Determining Arc Flash PPE for Tie-Breaker Scenario

[mull982]

We are currently performing an Arc Flash Evaluation on two or our plants electrical systems for two plants we have on site. We first ran all of the Arc Flash calculations and PPE levels for each plant individually and establised a PPE level at each piece of equipment with each plant isolated and operating seperately as they do during normal operation.

There is a tie-breaker between the two plants that is only closed during an emergency situation where one of the plants looses its utility feed. With the tie breaker closed both plants can be operated at reduced capacity off of the one remaining utility source.

With this tie breaker closed this may present a scenario where the fault contribution from one plant to the other through the tie breaker changes the Arc Flash level at the busses in each plant calculated when each plant was isolated. This may also change the PPE at each bus.

How is a situation/scenario like this usually handled with Arc Flash PPE labels to account for this emergency situation? Are there certain assumptions that are made?

Do you treat the tie-breaker as an emergency situation and therfore only establish PPE values on equipment for condition where plant is isolated?

Do you try to come up with a best estimate on what loads will be running on each plant and try to calculate a sepearate Arc Flash and PPE value and then use the worst case of this vaule and the one calculated when plants were isolated for establishing PPE requirement?

Do you just issue a blanket statement/requirment saying that during an emergency situation where tie-breaker is closed you must dress in highest rated PPE regardless of what was establised when plants were isolated to account for possible higher Arc Flash vaules.

I am interested to hear what others have done in a similar situation.

 

[charlie b]

With this tie breaker closed this may present a scenario where the fault contribution from one plant to the other through the tie breaker changes the Arc Flash level at the busses in each plant calculated when each plant was isolated. This may also change the PPE at each bus.

I infer that you have not yet performed a calculation in this "abnormal" (I would not use the term "emergency") operational configuration. I suggest running the analysis, and seeing what happens to the arc energy levels at each panel. If the results are higher in this configuration (my guess is that they will be), then I see two ways of handling the labels. The easy solution is to install the more conservative label (i.e., higher energy level), and use that PPE level even if you could use a lesser level if the plants are operated separately. The more complex solution is to apply the labels that apply to the normal operational configuration, and use an administrative control to increase the PPE level in the rare event that you need to work on a panel with the plants operating with the tie breaker closed.

 

[kingpb]

Charlie provides two excellent alternatives.

There is a third one, and that is, that the procedure could simply be that the there is not to be any working on or near live parts, when the tie breaker is closed. Remember, one option to meet 70E is don't ever work hot. This is a legitimate approach.

Now to the bigger issue you are facing, and one in which many other facilities are facing, and that is; are you really meeting 70E by doing a single arc flash study?

My response to that is a firm "maybe". Let me expand.:roll:

There are many under the impression that once they hire an engineer to perform an arch flash analysis, get the results, print/place some labels; all is said and done. Now, for the facilities that have only one configuration, and no multiple switching scenarios, i.e. no tie breakers or multiple feeders then it would be satisfactory, until there is a modification to the system.Then the arc flash, at least at the equipment in question, needs to be done again. IMO there have been many clients ripped off, by having someone perform a single study, at a cheap price thinking all is good. In some cases it may be good for an extended period of time. But for other locations, the study may be worthless before it's even complete.

Large facilities need to be able to perform arc flash studies on the fly, using the latest plant switching and connection scheme at the time the work will be done. If not, and you use old values or the values on the stickers, then how do you know the PPE is correct? Just as questioned in the OP, you don't, and that is the tragedy because it leaves the person most vulnerable with a false sense of security.

Best practice, never work hot. If in an emergency you must work live, then you better know what the arc flash levels are, at that given time, to be safe. And no, simply wearing level 4 as a precaution may not be good enough, as some arc flash levels exceed the maximum PPE level possible.

Mull982 is raising an excellent point and hopefully many will be enlightened to some common pitfalls.

 

[zog]

This is a very common question I run into. my fisrt question always is are you sure your AIC ratings support operation with the tie closed? Typicaly you would see key interlocks preventing such operations. Is this your MV or LV part of your system?

I just got back from meeting to large industrial facilities to discuss this same situation today, 2 plats, 400 miles, I am whipped