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

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

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

(OP)
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.

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

majesus, I am not familiar with this particular breaker, but typically you have 1-cycle for the trip device to operate and a 3-cycle breaker for a total of 4-cycles. I do not know of any breakers on the market that operate faster than 3-cycle. Anybody else?

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

majesus, Have you considered fusing it rather than trying
to chase down a breaker? Fuses are much faster and of
course keep the I squared T to less than half cycle. This
may get your catagory classification smaller.

Selective coordination may be a reason for the breakers to be slower at opening . Breakers usually don't have the short
circuit interrupting high thresholds like fuses do. That is
usually why breakers are slower so as to let downstream fuses to open the fault before the main breaker trips. That way the entire service isn't dropped out. Hope this helps.
 

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

(OP)
You are right! But I am just exploring so that is why I asked the question on the CB.

I am looking at fuses too... KTU fuses Class L made by Bussmann are extremely fast. The problem is in all the cases (different sub schenarios), I can't get the Category down with out bringing a fuse's currenting rating below the continous rating of the XFMR. For example:

3 XFMR continous rating = 2887A, 3 phase, bolted symetrical 1/2 cycle Fault Current at Secondary size of XFMR = 50kA

Secondary KTU Fuse at 3000A, Arc Flash Exceed Cat 4
Secondary KTU Fuse at 2400A Arc Flash Exceed Cat 4
Secondary KTU Fuse at 2400A Arc Flash Exceed Cat 4
Secondary KTU Fuse at 2000A Arc Flash Cat 2


I'm now looking at Chapter 27  of the C-H Catalog 2006 about current limiting Power CB. I'll play and see what happens :)

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


I've done a calculation with SKM.

Utility Contribution : 55.000 amps  X/R = 6.6
(I used 55 kA to obtain 33 kA Arcing Fault at the breaker)

Transformer with very low impedence
(to input the system voltage : 600 V)

Breaker with 520 LSI trip unit.
(Instantaneous set below 35 kA)

Downstream of the breaker, I have 5.4 cal/cm2 which gives
a category 2, using Switchgear as Equipment Type. If I use
Panelbord, then I get 9.2 cal/cm2 => cat 3.

With the type of breaker you have (DS-632 or MDS-632),
you should choose Switchgear as Equipment Type.


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

(OP)

Quote (Eleceng01 ):

majesus, I am not familiar with this particular breaker, but typically you have 1-cycle for the trip device to operate and a 3-cycle breaker for a total of 4-cycles.

I understand what you mean... and it makes sense, but the Federal Pioneer 50H-2 with the old USD-6 Solid State Plug (3000A) has an opening time of approx 1 cycle.

LINK of TCC Curve


Arc Flash using this CB is now a Cat 1.

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

With your 0.065 clearing time and 32kA of arcing current, I get about 7.5 cal/cm2 at 18 inch working distance, with no motor contribution. (using IEEE 1584 equations)

You might find a molded case breaker is slightly faster due to the difference in tripping mechanism.  

But either way, you are sacrificing any notion of coordination with the downstream feeder breakers.  

I agree that a 3000 A fuse isn't going to help much.

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

If this is a main breaker then having an inst trip may cause coordination issues. Utillity relay has a solution, Quick Trip.

http://www.utilityrelay.com/QUICK-TRIP_Page.html

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

(OP)
Thanks guys for your input.

I made a slight typo error with the arcing current. I'll post the following results since a few of you posted some simulation results as well (DPC, UncleBob)


Bolted fault current at the 600V PDC, 1/2 Sym: 50.1kA
Upstream XFMR 3MVA, Z=5.5, X/R=3.96
Main CB: RMS 250 with 3200 Sensor
L.T.P.:0.95
Band: 4
Inst. Pickup 4, 12.8kA



When PDC is rated as Switchgear (32mm Gap, 1.473 distance factor):
Arcing Current 32.21kA
7.41cal/cm2 @18"
Cat 2
Boundary 5.16ft
CB Clearing time: 0.065sec

When PDC is rated as MCC or Panelboard (25mm Gap, 1.641 distance factor):
Arcing Current 34.71kA
8.29cal/cm2 @18"
Cat 3
Boundary 4.87ft
CB Clearing time: 0.065sec


DPC, I am realizing that Thermal-Magnetic CB seems to be faster. Can I say that: In general, Thermal-magnetic is faster than Solid State?  Is it correct to rate CB speed wise from faster to slow: Thermal-Magnetic, Electro-Mechanical, Solid State? Or is that too General and not true?

That Federal Pioneer 50H-2 I was talking about earlier with the old USD-6 Solid State Plug (3000A) seems to be fast. It's an old product thou.

ZogZog, I am going through the QUICK-TRIP website now... Looks cool. As always, everyone I appriciate the input. Helps me learn :)

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

It is more a difference between molded case and power circuit breakers.  The molded case breakers have no (or almost no) withstand rating so they have to trip above a certain current.  Newer molded case breakers are designed with "blow apart" contacts designed such that the magnetic forces caused by the high current overcome the spring forces and the contacts open before the tripping mechanism even operates, speeding up the clearing time, and the higher the current the faster the clearing time.  You will see this is you look at the updated TCC curves that C-H has done for their Series C breakers.

A power circuit breaker has a 30 cycle withstand rating, so its contact must stay closed until the trip solenoid releases the spring.  Plus the power breaker has larger contacts and greater mass in the mechanism so they probably take longer to mechanically open.  

But I'm not sure how true this is in these very large 3000 A and 4000 A molded case breakers.  Certainly applies up to about 1200 A frame.  You'll need to look at some curves to be sure.  

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

(OP)
Roger that... Thanks.
The USD curve I showed earlier, I don't think it is right. It's too fast. I bet that is just for the Tripper... Etap is not taking account the Breaker opening. I'm looking into this...

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

(OP)
Yup it was... After digging:

http://www.schneider-electric.ca/www/en/products/switchgear/USR_RMS_Brochure.pdf

On pg 3:
Instantaneous Operating Time

When the circuit breaker is closed on a phase to gnd, phase fully offset fault at the operating capacity of the circuit breaker, the max operating time of the relay is 16mS. The operating time represents the time taken by the realy to energize itself, sense the fault, activate the trip energy accumulation circuit and transmit this energy to the terminals of the trip solenoid.


Sometimes, computers simulation programs like ETAP are great, others time, they give you false info... I'm glad my experience is building. I knew it just felt too fast when I was comparing them with other CB.

Typical Power CB opening time is about  x5 Cycles, 1 to two cycles... something seems off.

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

That Quick-Trip relay is almost the same thing as the ARMS module that can be installed on the Digitrip 520M. ARMS stand for ArcFlash Reduction Maintenance Switch.

In maintenance mode, the instantaneous setting can be as low as 2x, thus making the operating time faster.

However, in this particular case, since the arcing fault is pretty high, having the Instantaneous at minimum don't speed up the operating process.

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

majesus,
You might want to look at the new WL power circuit breaker from Siemens. It has a unique (as far as I know) optional feature called Dynamic Arc Flash Sentry, which allows a user to select two different sets of trip settings; one for normal operation and another for when Arc Flash potential needs to be lowered.

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

I may be missing something, but isn't the breaker in this case already operating in the instaneous mode at the available arcing current?

I don't know if shaving a couple of cycles off is going to get you much more.  You've got a high available fault current.  Add some wire or a reactor.  Just be sure you stay within the instanteous portion of trip curve at arcing current.  And classify it as switchgear.  That seems more likely.  In that case, you're already at Cat 2.

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

(OP)

Quote (WDeanN):

I may be missing something, but isn't the breaker in this case already operating in the instaneous mode at the available arcing current?

I don't know if shaving a couple of cycles off is going to get you much more.  You've got a high available fault current.  Add some wire or a reactor.  Just be sure you stay within the instanteous portion of trip curve at arcing current.  And classify it as switchgear.  That seems more likely.  In that case, you're already at Cat 2.

Exactly... you can always add more impedance to the system, hence change the fault characteristics. Just in this context, we are talking about CBs as I was exploring the cycle times of POWER CBs and its affects on Incident Energy.

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

Majesus -
That was one of the things I prefer about ETAP.  They keep circuit breaker data separate from trip device data in their libraries.  It's been a couple of years since I used it, but you can chose the breaker, then select your trip device, with ETAP only providing trip devices that fit that breaker, or was it the other way around?

In any case, you should be able to search the library and match the trip device with breakers that it will fit.  That may narrow down the selection process.  Otherwise, it's off to the vendors to find the fastest breaker...

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

Majesus -

Since your breaker already show the fastest clearing time by standards, I won't even bother trying to find a faster CB. I would instead put time on Working Procedures to lower the Hazard category by keeping the worker far enough. Or eliminating the "exposed to live parts" condition.

I don't think you can do more about fault clearing time in your case.

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

"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.

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

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.

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

(OP)

Quote (WDeanN ):

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

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

Zogzog

Quote:


"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.



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

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.

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

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.

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

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!

 

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

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.

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

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

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

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.  

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

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.

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

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.


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

Think whatever you want, I give up.

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


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....


 

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

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.

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

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.  

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

[quote]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. :D

Quote:


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.





  

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