Arc Flash Calculations 2

[richanton]

Has anyone done any of these. I am doing one for the first time using SKM Systems software. I am confused by the intent of the printable signs. I would think you would have one category for each piece of equipment such as a switchgear lineup or motor control center. The software however, calculates by bus, giving different values for different feeders. This would mean I would have to put different signs on each switchgear compartment, because each compartment would have a different requirement. This seems like overkill. Has anyone completed an installation in the field? I would be curious as to how you set up your signs.

 

[mpparent]

Personally, I think posting all the flash information on a placard is opening a can of worms. Do you use "line" side or "load" side? If your system has multiple operational schemes, you would then have to post for each. Currently, the NEC only requires that the sign say (I'm paraphrasing) "....arc flash harzard present". This forces maintenance and engineering to evaluate the work being done and decide upon proper PPE based on whether you are calculating the energy levels based on the NFPA 70E or IEEE 1584.

Mike

 

[richanton]

We are implementing a scheme where we place a bypass switch on the MCC or switchgear. When maintenance is required, the switch will be turned to the ON position which will place the feeder under the control of a separate instantaneous relay with a very low setting. This will supposedly reduce the energy available. The placards can be relatively simple in that they just identify the clothing requirement to work on the particular piece of equipment. I want to just have one placard per switchgear, such that any work on the gear will default to the worst case condition that the calculation comes up with.

 

[PWR]

Typically all the feeder breakers in a single switchgear will have the same hazard level. The main breaker, if there is one, will have a different (higher) energy available. The hazard is based on the energy available on the line side of the breaker(s) and the feeders will all be essentially the same (motor contribution can cause some differences but typically not enough to require different labels)

 

[MJJBEE]

It really depends on the type of switchgear you are using. If you could contact buswork not protected by the main I would use the value of arc flash based upon a claculation based on no main breaker. However if you have a seprate main you might be able to use the lower value. In my company we are labeling that there is a hazard. Then putting together a book with diffrent operating scenario one lines and refering people to that book for protection requirments. That way we don't have to go changing labels every time we make a system change.

 

[dpc]

I agree with PWR. All the feeder breakers on a bus will have the same arc-flash hazard. If there is a main breaker section, it may have a different arc-flash level. Each compartment will not have different requirements but each feeder is depending on the same upstream device for clearing the fault.

 

[richanton]

Actually, all the breakers don't have exactly the same values because it depends on your scenario. In my case, we are modelling with a 1250kw standby generator connected in parallel to one side of a double-ended substation, since this is a worst case scenario when the generator gets tested once a month.

I end up at Category 2 for all the breakers except the one connected to the standby generator which goes up to Category 4.

I will probably have a sign for Category 2 with a separate sign on the breaker cubicle feeding the generator identifying the special requirement if generator is being tested.

 

[dpc]

You need to determine the worst-case scenario in terms of arc-flash energy and base your signage on that, unless it is some extreme, or so unlikely, that you want to have a more detailed labeling or hazard assessment.

I would try to keep it as simple as possible.

 

[mpparent]

The only problem with a "worst-case" sign is that, to be nice to your maintenance crew, you may not want them wearing for instance 30cal gear (if that's worse case), when they could be wearing 8cal for the job at hand. That's why I feel an "arc flash hazard is present" type of sign is better, and that way, a review needs to take place so that a sensible amount of PPE is worn for the task.

Mike

 

[dpc]

Oh, there are a lot of problems with a worst-case sign. That's why I mentioned the "extreme" worst-case condition. But there are a lot of problems and liabilities associated with having multiple levels of PPE depending on the task.

The problem with a task-based approach is that the task often evolves as the work is done and there will always be a tendency to define the task so as to be able to wear the lowest level of PPE.

Another major problem with the task-based approach is that it really doesn't make any sense in a lot of cases. If the door of an MCC bucket is open, it won't matter what task was being done if an arcing fault occurs - the same energy will be released. The arc doesn't care what you were doing, or trying to do. NFPA-70E allows lower levels of PPE for certain tasks because they figure the probability of causing an arc is reduced, not because the impact of any arc created will be less. That's a little like saying you only need to wear your seat belt in inclement weather.

I think we're all trying to make sense out of the NFPA 70E requirements and come up with a workable, logical approach. I expect these requirements will be evolving quite a bit in the next few years.

 

[mpparent]

dpc,

To get around tasks evolving, at our site, we always have a Maintenance Operating Procedure. This has kept our eye on the ball, so to speak. But, I do hear where you're coming from.

Mike

 

[dpc]

If you can develop a culture of strict adherence to procedures, then I agree it's nice to have some flexibility.

 

[richanton]

I'm glad my question "sparks" so much discussion.(Pun intended.)

But seriously, I think what has to be done is you evaluate say everything in an electrical room. If everything in there is Category 0, 1, or 2, maybe you decide to make everything Category 2. Then the maintenance tech will know that to work on anything in that room requires certain clothing, and you put a sign on the door going into the room. Maybe you remain specific to each piece of switchgear and MCC if the categories are higher.

This is not to say that this approach will always be valid. There are always special cases. I just think you need to have a global view and just separate out things that don't fit the envelope.

I don't think it should get to the point where the maintenance guy has to remember what kind of underwear he put on this morning depending on what piece of equipment he is going to work on.

 

[dpc]

Underwear requirements always generate some laughs, but the requirement is a serious one.

Electricians should be required to wear cotton underwear all the time and then they won't have to worry about it. They should also be forbidden to wear watches, rings, necklaces, metal-frame glasses, etc.

The problem with a single PPE requirement for an entire room is when the PPE level is 3 or 4. At these levels, an arc hood is required and these are not comfortable to work in for long periods of time.

At PPE level #2, only a face shield is required and this is much cooler and much less restrictive than the hood.

I'd do whatever you can to get arc-flash energy below 8 cal/cm2.

 

[mrbrue]

Why not just consider installing current limiting fuses?(UL has specified good standards clearing the fault current before half cycle and melting the fuse before even reaching the first peak), then you can do away with the calorie calculations and the hazard labels.

 

[wbd]

mrbrue,
You would still have to do some calculations on the bolted fault current to determine if the current limiting fuse will limit the current. For example: Paragraph D.8.6 in Annex D of NFPA 70E, has the equations for CL fuses. For a Class L Fuse, 1601A - 2,000 A, if the bolted fault is less than 22.6kA, the arcing current needs to be calculated and then the time current curves used to determine the incident energy using the appropriate incident energy equations.
It's not as simple as just installing some CL fuses and walking away with what might be a false sense of security and safety.

 

[davidbeach]

Expanding on what wbd said: While a certain fuse manufacturer trys to suggest that fuses are the one and only solution to all protection problems, that ain't necessarily so. If you have situations where the minimum available fault current is below the current limiting value of a fuse, the fuse may well have a higher arc energy let through than a circuit breaker. In other cases, the fuse may provide benefits. In all cases, the calculations need to be done and alternatives evaluated; there are no silver bullets.

 

[dpc]

I agree with David Beach regarding current limiting fuses. They do not always limit current and do not alway reduce arc-flash energy. This is especially true for the larger fuse sizes.

Relying on fuses for arc-flash reduction can also create significant coordination problems as well.

 

[alehman]

I agree with the above. For fault current less than about 5 to 10kA, larger fuses can let through enormously more energy than typical MCCB's. The fuse mfg's like to talk about protection for very high fault currents, where their current limiting functions work well. Many faults involve arcing conditions and current well below the current limiting threshold where typical fuses are relatively slow. That's what causes terrible injuries. (No, I don't work for a c/b mfg).

 

[Electic]

Not an answer but another question pointing out the unsettled nature of arc flash calculations in general.

What about arc resistance and it's unpredictable nature: a high resistance fault can cause less current than what would trip the instantaneous pickup. Instead of clearing at a predicted 6-7 cycles, this could increase the 'cook time' (albeit at lower current levels)to 60 cycles or more. We have modelled real life situations where this could occur.

With modern protective relays it is easy to analyze that most ground faults do not reach maximum calculated available fault current. This could have a deletrious effect on true arc flash exposure by extending clearing times.

At this time we are interpreting our assignment as following the recommendations of IEEE which is a substantial safety improvement over no warnings or PPE, but with many disclaimers (as also exist in NFPA 70E) that following these guidelines does not assure absolute safety. Seems like a cop out, but I know of no better.