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 Post subject: Labeling for scenarios that will rarely if ever occur
PostPosted: Fri May 05, 2017 5:56 am 
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Joined: Fri Jul 08, 2016 10:01 am
Posts: 132
I already know what I'm going to do but wanted to get some opinions.

Say you have a 2 pad mount utility transformers along with a manual, kirk-keyed main-tie-main double ended switchboard and also a generator large enough to feed the entire building via a "back-feed" manually operated breaker also kirk-keyed. Transformers are identical as is the available primary fault current and secondary cable footage to the mains.

You run multiple scenarios.

Main 1 closed, Main 2 closed, tie open, generator out of service
Main 1 closed, Main 2 open, tie closed, generator out of service
Main 1 open, Main 2 closed, tie closed, generator out of service.
Both utilities out of service, generator in service, transfer switches in emergency position
Both utilities out of service, generator in service, Main 1 open, Main 2 open, tie closed generator backfeed breaker closed. Entire building fed by the generator.

You find the "worst case" scenario for incident energy at several boards is when the generator is feeding the entire facility via the back-feed breaker due to lower fault current and longer clearing times. However, this scenario rarely happens. It is only intended if there is a major ice storm or similar event when utility power is lost and expected to be down for many hours or days. In the last 15 years you've only run in that mode once during an ice storm that took out the utility for 4 days. When it happened only qualified, trained personnel were on site and there was no reason to work on exposed live equipment within the building during the utility outage while running on the generator.

Do you label for the worst case (on generator) knowing full well the labels are wrong 99.999% of the time or do you label for the normal condition/worst case of the other scenarios (which are all very close to one another due to the identical services) and put some signage at the back-feed breaker warning that incident energy labels for the facility are inaccurate in this mode?

Thanks,
bbaumer


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 Post subject: Re: Labeling for scenarios that will rarely if ever occur
PostPosted: Fri May 05, 2017 12:25 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 439
Location: Wisconsin
I have customers that perform the NFPA70E required risk analysis and decided to use the normal scenario. They rewrote their ESWP to say tha when on generator the higher value applied.

Other customers have taken an opposing view and labeled per the worst scenario, then put the possible lower level into their Energized work permit.


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 Post subject: Re: Labeling for scenarios that will rarely if ever occur
PostPosted: Fri May 05, 2017 7:55 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 1779
Location: North Carolina
Is the label that you are applying supposed to be used for anything or simply scare someone into not performing required and necessary maintenance on the equipment?

If the latter then by all means, put the most extreme over the top scenario on the label. In fact why stop with the generator? Why not insert arbitrary amounts of impedance and use the scenario with all motors running in the plant, whether or not the generator can support the load, just to make sure that it is extreme?

Or do you try to be as helpful and instructive as possible and thus put the "normal" case on the label and keep the others in say work instructions for emergency operations where they belong?

An outside consulting engineer of course goes for the "most conservative" route and picks the most extreme cases such as assuming that the ties are closed as well as the mains in a double ended substation without a generator feed simply because it makes the available fault current via 2 transformers as high as possible, never mind the fact that it might be Kirk keyed or simply never done in the first place. So it becomes the responsibility of the reviewing engineer at the plant to fix these screw ups and ensure that the report is not only technically accurate but also of practical value.


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 Post subject: Re: Labeling for scenarios that will rarely if ever occur
PostPosted: Sun May 07, 2017 2:06 pm 
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Joined: Fri Jul 08, 2016 10:01 am
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PaulEngr wrote:
Is the label that you are applying supposed to be used for anything or simply scare someone into not performing required and necessary maintenance on the equipment?

If the latter then by all means, put the most extreme over the top scenario on the label. In fact why stop with the generator? Why not insert arbitrary amounts of impedance and use the scenario with all motors running in the plant, whether or not the generator can support the load, just to make sure that it is extreme?

Or do you try to be as helpful and instructive as possible and thus put the "normal" case on the label and keep the others in say work instructions for emergency operations where they belong?

An outside consulting engineer of course goes for the "most conservative" route and picks the most extreme cases such as assuming that the ties are closed as well as the mains in a double ended substation without a generator feed simply because it makes the available fault current via 2 transformers as high as possible, never mind the fact that it might be Kirk keyed or simply never done in the first place. So it becomes the responsibility of the reviewing engineer at the plant to fix these screw ups and ensure that the report is not only technically accurate but also of practical value.


Your way of thinking is in alignment with mine, Paul. Thanks for taking the time to reply.


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 Post subject: Re: Labeling for scenarios that will rarely if ever occur
PostPosted: Mon May 08, 2017 7:40 am 
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Label it for normal operations of worst scenario under utility power. But add a note on the label or second label that instructs them to see arc flash levels in the AF Study when operating on generator. Then have that Scenario as a separate print-out in the AF Study. After that, it goes to the risk assessment.


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 Post subject: Re: Labeling for scenarios that will rarely if ever occur
PostPosted: Mon May 08, 2017 4:37 pm 
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Joined: Sat Apr 04, 2009 9:50 pm
Posts: 83
Location: San Antonio, TX
Fully agree with PaulEngr.

I am a consulting engineer performing arc flash studies. I have to find the most REASONABLE worst case. I have never done a worst case with a tie breaker closed and both sources of power active at the same time, if this scenario is not a standard operation (which it rarely is). If we continue to strangle the chicken of the golden eggs, we are not going to have more golden eggs.


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 Post subject: Re: Labeling for scenarios that will rarely if ever occur
PostPosted: Mon May 22, 2017 9:28 am 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 115
Location: Maple Valley, WA.
It is very common for a double ended substation (two transformers feeding two switchgear line ups with a tie in the middle.) to have a bus tie breaker that is open but has two live sources inside the cubicle. Racking in or out this breaker will have almost twice the arc flash energy as racking in/out a feeder breaker. So for our studies, the label that goes on the Bus Tie cubicle will have a much higher arc flash energy than the main buses of the switchgear.

We always include a report that sorted by each bus (equipment/location) and lists the AF energy for each operating scenario. So if the the electrician or technicians have access to the report, they can then determine the arc flash energy for exact operating scenario that they working in.

_________________
Robert Fuhr, P.E.; P.Eng.
PowerStudies


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