15kva Switch and 480 volt Grounding

[jerry h.]

Good Morning,
I am inquiring into the proper Grounds Requirements for our 15kva Switchs and the Proper Grounding of 480 volt Switch Gear during our PM's. The gear is de-energized at the 15 kv level and both the Mains and Tie have been removed for testing.
After much investigation, on my part, I found that no manufacture will make approved grounding sets for the 480 volt gear. How do I work between grounds? Also, by applying approved grounds on the Medium Voltage Switchs, load side of the Fuses, switch LOTO'd, how does this protect me on the Secondary side of the Transformer? Any help would be greatly appreciated.
Thank You,
Jerry

 

[zog]

Good Morning,
I am inquiring into the proper Grounds Requirements for our 15kva Switchs and the Proper Grounding of 480 volt Switch Gear during our PM's. The gear is de-energized at the 15 kv level and both the Mains and Tie have been removed for testing.
After much investigation, on my part, I found that no manufacture will make approved grounding sets for the 480 volt gear. How do I work between grounds? Also, by applying approved grounds on the Medium Voltage Switchs, load side of the Fuses, switch LOTO'd, how does this protect me on the Secondary side of the Transformer? Any help would be greatly appreciated.
Thank You,
Jerry

Yep, you found a real issue. I would need to check but I think this was addressed in the 2011 70E proposed changes.

I assume your issue on the LV side is the available fault current exceeds the 15 cycle rating of any available ground sets. I think 45kA is about the highest rating you can get (4/0). The grounds on the 15kV side will not protect people working on the LV side of the transformer. So you need grounds on the LV side, the trick here is to understand ASTM F 855, which requires the grounds to carry the available fault current for the time necessary to clear the fault. As long as any potential backfeed source on the LV side has an INST trip you won't have a 15 cycle duration, more like 4 or 5 cycles. The problem is the ground sets are rated for 15 and 30 cycles ratings.

So in reality your people will be protected, probelm is unless you get your ground sets rated for 5 cycles it is hard to prove.

Interested to see what other people have to say as I have always struggled with this issue before.

 

[richxtlc]

The need to provide grounds on both sides of the transformer is important. What you have to establish is an equipotential zone for the worker, so that, even if the circuit becomes energized, the worker is like a bird on a wire and therefore protected.
Also, the best way to apply grounds is (after testing for lack of potential) to short circuit the three phases and bring one cable to ground.
On the max fault and time, my info shows 59ka at 15 cycles and 42ka at 30 cycles for a 4/0 cable.
If would help if you can show us a one-line of the system you are trying to protect.

 

[Girl Engineer]

I was able to find this paper on IEEE.org, but do not have the right subscription to access it.

Factors in sizing protective grounds-IEEE engineering in the safety, maintenance and operation of lines subcommittee report
This paper appears in: Power Delivery, IEEE Transactions on
Issue Date: Jul 1995
Volume: 10 Issue: 3
On page(s): 1549 - 1569
ISSN: 0885-8977
INSPEC Accession Number: 5024940
Digital Object Identifier: 10.1109/61.400940
Date of Current Version: 06 August 2002

From what I can see, NFPA 70E only talks about inspection and maintenance of safety grounds.

We put 4/0 grounds from AB Chance (http://www.hpsapps.com/groundset/default.asp)on the 13.8kV knife switches, and work on the other side of the transformer. However, the transformer and knife switch are adjacent to the gear we are working on, there is no backfeed on these lines, and the disribuiton downstream is all within the building.

I can see where you would need to ground the secondary side of the transformer if you are worried about backfeed, the feeders run outside, there are capacitors, or some issue with induction or static. In that case I would just install ground studs on the 480V bus and hang the 13.8kV grounds on those studs, basing the ground size off the main of the 480V distribution equipment.