# hv capacitance



## guest (Feb 21, 2009)

nolabama said:


> I have always grounded transformers before working in or around them. I have always heard this is not only for personal protection but also to discharge capacitance. True untrue
> ? I kinda vaguely remember Amish electrician has some kind of discharge device.


If you're talking oil-filled transformers, it is very true.

Think of the construction of a HV capacitor: Metal plates separated by paper or mylar film insulated with oil. 

Now what about a HV oil-filled transformer? Copper (or aluminum) windings, covered in paper or similar material, insulated with oil. 

Ground them out at ALL terminals. :thumbsup:


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## nolabama (Oct 3, 2007)

We made a fancy ground with hot line clamps and big dlo


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## Big John (May 23, 2010)

Windings can hold enough charge to give you a nasty surprise because of the capacitance of the insulation, especially MV and above. But I've had it happen even megging small 600V generators where it will polarize the insulation and it can still give a little bit of a jolt minutes after the megger has been removed. 

Some guys have discharge sticks like these that they always use before applying grounds. On capacitors and extremely long underground cables, I can definitely see using it prior to grounding, because there can be some serious energy there. But I've never seen a problem just applying grounds directly to transformers. Remember that often these things are grounded through PTs and neutral points anyway, so they tend to bleed themselves down.


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## Vintage Sounds (Oct 23, 2009)

I was working on a job which involved disconnecting a 13.8kV to 347/600v dry-type transformer in a sort of electrical shed. I can't remember the KVA but it was big, about the size of a Ford Fiesta. First we waited outside while the utility came to disconnect the line side, then the foreman put on an arc flash suit and went inside to ground the windings so we could go in and disconnect it. Once he gave us the all-clear we went in and got to work. A few minutes later one of the journeymen with us let out a loud groan and said he got a pretty bad shock. 

It turns out the foreman only grounded the transformer secondary and not bothered with the primary because he for some reason thought the residual energy would have dissipated when he grounded the secondary. The JM who got zapped made contact with one of the primary lugs with his elbow. He did not feel the shock until the moment he removed his elbow from touching the lug.

Always measure, always ground.


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## Semi-Ret Electrician (Nov 10, 2011)

How about when one meggers a cable? Discharge required?


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## guest (Feb 21, 2009)

Semi-Ret Electrician said:


> How about when one meggers a cable? Discharge required?


In many cases, yes. 

I would do it in all cases...only takes once to get bit hard.


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## KDC (Oct 19, 2012)

Typically at work we always ground the high and low sides, and only work between the grounds. 

Of course, some of the guys have a bad habit of once we take the risers off to do testing, not worrying about re-grounding after/between tests until it's time to replace the risers.


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## Michigan Master (Feb 25, 2013)

*Good Article on Grounding by Jim White*

*Applying Personal Protective Grounds*
_By James R. White Jun 01, 2013_
Always test circuits for the absence of voltage before placing personal protective grounds. Just because you know it's de-energized doesn't mean that it really is.

http://ohsonline.com/articles/2013/...nds.aspx?goback=.gde_1769997_member_250601239


_About the Author:_
James R. "Jim" White is the training director of Shermco Industries, Inc., in Dallas, Texas, and has been with Shermco since 2001. He is the principal member of the NFPA technical committee "Recommended Practice for Electrical Equipment Maintenance" (NFPA 70B), represents NETA as an alternate member of the NFPA Technical Committee "Electrical Safety in the Workplace" (NFPA 70E), and represents NETA on the ASTM F18 Committee "Electrical Protective Equipment For Workers," as well as NEC Code Making Panel 13. White is an IEEE Senior Member and in 2011 received the IEEE/PCIC Electrical Safety Excellence award. He is a past chairman (2008) of the IEEE Electrical Safety Workshop (ESW) and author of two books available through American Technical Publishers, "Significant Changes to NFPA 70E -- 2012 Edition" and "Electrical Safety, A Practical Guide to OSHA and NFPA 70E."


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## Lone Crapshooter (Nov 8, 2008)

This article is very informitive.
http://ohsonline.com/articles/2013/0...mber_250601239
I have often wondered if the way we grounding was adiquate. 
Now I know it was not.

MANY THANKS MICHIGAN MASTER

LC


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## SteveBayshore (Apr 7, 2013)

Semi-Ret Electrician said:


> How about when one meggers a cable? Discharge required?


The new electronic megger that I just started using a couple of months ago has an audible alarm sounding after the test is complete letting you know it is safe to remove the test lead.


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## EB Electric (Feb 8, 2013)

I disconnect transformers for maintenance and testing quite regularly. We check for potential in arc flash gear on primary and secondary side using a non contact tester. We take a jumper and touch each cable termination to ground, on primary and secondary side. This dissipates any charge that is left on the cables. It may sound trivial but when there is a 300' run of 750 mcm 13.8kv there can be a significant amount of charge still in the conductor. We also megger feeder cables, and switchgear bus and discharge the cable before testing. If nothing else I think it's good practice.


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## Semi-Ret Electrician (Nov 10, 2011)

EB, I've heard but not seen, linemen would megger a spool of wire to zap a rookie just before he had to handle it. True?


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## EB Electric (Feb 8, 2013)

Semi-Ret Electrician said:


> EB, I've heard but not seen, linemen would megger a spool of wire to zap a rookie just before he had to handle it. True?


I could believe it. Most of the guys at work have hit someone with the megger before. Never like that, but testing a cable with someone on the other end in another room.


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## Big John (May 23, 2010)

Semi-Ret Electrician said:


> EB, I've heard but not seen, linemen would megger a spool of wire to zap a rookie just before he had to handle it. True?


 Never seen it, but if it was a spool of direct-burial, I'd bet there would be something to that.


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## Zog (Apr 15, 2009)

Semi-Ret Electrician said:


> How about when one meggers a cable? Discharge required?


Yes, specifically MV cable which is a big long capacitor. Think of a LV cable laying in a cable tray, 2 conductors seperated by an insulator......


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## Zog (Apr 15, 2009)

nolabama said:


> I have always grounded transformers before working in or around them. I have always heard this is not only for personal protection but also to discharge capacitance. True untrue
> ? I kinda vaguely remember Amish electrician has some kind of discharge device.


3 reasons you install grounds
1. Discharge stored energy or induced voltage
2. Provide a low impedence path to operate breakers in the event the circuit becomes re-energized
3. Provide a equipotential zone of protection

It is important the grounds are rated for the available fault current in the system, otherwise they may not stay on the line long enough to actuate the OCPD (15 cycles)

It is also important they are properly connected to perform function #3 (Touch potential protection for that 15 cycles)

This is a pretty good video, used to use it in my training classes, it gets really interesting at the 6 minute mark.


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## Big John (May 23, 2010)

Neat video Zog. I've never used the temporary-rod method, and darn sure wasn't expecting it to react like that.

One thing I wasn't clear on: Were they actually able to clear their fault with that method or were they just stopping the test after so many cycles?


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## Zog (Apr 15, 2009)

Just stopped the test


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## guest (Feb 21, 2009)

Great video Zog and thanks for posting it. :thumbup:

Some questions come to mind about the tests seen in the video: 

1: What is the power source? I would _think_ it is on-site generation as such tests would create some nasty effects on the POCO's lines....
2: What kind(s) of switchgear can be "programmed" to interrupt faults in units of CYCLES? and
3: HOW does said gear interrupt such faults without contact restrikes adding to the duration of the fault?


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## Zog (Apr 15, 2009)

mxslick said:


> Great video Zog and thanks for posting it. :thumbup:
> 
> Some questions come to mind about the tests seen in the video:
> 
> ...


1. Yes, they use a generator similar to what KEMA labs use
2. Not sure what you mean, 15 cycles is a max design criteria, we have a new MV breaker that is rated for 5 cycle clearing at max fault rating
3. That is only a concern with VCBs in certain applications


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## guest (Feb 21, 2009)

Zog said:


> 1. Yes, they use a generator similar to what KEMA labs use
> 2. *Not sure what you mean*, 15 cycles is a max design criteria, we have a new MV breaker that is rated for 5 cycle clearing at max fault rating
> 3. That is only a concern with VCBs in certain applications



For example, let's say you want to limit a fault to a seven cycle duration. Is there some kind of trip unit that can be programmed to trip in seven cycles? (or 3, 5, 10, 200 or whatever?)


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## Zog (Apr 15, 2009)

mxslick said:


> For example, let's say you want to limit a fault to a seven cycle duration. Is there some kind of trip unit that can be programmed to trip in seven cycles? (or 3, 5, 10, 200 or whatever?)


You just send a signal to the trip coil when you want it to trip, minus opening time of the breaker


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## Bipeflier (Jan 16, 2013)

I was a design engineer at Chance during the time those tests were being done.

They have a 762 MVA short circuit (up to 71,000 Amp Asym) generator built by GE in 1959. Powered by a 1000 HP motor with staged starting resistors. The lab and engineering facility was completed in 1960. Link: http://www.hubbellpowersystems.com/literature/general/19-9101.pdf

The circuits are controlled by a "cycle counter" the closes and opens HV circuit breakers.

As for clearing a fault through a temporary ground rod, it depends a lot on soil resistance, distance to the up-stream protective devices, etc. If soil resistance is high enough the protection will never see enough current to operate. Much better to ground to the system neutral.

Here is a link to the CHANCE (now Hubbell Power Systems) grounding encyclopedia. It is aimed at utilities but the principals apply to all electrical circuits. You can't be too safe.

http://www.hubbellpowersystems.com/literature/encyclopedia-grounding/

Take some time to root around on their website. Lots of information in there.


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## nolabama (Oct 3, 2007)

I have ordered that Chance grounding encyclopedia. Can't wait.


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## Jstanton27 (Jul 19, 2011)

nolabama said:


> I have ordered that Chance grounding encyclopedia. Can't wait.


Do they charge for that at all


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## nolabama (Oct 3, 2007)

Jstanton27 said:


> Do they charge for that at all


No they did not


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