Tips to finding faster CB for the puprose of Minimizing Arc Flash?

[majesus]

Just more looking for some tips or advice to increase my knowledge, expertise:


I have a CB protecting a 600V main PDC coming from a 3 MVA Sub XFMR. The CB is a 3200A C-H RMS 520 and has an Instantaneous total clearing time of approx 0.065sec. With a 32kA Arcing Fault, this results in a Cat 3 hazard. Suppose I want to find a faster CB (retrofit) to limit the Arc Flash below a Cat 2. The only method (in terms of CB searching) is looking at TCC of various CBs in Etap and see if they are faster. Then I check to see if retrofitting the existing with the new is compatible.

TCC CURVE of PDC



Is there a better method in finding faster CBs? Also 4 cycle 3200A breakers seem to be the norm. Why?
Is this a question of speed vs price, or physical upper boundary, ie larger contacts make the CB operate slower and 4 cycle is pretty fast for this size CB.

 

[majesus]

One other note to reduce the AF Category.
You quoted 18" for your study. I normally use 24" for LVPCB locations. That's 18" for arms + 6" for distance from front of CB to current carrying parts within the breaker itself. It's still conservative enough to allow full protection, without having to use a stick to operate the breaker.
It pays to understand why all those variables are chosen.

Very good point. You are right. There is a bunch of variables that one needs to be aware off. I'm glad I'm enjoying the learning process!


Side note:
SquareD Masterpact NW/NT Low Arc Flash Circuit Breakers:

http://ecatalog.squared.com/techlib/displaydocument.cfm?id=0613CT0001&action=view

On Pg 178

They look pretty cool (assuming your fault it higher than 30kA)


BTW, thanks everyone for the insights. Appreciate the help

 

[Morquea]

Zogzog

"Or eliminating the "exposed to live parts" condition. "

That has nothing to do with arc flash protection, PPE is required regardless of the parts being exposed or not.

Well, I disagree. Oh! Yes, it has nothing to do with the calculations of the Arc-flash hazards energy. It's more about coping with it.

Mostly based on what my senior colleague and I rule 2 years ago about NFPA 70E semantic revising work procedures for a client.

Here's a short explanation of the case with the most significant details.

Context
Maintenance had to be done on medium voltage breaker of medium voltage switchgear. Arc-Flash calculations shows that a category 4 PPE is required when exposed* to live parts. Since the duration of the maintenance procedures for one breaker is about 4 to 8 hours, we agreed that the worker would likely not keep it during the work and put it off at some point. So exposing himself to more danger.

*Exposed (as applied to live parts): Capable of being inadvertently touched or approached nearer than a safe distance by a person. It is applied to parts that are not suitably guarded, isolated, or insulated. (NFPA 70E Article 100 p.70E-11)

Design of the switchgear allowed to fully apply Article 120.1 and so deenergized the breaker. switches, CT, etc. with the exception of the bar at the top of the cell (6 feet above cell floor, breaker being at 3 feet above). According to the more restrictive boundary rules of the client, deenergized parts of the switch above the breaker where in the forbidden zone. The maintenance procedure had to reach that zone.

Interpretation of NFPA 70E
Consulting table 130.7(C)(9)(a) for Metal Clad Switchgear, 1kV and above, we could notice a difference between recommended Hazard/ Risk Category from a task with enclosure doors closed and enclosure door open.
Ex.:
CB or fused switch operation with enclosure doors closed - Hazard/risk Category 2
CB or fused switch operation with enclosure doors open - Hazard/ Risk Category 4

We deduced that the spirit behind that difference where the "exposed condition" stated in 130.1 " Live parts wich an employee might be exposed shall be put into electically safe work condition..."

The solution
So we suggest to installed a temporary (of more permanent) "cover" between the live parts and the switches' blades, removing so the exposed conditions according to the definition.

Then, we allowed the worker to reach the forbiden zone since the risk to provoke a fault was appropriately mitigated. Also, we recommended the use of long range V-rated tools to keep the worker far away as an added safety measure (and keeping him more or less at distance of arc blast hazards).

End word
That was the short explanation of how we used the "exposed" definitions to workout a maintenance procedure that required only a category 2 PPE where it would originally required a category 4 PPE.
And yes, it has nothing to do with the calculations of the Arc-flash hazards energy. It's just about living with it.

 

[dpc]

You are free to interpret NFPA 70E however you want at this point. Consistency is probably the most important thing OSHA will be interested in.

My criteria is to make sure my interpretation is something I'd be comfortable explaining to a jury if some poor soul gets blown up.

 

[Zogzog]

OK, just because you "assumed" something dosent make you right.

The basis of the difference the tables show is the different "assumed" working distances to the potential arc source. When the covers are removed (and live parts exposed) a 18" working distance was assumed, with the covers on, a further working distance was assumed, dependant on the type of equipment being used. This is explained in the notes section of the 70E handbook.

You are also mixing the tables with the result of an anylsis, cant do that, I have discussed this with more than one 70E commitee member. I am not saying that your solution to your maintenance program was wrong, it was a very good solution in fact, but some of the assumptions made were a little off.

The 70E (2004) IS confusing about when the arc flash boundary applies, that has been addressed in the ROP for the 2009 70E due out October 2008. The ROP's are available on the NFPA website and the next version will clear up this commonly confused concept. After you read these, then see if you still disagree with my statement.

 

[Electic]

I disagree that covers in place make no difference.

If that were the case every pad mounted utility transformer would need barricades to keep the public outside of the safe approach distance.

And if that were the case, NFPA 70E 130.7(C)(9)(a) would not have different risk categories given for the same class of equipment, based on type of work being done. It does!

 

[dpc]

Covers reduce the risk, but not the hazard.

The original draft of NFPA 70E-2004 contained provisions for reducing HRC based on the task, but this was pulled from the final version, leaving the mess we have now.

The pad-mounted transformer example is not a good comparison unless the doors of the terminal compartment are open and some type of maintenance activity is taking place.

(Almost) no one maintains that PPE is required just to walk by buttoned-up deadfront equipment when nothing is being done to it. Of course it could blow up, and the doors can come off, but the risk is very low and you would have had a very, very bad day. **Most** faults occur when something is being done the equipment.

By using Energized Work Permits, you have a mechanism for lowering the HRC category for low-risk tasks, if you believe that is a valid approach.

 

[Zogzog]

Yep the 2009 edition will use the phrase "Interaction with equipment" or something similar to taht when the AFB and PPE requirements apply.

 

[Electic]

To make such a distinction between "risk" and "hazard" is at some level playing a word game. Because of such word games I have encountered professionals advising against approaching equipment with louvres such as dry type transformers where energized terminations can be seen thru a vent.

The quoted NFPA 70E 130.7(C)(9)(a) table does apply a different risk category for similar types of equipment, depending on the type of work being done. Whether that is a hazard or a risk, it does change, based on assumed proximity of covers.

 

[Zogzog]

Its not the assumed proximity of the covers its the assumed working distance of the worker. There is a major difference between risk and hazard and "professionals" gets used pretty loosely these days.

I have witnessed many real arc flash events at the KEMA test lab and seen the aftermath of dozens of serious arc flash events and I have never seen a panel, door, or cover contain the arc.

 

[Electic]

NFPA table NFPA 70E 130.7(C)(9)(a) cites risk category '0' for operating a 240-600V breaker with the covers on; and risk category '1' for the same deal with covers off. Working distance would be the same (though likelyhood of disturbing the field might increase with covers off) yet the risk category increases.

The same table also has a heading "hazard/risk" implying an interchangeablity of the terms in this application.

I use the word professional as do most state licensing agencies.

 

[Zogzog]

Think whatever you want, I give up.

 

[dpc]

The distinction between hazard and risk is pretty well accepted among safety professionals.

But that's really beside the point. My point was that if a fault occurs, the energy released is the same whether the cover is on or not. Is it better for the cover to be on? Of course. Should you expect the cover to protect you? No.

Certainly there are overzealous people who get carried away when looking at arc-flash issues (and every other safety issue). But that doesn't make the hazard (or risk) any less either.

This is a done deal. The days of going into an MCC bucket armed with a Fluke DMM to check voltages wearing cutoffs and a pair of flip-flops is rapidly coming to an end (and yes, I have witnessed this). Even though the new reality is often a PITA, it should have come about a long time ago. Back when men were men, linemen wore leather hats instead of hardhats and electricians checked for a live 120 V circuit by slapping it with the back of their hand. I don't think anyone is advocating those practices any longer.

In the end, manufacturers will design new equipment that will provide better protection - but that process will take a long time.

Ten years from now, electricians will think back in disbelief at some the activities they used to do and what they were wearing when they did it. And I'll be retired....

 

[alehman]

And the U.S. will have completely eliminated itself from the world market because we invest so much protecting ourselves and providing parity for all that we can't compete with those who don't. What's worse? Tough to say.

Sorry I just had to tell a client that they have to do a major remodel of their entire building to comply with accessibility codes just because we did a lighting retrofit.

Safety and accessibility aren't bad. My beef is that the concepts are so often mis-applied due to laziness on the part of governments and fear legal reprisal on the part of users.

I'll get off of my soap box now.

 

[dpc]

I agree that some of this will increase the cost of doing business day-to-day.

The handicap access issue is a real can of worms, especially in an industrial setting, but not really a safety-related issue.

But I still think the arc-flash protection requirements are here to stay.

 

[Morquea]

but some of the assumptions made were a little off. {/quote]

True. But I would have the write a complete paper to show all the consistency.

The 70E (2004) IS confusing about when the arc flash boundary applies, that has been addressed in the ROP for the 2009 70E due out October 2008. After you read these, then see if you still disagree with my statement.

I will look forward to this.