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.

 

[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 do not know of any breakers on the market that operate faster than 3-cycle. Anybody else?

 

[jahrtrix]

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.

 

[majesus]

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

 

[unclebob]

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.

 

[majesus]

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.

 

[dpc]

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.

 

[Zogzog]

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

 

[majesus]

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

 

[dpc]

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.

 

[majesus]

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...

 

[majesus]

Yup it was... After digging:

http://www.schneider-electric.ca/

http://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.

 

[unclebob]

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.

 

[jraef]

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.

 

[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.

 

[majesus]

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.

 

[WDeanN]

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...

 

[Morquea]

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.

 

[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.

 

[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.