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 Post subject: Buses tied together
PostPosted: Wed Sep 04, 2013 11:49 am 
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Joined: Wed Mar 13, 2013 9:12 am
Posts: 12
Hello,

I am reviewing an electrical study which includes and was primarily done to determine arc flash hazards. In this facility there are three (3) 1500kVA transformers, 13.2kV-480/277V all feed from the same utility source. The three 480V buses all have cross ties between the 3 of them (breaker-cable-breaker). The facility normally runs with the buses separated and would tie together for transformer outage.

The report I have modeled the arc flash hazard for the buses and everything else with all 3 buses cross tied. The report states that this is the most conservative results even though this is not how the plant normally operates. This leaves it impossible to operate any breakers on that bus for lock out/tag out purposes.

So, is this the normal way to model and label a facility? If so, why not model and label for normal operating configuration? The way it is modeled now, this configuration may never occur but it is causing heartburn here with the labels on the equipment.

Thank you in advance for any of your thoughts and comments on this post.


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PostPosted: Wed Sep 04, 2013 1:17 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 445
Location: Wisconsin
Tying the buses will provide a conservative result for available fault current and resulting Short Circuit evaluation.

It may not provide the most conservative results for arc flash, as the clearing times of protective devices may be slower when the fault current values are 'low'.

You should always evaluate multiple scenarios such as: High fault current, normal operating conditions, low fault current.


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PostPosted: Wed Sep 04, 2013 1:33 pm 
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Thank you. It is stated in the report that the buses tied together produces the worst case arc flash hazard with 157 cal/cm2.

So my questions in paragraph 3 of my original post still remain......


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PostPosted: Wed Sep 04, 2013 7:47 pm 
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Joined: Mon Jan 18, 2010 11:35 am
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Location: Wisconsin
You get to determine what you want to put on your labels. NFPA 70E purposely leaves this as a grey area, so that it can be addressed in your Electrical Safe Work Practice.

I have several customers, which label per their normal configuration, their higher maintenance mode is covered via their live work permit.


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PostPosted: Fri Sep 06, 2013 6:19 pm 
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Joined: Sat Feb 27, 2010 5:59 pm
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Location: Cincinnati, OH
My company's practice is to discuss the various operating modes with the customer, calculate a set of arc flash results for each operating mode the customer defines, and label the equipment with the worst case result. This does not mean that we study all possible operating scenarios. In the case above you are indicating that when you tie buses together you do not actually parallel more than one transformer, but only tie the buses together when one of the upstream transformers is out of service. Our scenarios in the model would mimic this explanation. All individual scenario arc flash results are included in our report as well as the combined worst case results which are used to print the labels.


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PostPosted: Mon Sep 16, 2013 7:45 am 
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Joined: Fri Apr 15, 2011 7:43 am
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Location: Colorado
I agree with JBD, this will give conservative fault values and could give you conservative arc flash values. Do you want conservative arc flash values? or do you want worst case?

The best method would be run various scenarios with one xfmr/bus, two xfmrs bus, three xfmrs/bus (tied). At minimum it should be modeled as it would be run.


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PostPosted: Mon Sep 16, 2013 8:06 am 
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Joined: Thu Oct 18, 2007 7:10 pm
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Location: NW USA
There are several ways this may be handled. Two methods we have used (at different facilities that had different needs)

1) label the equipment for worst case "normal" configuration, and forbid work on energized exposed conductors if the system is not in "normal" configuration. A good example of this would be a system feeding from a standby generator, with low available fault current that will not allow devices to clear on instantaneous curves. It would be stupid to prohibit work based on this abnormal condition that might occur perhaps once every two years, and that can be scheduled around. This seems similar to your transformer failure hypothesis.

2) Another method used where the facility had MANY sources of power and operating configurations: we put a generic label on all the equipment with a reference to a book provided of arc flash calculations. This book had the calculations listed for each piece of equipment, for each of 12 different electrical system operating configurations. Several of the configuration resulted in unworkable arc flash exposure, so the work rules given included arranging the electrical system to result in a workable configuration before work on exposed energized systems was commenced.


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