2005 Section 700.27 & 701.18

[ryan_618]

From the 2005:

700.27 Coordination. Emergancy system(s)overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

701.18 Coordination. Legally required standby system(s)overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

Definition from Art. 100

Coordination (selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment effected, accomplished by the choice of overcurrent protective devices and their ratings or settings/

Engineers/designers: How will you address this?

Inspectors/plans examiners: How will you ensure this is complied with?

The only thing I can think of is to do a fault current study on the design then study inverse time circuit breaker curves to make sure the branch circuit OCPD opens before the feeder or service OCPD opens. Or, I geuss you might be able to use current limitting breakers on the branch circuits??? What do you think?

 

[charlie b]

Re: 2005 Section 700.27 & 701.18

I don?t have the 2005 yet (none of my clients use it, so I?m in no hurry). But these new sections will have a definite impact. Selective coordination is no easy task. It?s not easy to design, nor is it easy to maintain.

From the way I read these two new articles, the 2005 is requiring coordination at all levels of the emergency system. The branch circuit breaker (on a sub-panel) must open before the main breaker (on that same sub-panel), which must open before the feeder breaker on the upstream panel, which must open before the main breaker on the upstream panel, and so it goes all the way to the main service breaker. The rules apply at each and every point along the emergency distribution system. It is hard enough to select and set any two breakers such that they selectively coordinate. But to do three or more becomes a real challenge.

This will also remove much of the flexibility that the installer and the maintenance tech once had, with regard to buying breakers for the initial installation and for future replacements. If I, the engineer, create a design that coordinates at all levels, then the installer and future maintenance techs must not be permitted to offer any substitutions. If you put in a different type of breaker, then the coordination study becomes invalid. I am curious as to how the owners will establish controls to ensure no substitutions are made throughout the life of the facility.

One final word: If you add a ?large? motor to an existing facility, you have to perform the coordination study all over again. How big does the new motor have to be, before the amount of current it contributes to the ?available fault current? would invalidate the results of the coordination study? That depends on many factors, and there is no simple answer.

 

[ron]

Re: 2005 Section 700.27 & 701.18

I'll start by asking how you verified selective coordination as part of the 2002 code for elevators.

From NEC 2002
620.62 Selective Coordination.
Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.

It is surely possible for the AHJ to request to see the short circuit study and copies of the time current coordination studies performed. Usually coordination studies are completed after construction is underway, so that the manufacturer and model of the OCPD's are known.

As a side note, from the 2005 definition, would you agree that an overcurrent condition does not include a fault condition? That's my interpretation, for very important reasons.

 

[charlie b]

Re: 2005 Section 700.27 & 701.18

Originally posted by ron:As a side note, from the 2005 definition, would you agree that an overcurrent condition does not include a fault condition?

In my view, an ?overcurrent condition? would cause actuation of a breaker?s long time (i.e., thermal) device, and a ?fault current? would cause actuation of a breaker?s instantaneous (i.e., magnetic) device. So yes, they are different.

But is that relevant to the new rules? You cannot reliably coordinate two molded case circuit breakers in their long time range. There is too much overlap in their time-current curves. Put a 100 amp main on a panel and a 40 amp branch circuit breaker, then impose an overcurrent condition of 150 amps on the branch circuit, and either breaker could go first. So I have to believe the new rules are addressing fault conditions.

Does that interpretation support, or go against, your very important reasons?

 

[ron]

Re: 2005 Section 700.27 & 701.18

I was being dramatic with the "very important reasons".
In a nutshell, it is practically impossible to selectively coordinate OCPD's in the instantaneous region (magnetic), or less than 0.1 seconds.

I believe (or hope) that the intent of the code would be selectivity in the long and short time regions (overcurrent condition) and not fault conditions which I consider in the instantaneous region, of course depending on the amount of fault current available.
When selectivity is important, I specify LSI adjustable circuit breakers down to the main breaker of every panel. A fixed characteristic branch breaker can generally be made to be selective with all of the upstream LSI adjustable devices with some planning (except for instantaneous region). I try to keep the ratio of main to branch at least 2 to 1, and I can almost aways get selectivity for the entire distribution.
In data centers, we strive for selectivity of all distribution, so when possible be specify LS only breakers when we are in switchgear (30 cycle rated), to help avoid a selectivity problem with the next set of distribution OCPD's.

Charlie, I agree with you that substitutions and future maintenance activities sometimes put to squash on a good plan laid out from design.

[ September 29, 2004, 09:34 PM: Message edited by: ron ]

 

[rbalex]

Re: 2005 Section 700.27 & 701.18

Unfortunately for those of us who will have to design for this,"overcurrent" is also a defined term and it includes short circuits and ground faults (SC/GF). For LV systems, selective coordination with standard molded case circuit breakers (MCCB)is virtually impossible at SC/GF levels. There is some chance (slim) with electronic sensored insulated case and air circuit breakers with definite time delays. It will all depend on available fault currents throughout the system, complexity of the network and the "forced magnetic interrupting" levels of the breakers.

There is a fair chance with some classes of fuses and if the fault duties are high enough you may as well use them because the MCCB's are likely to be good for only one "shot" anyway.

Overcurrent. Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

FPN: A current in excess of rating may be accommodated by certain equipment and conductors for a given set of conditions. Therefore the rules for overcurrent protection are specific for particular situations.

 

[ron]

Re: 2005 Section 700.27 & 701.18

Bob,

if the fault duties are high enough you may as well use them because the MCCB's are likely to be good for only one "shot" anyway.

Why do you feel this way? I'm not aware of any UL testing or maintenance standard that state that a fault requires replacement. I've seen many faults cleared by an upstream breaker, and the upstream breaker survived nicely.

 

[ryan_618]

Re: 2005 Section 700.27 & 701.18

This is a sticky subject indeed. The reason I brought it up is that, compared to elevators, emergancy systems (legally required) are very, very common, as you all know. Honestly, I don't remember the last even small commercial job that didn't require some level of emergancy lighting.

I really don't know how to approach it from a plan review/inspection point of view. It will be interesting.

 

[rbalex]

Re: 2005 Section 700.27 & 701.18

Originally posted by ron:
Bob,
Why do you feel this way? I'm not aware of any UL testing or maintenance standard that state that a fault requires replacement. I've seen many faults cleared by an upstream breaker, and the upstream breaker survived nicely.

Ron, my opinion is derived from IEEE Std 1015-1997(Blue Book-Molded Case applications)

3.35 Circuit breaker useful life
It is prudent to replace any MCCB that has interrupted, at most, two faults at rated maximum current. This is so because the MCCB short-circuit proof test consists of an ?O-t-CO? sequence, which means that in proof testing of the circuit-breaker design and in periodic follow-up testing thereafter, the circuit breaker is required to open a fault from an initially closed position (corresponding to the ?O? operation), then after a period of time (t) to reset is allowed, to be closed into a maximum fault and trip open for a second time (corresponding to the ?CO? operation). This demonstrates a circuit breaker?s ability to perform at least two maximum level fault interruptions with the second at least a little worse than the first. No maintenance of the circuit breaker on test is permitted between interruptions.

The "No maintenance of the circuit breaker on test is permitted between interruptions" is significant because if you go to NEMA 4, Molded Case Circuit Breaker Maintenance, it hints all around but never actually admits you can't maintain an MCCB after its cleared a fault.

 

[charlie]

Re: 2005 Section 700.27 & 701.18

We have argued this issue in Panel 10 and have come to similar conclusions about the coordination. In my opinion, you can achieve coordination with larger circuit breakers where they can be adjusted to hold in for a few cycles of fault current while giving the smaller circuit breaker time to operate. The other way is to use fuses and work the TCC curves. Large industrials have some additional options. This sure leaves series ratings out in the cold, doesn't it?

 

[ron]

Re: 2005 Section 700.27 & 701.18

Thanks for the extra IEEE info. I think it is relatively clear that a molded case breaker can't be maintained.
The IEEE quote indicates that it can clear two maximum level faults. It is unusual to have a fault that approaches the maximum level fault, as dynamic impedances and terminations in the circuit are IMHO, resulting in fault currents much less than what we calculate in the ideal three phase bolted fault scenario world. Of course we could never measure what an actual fault current is, unless we coincidentally happen to have a disturbance meter on the circuit.
Great discussion. Thanks

 

[rbalex]

Re: 2005 Section 700.27 & 701.18

Ron,

It?s my personal opinion that current versions of both IEEE 1015 and NEMA AB-4 have been "sanitized.? Previous versions were less vague about the recommendation of when to replace an MCCB after a fault. BTW The NEMA MCCB standards are available here as free downloads.

You are correct, of course, that it would be unusual for faults to reach maximum rating. The problem is you don?t know. So you are left with MCCBs of indeterminate capability. And, if you go through the NEMA recommended inspection procedure after an MCCB has interrupted a fault, the last thing you do is perform a ?maximum level fault interruption with the second at least a little worse than the first; i.e., the "second fault" after which," it is prudent to replace any MCCB ..."
[Edit Add:]
I particularly like the final step:

6.6.6 Reinstall Breaker
If applicable , reinstall the circuit breaker per manufacturers instructions.

You may very well find it isn't "applicable." [Note the instuction's number ]
[Edit End]

When the MCCB is an instantaneous only type, the situation is even worse. A few trips on inrush while trying to determine the proper setting may be enough to damage it. That?s one of reasons I insisted on the last two sentences of 430.52(C)(3) Ex 1 when I was on CMP11.

...Trip settings above 800 percent for other than Design E motors or Design B energy efficient motors and above 1100 percent for Design E motors or Design B energy efficient motors shall be permitted where the need has been demonstrated by engineering evaluation. In such cases, it shall not be necessary to first apply an instantaneous-trip circuit breaker at 800 percent or 1100 percent.

[ September 30, 2004, 12:43 PM: Message edited by: rbalex ]

 

[ryan_618]

Re: 2005 Section 700.27 & 701.18

Originally posted by charlie:
This sure leaves series ratings out in the cold, doesn't it?

Wow. It just hit me like a ton of bricks what you are saying here Charlie. A series rated system must have the upstream OCPD device open when there is a fault down stream of it. Since this is true, series rated systems cannot be selectivley coordinated, and therefore no emergancy loads could be installed on a series rated system.

That is going to make for a complete re-design of some of the larger buildings with high amounts of fault current. I just got done doing a plan review of a large building, and under the 2005 the design would have to go back to the drawing board.

 

[charlie]

Re: 2005 Section 700.27 & 701.18

That is exactly what I meant.

 

[ryan_618]

Re: 2005 Section 700.27 & 701.18

Originally posted by charlie:
That is exactly what I meant.

This is a bigger change that I originally thought

 

[ryan_618]

Re: 2005 Section 700.27 & 701.18

Does this requirement for selective coordination include branch circuit breakers for unit equipment? I'm trying to determine if unit equipment on a general lighting branch circuit is really considered a component of an "emergancy system" or of a "legally required standby system", or if perhaps it is just a plain old branch circuit with some unit equipment installed on it. What do you guys think?