Series Ratings and NEC 2008 240.86A 6

[HenryOhm]

Following up thread238-177655, in the 2008 NEC, this sentence has been added to 240.86(A) regarding series rated OCPD's:

"For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current limiting device."

Seems overly conservative. If the upstream device, ie high speed fuse, caps current at a level below the kAIC of the downstream device, what does it matter if the downstream device then begins to open during the "downhill" portion of fault current extinguishment? How could the downstream OCPD's opening only diminish the fault level further? (My understanding of a high-speed fuse is it finishes it pre-arcing phase very quickly but that the time to full arc extinguishment is relatively much longer.)

A white paper by GE engineers (see

http://www.geindustrial.com/publibrary/checkout/White Papers|OCPD1|generic

) stated:

"In practical terms, this will require low-voltage power circuit breakers with 30-cycle withstand ratings on the load side of a current-limiting fuse. Many power systems will not meet these prerequisites. It is doubtful that the engineered series combination solution will become very popular."

Curious, does this recent inclusion in NEC 2008 represent a guideline by which most system coordination PE's would have held anyway? Or is it really more of a victory of, say, circuit breaker manufacturers over fuse manufacturers in the lobbying for NEC code changes?

Thanks much for any help.

 

[davidbeach]

It would appear to be a return of a certain amount of common sense to this section of the code. Frankly, this provision was a serious mistake when originally added.

A circuit breaker will either remain passive for well more than the first half cycle - rare any more; or the breaker will - the magnetic portion of the thermal magnetic trip element - begin to move the contacts at the point on wave that the current exceeds a value characteristic to that particular breaker. There are breakers that are known to partially open - arc elongation but not arc extension - and reclose during the downstream interruption of high current faults.

The circuit breaker manufacturers can not determine series ratings by calculation, they must be determined by test. Known Fact. Why then should someone be able, per the code panel, to calculate a series rating for a breaker that they don't even have any of the design data for? Anybody that knows enough to some chance of making the calculations allowed by the code also knows enough to stay far, far away.

 

[dpc]

Agree with David. The 2005 version was seriously flawed, and I don't know of any reputable engineers in the US who were willing to take the liability.

This is as much about risk management and liability as it is about physics. It doesn't make sense for a consultant working on a $5,000 fee with $500 of profit to assume an almost infinite downside liability.

I'd stick with the tested combination approach.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[rbulsara]

Agree with both dpc and davidbeach above. Not worth assuming the liability.

 

[alehman]

An emphatic 3rd!

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[HenryOhm]

All,

Thanks so much for the quick replies. I did look into the IEEE color book set enough to see where both IEEE 242-2001 and 1015-2006 essentially require UL type testing.

Thanks again!!

 

[RonShap]

... as the originator of the first post, I also only rely on tested assemblies.

 

[HenryOhm]

Sorry. The so-far unanimous consensus of the very knowledgeable responders is clear. But at risk of being a pain...

Should have mentioned I'm focused on high-speed fuse solutions for underrated power circuit breakers. Took a trip to the local university engineering library to learn more about testing requirements (ie UL 486). Came across ANSI/IEEE C37.27-1987. The date is over 20 years old but IEEE still considers it an active standard.

It requires a subtraction of 16ms from the downstream CB's stated clearing time and an addition of 4ms to the upstream fuse, but otherwise simply requires the adjusted fuse clearing time to be less than that for the adjusted "minimum" clearing time of the downstream circuit breaker.

Does a power circuit breaker vs MCCB change this equation for anyone or is C37.27 just collecting cobwebs?

Thanks again for the help!

 

[alehman]

Does C37.27 address general purpose fuses or just circuit breaker limiters? Even if you can justify application of that document, I would still want the c/b manufacturer to agree (in writing).

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[dpc]

Not sure what the scope of the standard is. MCCB behave differently under fault conditions than power circuit breakers because they have no withstand or through-fault rating (well, they have one, but it is very low), so they have to trip on a high current fault. As David Beach noted, a modern molded case breaker's contacts will part before the breaker mechanism even trips. A PCB will not do this.

The fuse-breaker problem is cause by the dynamic arc impedance within the breaker after the contacts part. Also, the breaker itself presents a fair amount of impedance to the fault current, even though we generally neglect it in calculations.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[rbulsara]

henryohm:

What part of your focus is different than widely available fused power circuit breakers, tested and listed by mfrs. already?

Are you interested in convincing professional engineers to recommend high speed fuses for series rated use with under rated PCB's without manufacturer's consent? You already got the answer to that.

 

[stevenal]

Designation: C37.27-2008

Sponsor: IEEE Power & Energy Society/Switchgear

Title: IEEE Application Guide for Low-Voltage AC Power Circuit Breakers Applied With Separately Mounted Current-Limiting Fuses

Status: Approved Publication of IEEE , Published Date: Mar 31, 2009 **Supersedes C37.27-1987 (R1998).

 

[HenryOhm]

Thanks so much, stevenal. The main search tool of IEEE Explore won't reveal this but it can be found here:

http://ieeexplore.ieee.org/xpls/abs_all.jsp?tp=&isnumber=4808344&arnumber=4808345&punumber=4808248

The standard clearly states it is not valid for MCCB's. It also makes widespread reference to current-limiting fuses and indicates that the start of the current limiting portion of that fuse must be lower than the PCB's short-circuit current rating. But, it also says, "In the absence of current limiting fuses, the peak current available is 2.3 times the system available short-circuit current..." seemingly allowing for a non-current limiting fuse to be used provided it meets this 2.3X requirement.

It also details a certain margin it wants to see between the fuse and PCB in the non-instantaneous portion of the PCB, ie ensuring the PCB will trip ahead of the fuse in these lower ranges within the PCB's rating.

Many cases of underrated PCB's may still not benefit, given the 16ms and 4ms adjustments. But, it does take a different approach than requiring the PCB to "remain passive."

Thanks again for all the input!