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 Post subject: Temporary Protective Grounds Sizing
PostPosted: Thu Jun 18, 2020 5:52 am 

Joined: Fri Aug 22, 2014 2:31 pm
Posts: 6
OSHA 1910.269(n)(4) requires temporary grounds to be sized to carry the maximum short circuit available current for the time necessary to clear the fault.

There are 2 philosophies on this requirement for working on switchgear for bus cleaning & inspections:

Philosophy 1: Assumes all breakers are available, and will allow current to flow to the bus, although they are racked out and LOTO'd. Essentially this assumes that the short circuit current is determined similar to the method used for determining arc flash incident energy.

Philosophy 2: Assumes that current cannot flow through the racked out and LOTO'd breakers. As a result, the only current that the temporary grounds need to be sized for is that caused by induced voltage.

As an example, see the attached single line.

Which philosophy does your company take on this?


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 Post subject: Re: Temporary Protective Grounds Sizing
PostPosted: Thu Jun 18, 2020 8:17 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
This is a layered belt plus suspenders approach. Isolate, test, and ground to ensure workers are safe even if one of the three tasks is performed imperfectly. Are you really saving much by providing undersized grounds?
I also observe that OSHA specifically mentions accidental energization. I would call induced voltage expected and not accidental.


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 Post subject: Re: Temporary Protective Grounds Sizing
PostPosted: Mon Jun 22, 2020 10:10 am 

Joined: Fri Aug 22, 2014 2:31 pm
Posts: 6
Hello Steve, thanks for your reply. I think one of the big questions is the philosophy for sizing the grounds, specifically clearing time. Do you assume that the protective device connected to the bus (and in our situation is racked out and LOTO'd) is unavailable, and that the upstream device has to operate? Would you be willing to share your company's philosophy?


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 Post subject: Re: Temporary Protective Grounds Sizing
PostPosted: Mon Jun 22, 2020 11:47 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
rn,

I work for a utility, and the personal protective grounds are carried around on trucks. Company philosophy is they are sized so they may be used anywhere on the system.

In your case, accidental energization could happen if either of the two breakers were racked in and closed. I would look at the worst of these two scenarios. Unless of course you can think of another that would delay clearing?


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 Post subject: Re: Temporary Protective Grounds Sizing
PostPosted: Mon Jun 22, 2020 2:01 pm 

Joined: Fri Aug 22, 2014 2:31 pm
Posts: 6
Steve,
I think the struggle for us is defining a philosophy. Does your company philosophy assume that, for the purposes of ground sizing, the LOTO is not in place?

As an example, on the attached single line, there are 3 protective devices that would allow a path of power to the SWGR:
1. Reserve breaker
2. Main breaker
3. MCC 129

Would your company assume, for ground sizing, that:
1. Fault current was supplied from all 3 sources at the same time
2. That each protective device was closed and that its associated relaying was in operation, and, consequently, that the fault current clearing time was based on the protective device's relay settings.

A couple other questions:
1. Does your company install any grounds larger than 4/0?
2. Does your company ever install more than one set of grounds to achieve the fault current capacity/trip time combination?

Thanks for your input!


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 Post subject: Re: Temporary Protective Grounds Sizing
PostPosted: Wed Jun 24, 2020 5:36 pm 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
I didn't realize the MCC was source. I would evaluate each possible source separately (not together) and take the worst case. I don't have answers to the ground cable questions.


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 Post subject: Re: Temporary Protective Grounds Sizing
PostPosted: Thu Jun 25, 2020 6:06 am 

Joined: Thu Apr 18, 2019 11:42 am
Posts: 16
Could be wrong here...but OSHA 1910.269 is applicable to power generation, utility work, etc. If this is a low voltage system, I'm not sure why you would need to throw ground clusters on the equipment. All sources of power/energy are locked out/tagged out.

If we are talking about a MV/HV system, then I can understand more the need for ground clusters.

Mike


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 Post subject: Re: Temporary Protective Grounds Sizing
PostPosted: Mon Jun 29, 2020 7:08 pm 

Joined: Fri Aug 22, 2014 2:31 pm
Posts: 6
Hello Mike,

Yes sir, this is MV. 6.9 KV and 13.8KV nominal.


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