700.27

[charlie b]

Two part question.

700.27 (new to the 2005 edition) says that for emergency systems, all overcurrent devices must be selectively coordinated with supply side overcurrent devices. That is hard to achieve, especially with branch circuit breakers in the instantaneous region.

Situation:

In a 400 amp panel, there is a 225 amp breaker serving a feeder to a 225 amp MLO panel. In the 225 amp MLO panel, there is a 20 amp breaker serving a branch circuit for emergency lights. I can get the two breakers in the upstream panel (i.e., the 400 amp main breaker and the 225 amp feeder breaker) to coordinate at all fault levels. I can also get the 20 amp branch circuit breaker to coordinate with the 225 amp feeder breaker, but not in the instantaneous region. There is, in fact, enough fault current available to risk having the fault cleared by the upstream 225 amp feeder breaker. The AHJ is adamant about enforcing the coordination rule throughout the range of available fault current.

Proposed Solution:

Install a fuse immediately downstream of the 20 amp branch circuit breaker. Use an in-line fuse (attach by wire nuts to the branch circuit conductors), not a fuse block (which would have had to be attached to the panel. I can easily achieve coordination. I would declare that the branch circuit?s overcurrent device is the fuse, not the breaker, and that the breaker is merely there to isolate the circuit for maintenance.

Questions:

1. Would you accept this proposal as meeting the requirements of 700.27?

2. Is it acceptable to use this type of fuse connection method internal to a panel?

p.s. If you like this solution, then it was my idea. If you don't like it, then it was my boss' idea, and I just told him I would post it for him. :grin:

 

[ron]

It is good to be innovative, but here would be my concern ....

What is the relationship between the fuse and the 20A breaker. During a fault, will the fuse not open as fast as you think, because the 20A breaker contacts have begun to part? If that is possible, then there may be enough fault current left over, while the fuse is interacting with the dynamic impedance, that the upstream 225A CB may begin to open.

Although on a TCC, the fuse will look selective with the upstream breakers (225 and 20A) since you don't get to see the time on the curve <0.01 seconds.

I don't know, your in a tough position. I think the requirement is silly to get enforced in the instantaneous region as the reality of actually getting the calculated amount of fault current is so small, considering the impedances that are present (breaker contact impedance, wire termination impedance, connection and bus impedance, etc) which are not considered when using the standard ANSI method of calculation.

Can you work on reducing the calculated fault current below the range that the non-selective area occurs. This would be done by ensuring that you are calculating/considering all impedances that will be involved. The drawing of the TCC's end at the maximum fault current calculated, so if the calculation is lower, then you may not see the non-selective nature of the 20A and 225A breakers.

BTW, one thing I tried a few years back, was consider that a fault in the panelboard would effect the 225A main only. A fault that would involve the 20A branch breaker would probably involve a fault someplace downstream of the actual branch circuit breaker. During UL testing, they test the aic ratings at the end of a 5' piece of wire rated for that size breaker. Considering the impedance of 5' of #12 wire would bring the fault current down significantly.

 

[benaround]

Charlie,

Are all the panels supplied by the EM Generator? 700.18

IMO, if the 20a circuit is for battery back-up " bug eyes " then 700.27 is n/a.

edit: 700.18 is for power not lighting, sorry next time I'll read 1st. How many times

have I said that!!

 

[don_resqcapt19]

Charlie,
Bussman submitted the proposal that resulted in that rule and they have a solution.
Don

 

[ron]

That is very convenient that Bussman has come up with that panel. :roll:
I wonder if those branch switches are molded case switches with inherent instantaneous protection?

 

[don_resqcapt19]

Ron,
They are standard circuit breakers as far as I know. When I saw it a a trade show they said that the circuit breaker provides the overload protection and the fuse provides the short circuit and ground fault protection. They even made mention of the OSHA rule that says you cannot reset a breaker without an investigation of the circuit unless you know that the breaker has tripped from an overload. They said if the fuse has blown you had a ground fault or short circuit and if the breaker is tripped you had an overload.
Don

 

[tom baker]

Charlie- there is a wac rule that only requires that 700.27 for new installations, I believe after July 1, 2006 (thats a washington rule). But you probably know that.

 

[bphgravity]

I am struggling with the same issues right now and wish I could provide you a solid answer either way.

Here is a comment a posted on a recent topic, perhaps it will provide some help with our situation:

At this particular time, we are not requiring coordination in the instantaneous regions for three primary reasons:

1. Bolted fault currents at those levels are highly unlikely.

2. AHCA only requires coordination in the short time and long time regions above 100ms.

3. Section A.6.5.1 of the NFPA 110 states, "It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination is not practicable without using equipment that could be prohibitively costly or undesirable for other reasons. Primary consideration also should be given to prevent overloading of equipment by limiting the possibilities of large inrushes due to instantaneous reestablishment of connections to heavy loads."

I check coordination to the 100ms level, anything beyond that is at the discretion of the design professional (engineer) whom is ultimately responsible for proper system operation.

 

[davidr43229]

Ron

That is very convenient that Bussman has come up with that panel.

http://www.bussmann.com/library/bifs/3115.PDF
Before the entry of these code articles selective coordination was not required or sometimes, even considered.
The new panel 20 (COP) Critical Operations & Power Systems, made up of firemen & poilcemenwhich expressed a primary concern in a 911 Senario which includes Hospitals, Mass Transit, Large data collection centers and some gas stations. COP has endorsed the use of Selctive Coordination and believe it to be an important issue in another 911 incident.
A product ?, sure....
An Idea? even better...
An acceptance? pricless....
Just my $.02

 

[ron]

David, I wish I considered it that clear.
In order to achieve selective coordination, you almost always have to use fuses in some way.
By the nature of Article 700, the loads are important. The code does not mandate redundant important loads or redundant circuits for those single loads.
The first time the fuses blow on those very important loads and there are no spares around, that very important load will likely be useless.
Wouldn't it be nicer if that important load was served by a breaker, that may not be selective in the instantaneous region, but since that type of fault doesn't happen all that often, that when it tripped it could be quickly reset and the important load could be restarted?

I think the selective coordination requirement, especially when required in the instantaneous region is counter intuitive to what the intent was.

I do coordination studies for critical facilities, and I often have to make compromises between arc flash reduction, selectivity and the ability for the operation to continue. It is not like a circuit feeding a kids desk in a school, it is more important and requires some common sense (performance based design) rather than just prescriptive methods that lock out the ability to do it right.
Just my 0.02 dollars.

 

[don_resqcapt19]

David,
I think that there will be very serious legal problems with the implementation of new Article 585(Critical Operations Power Systems) to non public buildings and facilities. For example if it don't apply it to all gas stations, it would be an illegal "taking" against the ones that you do apply it to. I expect that only critical public facilities will use this new article and then only when required by some other code. The NEC does not ever require the use of this new article, but only tells us how to make the installation when some other code requires its use.

Don

 

[don_resqcapt19]

Ron,

Wouldn't it be nicer if that important load was served by a breaker, that may not be selective in the instantaneous region, but since that type of fault doesn't happen all that often, that when it tripped it could be quickly reset and the important load could be restarted?

You are not permitted to reset a breaker without investigating the cause of the trip unless you know the trip was caused by an overload. Look at the picture from Brian in post #2 of this thread.
Don

 

[davidr43229]

Wouldn't it be nicer if that important load was served by a breaker, that may not be selective in the instantaneous region, but since that type of fault doesn't happen all that often

I would submit that in the instantaneous region is where the fault (shortcircuit) is sensed, thus causing the breaker to trip during that event. Can we dismiss this event, I would hardly think so.
The convenience of the relatching of a breaker as defined by UL489 was to reset in an overload (functionally), not in a fault, which maintainence and possible replacement exists, since the breaker is only rated for 1 trip in this type of event. Why would you want to reset that breaker, while ignoring the integrity of that device?
All the specs I have viewed or helped design call out for spares to address your hypathatical event, so the integrity of the design remains intact.
Just My $.02

 

[iwire]

Ron,

You are not permitted to reset a breaker without investigating the cause of the trip unless you know the trip was caused by an overload. Look at the picture from Brian in post #2 of this thread.
Don

Don, I don't disagree that is an OSHA issue but lets look at the real world.

People who own buildings and those that work in them do reset breakers every day.

The instructions on how to reset the breaker are often on the cover of the panel.

And in a case such as Ron was describing (an emergency) I think OSHA rules will quickly be forgotten.

 

[davidr43229]

Section A.6.5.1 of the NFPA 110 states, "It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination is not practicable without using equipment that could be prohibitively costly or undesirable for other reasons. Primary consideration also should be given to prevent overloading of equipment by limiting the possibilities of large inrushes due to instantaneous reestablishment of connections to heavy loads."

I check coordination to the 100ms level, anything beyond that is at the discretion of the design professional (engineer) whom is ultimately responsible for proper system operation.

Again, from a previous post:
The NFPA 110?s scope does not encompass the full emergency system. It encompasses from the generators to the load side terminals of the transfer switches. The NEC covers the entire system. so IMO the NFPA 70 (NEC) trumps NFPA 110 and should be checked by the Inspectors.
Just My $.02

 

[davidr43229]

I think that there will be very serious legal problems with the implementation of new Article 585(Critical Operations Power Systems) to non public buildings and facilities. For example if it don't apply it to all gas stations, it would be an illegal "taking" against the ones that you do apply it to. I expect that only critical public facilities will use this new article and then only when required by some other code.

Don,
I am in full agreement with you on this. How, when & who, do you differenciate. I will try to find out the details & post.
Just my $.02

 

[don_resqcapt19]

Bob,

People who own buildings and those that work in them do reset breakers every day.

And sometimes you get the result show in Brian's picture.
Don

 

[George Stolz]

I've wondered how often selective coordination is examined on the job I'm on.

Awhile back, a shorted elevator motor took down the emergency main, and when the ATS kicked over, it took that down too. Not very comforting for a hospital. :roll:

I guess a week or so ago, someone kicked a 20A breaker on a short or a ground fault, and both the branch circuit OCPD and the main utility breaker tripped, so the backup's OCPD didn't have to try to outlast the branch circuit's OCPD when it came on that time.

Anyway, events like that make you wonder.

 

[iwire]

Bob,

And sometimes you get the result show in Brian's picture.
Don

Don I believe we where taking about branch circuits, Brains picture was a 400 amp feeder.

I don't find many people messing with 400 amp breakers and I doubt many 20 amp branch circuit breakers have blown apart like that.

 

[iwire]

Awhile back, a shorted elevator motor took down the emergency main, and when the ATS kicked over, it took that down too. Not very comforting for a hospital. :roll:

George for us it is not unusual for us to wait till the end of the job to adjust the breaker settings per the engineers provided info.