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 Post subject: have any of you ever heard of this?
PostPosted: Thu Jan 11, 2018 12:25 pm 

Joined: Thu Jan 11, 2018 12:04 pm
Posts: 1
hi there. new member, first post. happy to find this forum. 8-)

have any of you ever heard of anything like this?
- techs were working to pull in new cable feeds with grounds into an MCC (MCC1).
- system config is as follows: source is an old 480V switchgear with drawout breakers feeding various 480V mccs. one of these breakers (call it oldmain) feeds MCC1 temporarily until a new 480v swgr can be installed.
- the new cables being pulled in are the new source cable from the new 480V switchgear.
- oldmain is opened up, locked out, but not racked off the bus.
- mcc1 main breaker (call it mccmain) is turned off. 4 hours later, cable pull work commences.
- the copper ground wire inadvertently makes contact with A-phase primary lug of mccmain, and theres a spark.
- tech is working right in front of mccmain, within a few inches, but gets no injuries. just a nice sized flash of light and an audible pop.
- a few strands of copper melt off, but the entire ground wire is not severed.
- we tested and verified zero energy before and after the incident. tested oldmain, did meggers, checked for rodents, looked for backfeeding possibilities, e.t.c. but found nothing plausible.

my questions are: have any of you seen anything like that? what could be the cause? if it could have been residual voltage, can you have residual voltage significant enough to burn through a few strands of copper?

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 Post subject: Re: have any of you ever heard of this?
PostPosted: Sat Jan 13, 2018 2:53 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2173
Location: North Carolina
I seriously doubt capacitive coupling. That's usually more of a medium voltage thing so we can skip that one.

So that leaves inductive coupling or direct contact. You said ground wire...what is the voltage between the two grounds? Sometimes it is not insignificant, particularly if bonding is not continuous everywhere. However you can quickly find these with megger checks in all the possible directions.

Second possibility is inductive coupling. Is the cable all running in parallel to another cable somewhere? All that is needed is a complete circuit (a loop) for this to happen and the loop can involve two ground rods and Earth in between. This usually requires a significant amount of distance since basically you have two one-turn transformers but not always...higher currents cause significantly greater fields.

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 Post subject: Re: have any of you ever heard of this?
PostPosted: Mon Jan 15, 2018 7:56 am 
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Joined: Mon Sep 26, 2011 2:17 pm
Posts: 21
Location: Metairie, LA - EIS
What is the grounding configuration of the existing switchgear (ungrounded, high resistance grounded?)
Just wondering if something weird is going on (i.e, an active ground fault one of the phases) while the work is being done.

Although paulengr's reasons could have caused the "spark".

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