grid there is a spark thrown. I think that the problem is that the service is not being grounded, but I would like the techincal reason for the sparking.

 

[don_resqcapt19]

Rich,
Even when the grounding is complete the same thing will happen. You are making a second connection to ground when you touch the neutral to the grid. There is some voltage drop on the neutral and that voltage will try to drive current back on the grid. This will be a very low voltage and very low current.
Don

 

[dbuckley]

Two possibilities

The less likely first: if if the proper bonding is not yet in place then you have a ceiling grid that is free to adopt any potential it likes, depending on what it can find paths to, rather than being a part of the equipotential zone.

Far more likely: The neutral is a small voltage different from ground, due to voltage drop across the neutral. The voltage is usually quite low, but the current that can flow is substantive, as you are effectrively paralleling the neutral.

Update: beaten to it

 

[stickboy1375]

Never mind... way to slow

 

[Rich Elec.]

Thanks guys,

Makes sense.

Rich.

 

[quogueelectric]

clear case

clear case

of voltage drop on the neutral