Circuit Breakers with different short circuit ratings 12

[uno]

I would like to know if anybody has used circuit breakers in a power distribution circuit where the upstream circuit breaker and the downstream circuit breaker ( maybe incomer to another distribution board) have different short circuit ratings. There is no fuse used anywhere in the circuit.

 

[davidbeach]

DanDel,

In your 4/29 post you stated "If the system was fully-rated, the 20A C/B would be designed to open and isolate the fault from the rest of the system. In a series-rated system, the whole 400A panel is de-energized."

It would take a fully rated system to in fact to that, but fully rated is not a sufficient condition. There can be many fully rated systems where the 20A and the 400A would race each other for fault currents in excess of 4000-8000A. In fact, to achieve a selectively coordinated combination of a 20A breaker and a 400A breaker you will most likely need to use an electronic trip on the 400A breaker that allows you to turn off the INST trip element of the 400A breaker; or somehow limit the fault current at the 20A breaker to less than the INST pickup of the 400A breaker.

I have run into a number of clients who think that fully rated sounds better than series rated and they insist on fully rated simply because in series rated the upstream device is required to trip to help the downstream device clear the fault. This is not a reason to decide between series rated and fully rated; there are other legitimate reasons to pick fully rated over series rated, and cases where series rated is not allowable (motor contribution between the upstream and downstream devices), but selectivity cannot be achieved simply by requiring all devices be fully rated.

 

[jbartos]

Comment: Alternately, protective device protection and coordination software may be applied. Software warnings can be issued if the protective devices are not properly selected and applied.

 

[EddyWirbelstrom]

IEC Standards permit the installation of a Moulded Case Circuit Breaker (MCCB) with an interrupt rating less than that of the prospective short-circuit current at its point of installation provided that :

The upstream MCCB (or fuse) / downstream MCCB combination is chosen from MCCB manufacturer's certified tables.

This ensures that in the event of a fault on the load side of the downstream MCCB, the upstream MCCB limits its let-thru current to below the interrupt rating of the downstream MCCB.
Note: Manufacturer's of current limiting MCCBs publish curves showing MCCB let-thru current versus prospective current. This reduced let-thru current can not be used to specify the required interrupt rating of a downstream MCCB.
This is because the tests are done with the MCCB load-side terminals shorted by a prescribed cable size/length.
When two MCCBs are in series, the let-thru current is different and can only be determined by tests.

'Cascading' is when both MCCBs in series trip for a fault on the loadside of the downstream MCCB. The upstream MCCB limits the let-thru current to below the interrupt rating of the downstream MCCB.
'Cascading' and discrimination is now possible.
See

http://www.schneider-electric.com/cahier_technique/en/home_en.htm

 

[EddyWirbelstrom]

I forgot to add another reference :
"Interplay of Energies in Circuit Breaker and Fuse Combinations" IEEE Transactions on Industry Applications, VOL 29, No. 3, May/June 1993.

 

[DanDel]

davidbeach, my post was simply an example of what is possible with a fully-rated system and impossible with a series-rated system.
I believe that the main disadvantage of a series-rated system is loss of selectivity, and so do many people who are responsible for industrial power systems. The loss of one small circuit is very often significantly more preferable economically to a larger outage that could have been prevented by the proper design of an electrical system, which, in my opinion, would start with fully-rated devices.
You may have other priorities and experiences, and that's fine; I understand what you are saying.

 

[davidbeach]

DanDel,

I agree 100% that a selectively coordinated system will be fully rated, and that a selectively coordinated system can be highly desirable. What I disagree with is the implication I read in your post (whether it existed in your mind or not) and have read many other places, is that since series rating means both devices trip, fully rated means that only the down stream trips. The implication seems to be that series rated vs. fully rated is a binary choice. There are really three choices: series rated, fully rated (but not selectively coordinated), and selectively coordinated (which is inherently fully rated).

Selectively coordinated is highly desirable in many instances, and can be a great deal of fun design, but can also be extremely expensive to implement for smaller circuits or for systems with several overcurrent devices between a significant transformer and the final load. To do selectively coordinated at low voltage with circuit breakers usually requires solid state trip units with the ability to turn the INST trip off or set it above the available fault current.

I think we agree more than we disagree, but could we agree that "fully rated" covers too much ground to be used without further qualification regarding selectivity?

 

[jbartos]

Comment: The selective protective device coordination is better accomplished by applications of protective devices that have I-t characteristics of the similar form. This usually means the protective device of one manufacturer and of the specific protective device category or line. However, the electronic circuit breakers with microprocessor solid state trips are more likely to be possible selectively coordinate.

 

[ScottyUK]

Hi jB,

Your statement is true up to a certain point, but most CBs have a high-set function for which the tripping time is not clearly defined. A heavy fault may well cause the high-set function of both the upstream and downstream CBs to operate, giving a 'race' condition where both CBs see the fault and react to it by initiating a trip. The high-set function is 'instantaneous' in so far as real world apparatus ever can be instantaneous, and this makes grading between devices impossible unless the downstream high-set level can be set to a level which allows the downstream CB to clear a fault while the upstream CB is on the I-t curve. Some of the larger and more expensive CBs allow the high-set to be turned off, which makes this possible.

I thought the US used standard curves - ANSI? - for the responses of protection relays. Does this apply to breakers as well? In IEC land, we find it easy to grade CBs from different manufacturers because they all conform to standard curves defined in BS7671 as Types B, C, & D up to a certain size. Larger breakers have their own means of 'bending' their response curve to suit the application, rather than a set of pre-defined curves. HV curves are defined in a standard whose number eludes me but are known as: EI - extremely inverse; VI - very inverse; SI - stanadrd inverse.



-----------------------------------

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Get it over with.

 

[alehman]

In the U.S., ANSI defines standard curves for protective relays (e.g. inverse, extremely inverse, definate, etc). Many electronic relays also have time-current curves that can be programmed to mimic typical low votlage breakers to improve grading. ANSI doesn't define curves for low voltage circuit breakers however. Each manufacturer sets their own. Electronic trip breakers allow a large amount of flexibility and have very similar settings among different manfacturers.

A full function electronic trip unit will have adjustable long time pickup, long time delay, short time pickup, short time delay, instanteous pickup and earth fault pickup and delay. Some curves can be definite time or I^2t. Various combinations of functions and ranges of settings are available. In the ANSI regions, elimination of instantaneous or hi-set function is allowed only on ANSI standard power circuit breakers (ACB's) installed in ANSI standard low voltage switchgear.

 

[davidbeach]

alehman,

Your statement "In the ANSI regions, elimination of instantaneous or hi-set function is allowed only on ANSI standard power circuit breakers (ACB's) installed in ANSI standard low voltage switchgear." was more definitive a few years ago than it is now. With going to ANSI type gear, you can use the new Square D (MG) Masterpact and Cutler-Hammer Magnum DS breakers as UL insulated case breakers with the same set of trip functions as are available when used as ANSI LVPCBs. The breakers have a fixed INST override function set at their short time rating, but if the breaker is selected such that the maximum available fault current is below the short time rating, the tripping will be the same for UL and ANSI.

 

[davidbeach]

My second sentance should start out "Without going..." Sorry for any confusion.

 

[alehman]

My understanding of the MasterPact is that there are two versions, one listed for UL 489 (MCCB), with inst. override and one listed under UL 1066 (LVPCB), without inst. override. UL 891 switchboards must use the UL 489 version and ANSI switchgear can use either. Do I have that confused?

See also thread238-78317

 

[jbartos]

Comment on ScottyUK (Electrical) May 4, 2004 marked ///\\Hi jB,
Your statement is true up to a certain point, but most CBs have a high-set function for which the tripping time is not clearly defined.
///Yes, below .o1 sec. Such devices can only be coordinated on enegetical basis needed for tripping, if electromagnetic and if electronic, then there is the short-time delay setting of upstream breaker serving as a backup of the downstream instantaneous setting of breaker or protecting cable between load side of upstream breaker to the line side of downstream breaker(s).\\ A heavy fault may well cause the high-set function of both the upstream and downstream CBs to operate, giving a 'race' condition where both CBs see the fault and react to it by initiating a trip.
///Yes, this is true for the circuit breakers that have no electronic trip unit, e.g. GE MicroVersaTrip, Square D Micrologic, Cutler-Hammer Digitrip, Siemens SensiTrip, etc. and those electromechanical breakers that have the same t*I*I peak through.\\ The high-set function is 'instantaneous' in so far as real world apparatus ever can be instantaneous, and this makes grading between devices impossible unless the downstream high-set level can be set to a level which allows the downstream CB to clear a fault while the upstream CB is on the I-t curve.
///Yes, this is still possible to see, engineer and design. However, the technology is there to avoid it. I always try to avoid it (even if it costs more money).\\ Some of the larger and more expensive CBs allow the high-set to be turned off, which makes this possible.
///Nowadays, even the small CB three phase circuit breaker from about 3A up, see GE site for breakers.\\I thought the US used standard curves - ANSI? - for the responses of protection relays.
///Yes, mostly for electromechanic and solid state. However, the integrated digital relays do seem to have various algorithms and logic for the more sophisticated protection.\\ Does this apply to breakers as well? In IEC land, we find it easy to grade CBs from different manufacturers because they all conform to standard curves defined in BS7671 as Types B, C, & D up to a certain size. Larger breakers have their own means of 'bending' their response curve to suit the application, rather than a set of pre-defined curves. HV curves are defined in a standard whose number eludes me but are known as: EI - extremely inverse; VI - very inverse; SI - stanadrd inverse.
///The electronic trip units marketed worldwide, e.g. Schneider Electric / Square D do use the long time setting, long time delay, short time setting, short time delay, ground fault, t*I*I, and instantaneous setting.\\\

 

[davidbeach]

alehman,

As far as I know, your understanding is correct. If one selects the UL 489 version such that the short time withstand rating (same as the INST override setting) is above the maximum fault current, the trip characteristics are the same as the UL 1066 (ANSI) versions. Without digging out the material, I believe that the UL 1066 versions also have INST overrides, but for that application the breaker AIC rating is the short time rating.