locating neutral-ground conneciton

[alehman]

A recent test on a client's low-voltage system in a data center indicated a neutral-ground connection somewhere downstream of the main disconnect. We have inspected everywhere we can think to look, but no signs of the connection. We could use current injection to try and locate the extraneous bond, but we would need to remove the main N-G bonding jumper to do that. The client will not allow us to shut the system down again so that's not an option for now.

I'm looking for other ideas to trace the extraneous N-G connection on an energized system.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[waross]

If you picked this up then there is a good chance that the ground is passing part of the neutral current. I would try checking the ground conductor for current. When current on the ground conductor drops, you are just downstream of the connection.
Take a close look at the neutral bar in any sub panels. If the green bonding screw has been replaced with a plain screw, or if the green paint has been removed from the screw it will look like a normal mounting screw, but will be bonding the neutral to ground.
Good luck Alan.

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

 

[davidbeach]

You could use a CT to inject a pulsing current into the neutral-ground bond. That current will then following the circuit formed by the extraneous neutral-ground connection. Follow that current and see where it leads.

 

[rcw retired EE]

277V lighting ballasts always gave us fits when testing ground fault systems for improper N-G connections. Our instruments would pick up something in the ballast as a N-G connection. Each ballast wasn't much, but a high-rise building full of them fooled the instruments.

We had luck measuring and chasing currents in the green EGC's going to each panelboard and major piece of equipment.

A common problem is the UPS system Bypass circuit. The UPS output neutral needs to be grounded and many times it is not done properly.

Good Luck!

 

[alehman]

Thanks for the suggestions. The UPS bypasses is one I had thought of as well. We'll double check those for proper connections.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[itsmoked]

rcwilson has a good point. Switching power supplies often have noise suppression capacitors internally connected between the neutral and the ground. Every power supply in a data center is a switcher..

Because of these suppression caps most switchers are now listing their "leakage" current in the spec sheets. Figure out which switcher or switchers are the dominant tree-in-your-forest and find the leakage number(s). Parallel them based on the quantity(s) and see what a whopping big capacitor of the resulting size would look like in your hunt for the extraneous ground.

Keith Cress
kcress -

http://www.flaminsystems.com