How would you explain this?

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kenman215

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albany, ny
We were finishing the final tie in of about 2000' of interconnected lighting counduit and boxes, conduit ranging in size from 1/2" to 1". Conduit is supported via tapcons and sammies to the poured concrete ceiling. When the feeder pipe running from the house panel to the first jb touched the metal components, it lightly sparked.

It sparked multiple times, ruling out static discharge.

The neutral and ground are bonded via bus jumper at the terminal cabinet, before the main switchboard.

The ground jumper between the terminal cabinet, passing through the CT cabinet, into the house panel was sized and torqued correctly.

Two 5/8" copper clad ground rods were driven seven feet apart, with a 3/0 CU wire running from the main grounding bus continuously through the first, to the second rod. All terminations were torqued correctly.

The 3/0 grounding conductor bonding the building rebar was installed and terminated on the main grounding bus.

The main water bonding has yet to be done.

The complete load on the building is currently about 160A, 3 phase 120/208.
 
Two possibilities come to mind: First is that a structure that large could easily cover a big enough area to cross a neutral gradiant cause by the POCO grounding their poles and connecting all that conduit to the system ground equalizes the difference (get ready for conduit to rust sooner in that installation). The second could be from capacitive coupling of that many feet of wire in the conduits all getting grounded out at once.
 
PetrosA said:
Two possibilities come to mind: First is that a structure that large could easily cover a big enough area to cross a neutral gradiant cause by the POCO grounding their poles and connecting all that conduit to the system ground equalizes the difference (get ready for conduit to rust sooner in that installation). The second could be from capacitive coupling of that many feet of wire in the conduits all getting grounded out at once.
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Just when you thought you provided all relevant information...

We piped for and POCO installed two new transformers, one a feed through that was moved and replaced from its original location, feeding a gas station on one side of the building. 2nd transformer is for the new structure on the other side of the site, about 400' away.

Conduit runs that we did have not yet been pulled through. Sparking occurred from simply metal to metal contact.

My thought was that our EMT grid provided a more appealing ground path then what had already been installed as part of the service.

I tested the grounding bus to earth and did get a reading of slightly over 1 volt. 80% of the 160A draw that we currently have is from 4 old 40A 3-phase electric heaters with fans that are bein used for temp heat.
 
Your new conduit was probably at Earth potential and the box you were connecting to was probably at an elevated neutral potential (NEV or Neutral to Earth Voltage). I have also seen sparks where stray currents can cause grounding sparks.

It is possible that these currents are not related to the local load as you could get sparks in some cases with no power on at the local site.
 
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