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Circuit Breakers with different short circuit ratings
12

Circuit Breakers with different short circuit ratings

Circuit Breakers with different short circuit ratings

(OP)
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.

RE: Circuit Breakers with different short circuit ratings

(OP)
Or have you seen a power distribution circuit where the upstream circuit breaker is from a MCC/PMCC and feeds another distribution board which has outgoing breakers having lower short circuit ratings. There is again no fuse used anywhere in the circuit.

RE: Circuit Breakers with different short circuit ratings

I'm confused; of course two different C/Bs can have two different interrupting ratings. And typically, you would have lower-rated C/Bs downstream and higher-rated C/Bs upostream. Can you clarify your question?

RE: Circuit Breakers with different short circuit ratings

Are you talking about a series rated system, in contrast to a fully rated one?

Mike

RE: Circuit Breakers with different short circuit ratings

How far upstream are you talking?  Is there any equipment in between?  Upstream breakers have less impedance between them and the source, therefore will see (and must be rated for)higher SC currents.

If there is no impedance in between the two breakers,the downstream device can be series rated as per manufacturer specs and testing. This has been discussed in this forum previously...maybe try keywords "series rated"??.




 

RE: Circuit Breakers with different short circuit ratings

(OP)
Thanks Pals

Should have used keyword search. It is available in :
http://www.eng-tips.com/viewthread.cfm?SQID=21916&SPID=238&newpid=238&page=3

Now I want to know if anybody has used series rating/ Cascading applications with selective discrimination in any of your industry. I have read some literature but would like to know if it has been carried out in real life situations especially in three phase power distribution schemes. I know it is being done for lighting .

Dandel
I would also like to know as to how you think it is possible. Have you seen it anywhere. I do not want to take the cable impedence or the presence of a fuse in the scenario to reduce short circuit levels.

Thanks to all

RE: Circuit Breakers with different short circuit ratings

uno,
If you use U/L labled molded case breakers check out what the interrupting rating really means. The U/L test method calls for about 10 feet of wire on the load side of the breaker during the test. The wire used will be the maximum size permitted for the breaker size. On the smaller breakers the 10 feet is sufficient to cut the fault from about 35kA down to 14kA. The breaker will be approved for use with a 35kA source. If you should happen to short wires while working near the breaker terminals it will probably not clear the fault. We found this out the hard way. This was a topic of interest in a past IEEE PCIC and IEEE IAS conferences where some folks tested this and photographed the results as part flew out of an open MCC bucket. Wire impedance can be significant.

RE: Circuit Breakers with different short circuit ratings

3
Uno,

Yes, discrimination is applied on almost all circuits, regardless of their purpose. It has to be, otherwise a fault on a small circuit would trip everything back to the utility transformer, if not back to the generating plant. This is achieved by discrimination on either a time or current basis on most LV systems. In addition to discrimation techniques, more specialised protection schemes are found on MV and HV systems and heavy-current LV systems, by restricting the zone over which the protection operates, or by making the protection sensitive to power flow in a certain direction, and so on. That is probably getting a bit too complex for this discussion, but protection is a fascinating field.

The use of cable impedance or transformer impedance to limit fault levels is standard practice. Fault level limitation by cable impedance is usually a fortunate result of using real-world cable sizes to supply real-world loads, where cable size is usually based on current-carrying capacity. Occasionally it is necessary to increase a cable to a larger size to withstand fault current, but while this is a prudent check for any design, the situation doesn't arise that often.

Transformers are deliberately designed with an impedance which restricts the prospective fault to a level which switchgear is cable of clearing. It is possible to design transformers with very low impedance, but switchgear becomes difficult to obtain or has to be designed for the application. In most situations the degradation in transformer regulation from no-load to full-load caused by increasing the transformer impedance slightly is of little or no consequence, while the benefits of limiting the fault level are apparent in reduction of conductor and switchgear size and cost.

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

Start each new day with a smile.

Get it over with.

RE: Circuit Breakers with different short circuit ratings

With respect to experience with the cascade approach, I have personally seen it applied successfully at in excess of 100 sites across a variety of industries.

In my 30 plus years of experience, most as a service engineer, I have not seen a single ciruit breaker failure that could be attributed to the fact it was not rated for the available fault duty and the upstream fully rated breaker failed to clear.

RE: Circuit Breakers with different short circuit ratings

uno, maybe I don't understand your problem but the 'short circuit rating' of a C/B is the amount of fault current it is rated to interrupt. This is a rating tested by the manufacturer and marked on the device. It appears you may be asking about fault current values being different at different locations, though this is true also.
Series-rated systems are based on an upstream device opening for a fault before the maximum fault current is reached. This technique can be used to 'limit' the fault current at a lower-rated downstream device, but it will only accomplish this by opening under the fault, which removes the selective coordination from the system(discrimination).
For instance, you may have a 400A fuse open the feeder to a 400A panel when the fault was on the load side of a 20A feeder C/B in the 400A panel. 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.

RE: Circuit Breakers with different short circuit ratings

Comment on uno (Electrical) Apr 27, 2004 marked ///\\\
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.
///Often, the power distribution system physical geometry covering length between different panels, power distribution panels, switchboards, switchgear is such that one level downstream circuit breakers and buses (panelboards, MCC, switchboards, etc.) have lower short circuit level rating. If the buses are were close, then the short circuit rating will be necessary to be the same. The short circuit rating is a safety related since the protector may explode. It is somewhat different from the selective coordination since the protectors may have the good selective coordination and be improperly rated short circuit-wise thus causing a potential hazard. This area is often very poorly understood and in some instance poorly engineered and designed.
Nowadays, any more important or noticeable size electrical power distribution system is modeled by suitable software before it is constructed. The software normally includes short circuit and voltage drop analysis programs, e.g. SKM, Inc. DAPPER, A_FAULT, etc.\\\
There is no fuse used anywhere in the circuit.
///Good to know.\\\

RE: Circuit Breakers with different short circuit ratings

Comment on uno (Electrical) Apr 27, 2004 marked ///\\\
Or have you seen a power distribution circuit where the upstream circuit breaker is from a MCC/PMCC and feeds another distribution board which has outgoing breakers having lower short circuit ratings.
///Yes, this is quite normal if the distribution feeder is sufficiently long, i.e. has an adequate impedance to reduce the short circuit current (fault) down to the next standard interrupting rating level of a circuit breaker and its panelboard, switchboard, cabinet, etc.\\\
 There is again no fuse used anywhere in the circuit.

RE: Circuit Breakers with different short circuit ratings

(OP)
Thanks, thanks to all..

PWR-It is a relief to know that this technology is being used somewhere in this world.With such an experience I would certainly like to consult you before pushing this application in my projects. Could you send me your email address at pandeys@mantraonline.com

Dandel- Your reply is very appropriate. I would like to add my views about my favorite topic IEC vs NEC. What I could gather that in the NEC world Series rating tests conducted by UL is such that panelboards/distribution boards are certified if both the incoming and the outgoing breaker trips on a short circuit in the outgoing feeder. However as you mention this can break power supply to the entire panel which has several outgoing feeders.In the IEC applications where UL listing is not required , supplier have started to gaurantee selective discrimination along with the series rating (also known as cascading).

jbartos- You certainly have a good view of the present philosophy that we engineers follow. Hoever I would like to bring about some change with increasing technological developments . Only one thing struck me. Why do you say that it is good to know that no fuse is being used?

Thanks again.
uno

RE: Circuit Breakers with different short circuit ratings

uno, I'm not sure how you could have selectivity and series-rated devices, at least with fuses and in the NEC use of the term. Perhaps you could give an example.

RE: Circuit Breakers with different short circuit ratings

2
DanDel,

In your 4/29 post you state/imply that fully rated and selectively coordinated might be related.  This is not at all true.  What could be true is that a selectively coordinated system is fully rated, but the converse is not true.  There are a number of examples where one can have both a fully rated combination of two breakers and a series rated combination where the manufacturer's trip curves are identical between the two.  I could even put together the same thing using a fuse and a breaker.

RE: Circuit Breakers with different short circuit ratings

To expand on the previous post, a fully rated combination isn't necessarily any better coordinated than a series rated combination. Most MCCBs have an instantaneous trip region, which traditionally cannot be coordinated by definition. Some manufacturers apparently claim that certain combinations of their MCCBs can be coordinated.

RE: Circuit Breakers with different short circuit ratings

davidbeach, what I said was that series-rated devices remove the selective coordination from the system. To have proper coordination, one of the first requirements is to have a fully-rated system.
Where did I say that all fully-rated systems are inherently coordinated, as you imply?

RE: Circuit Breakers with different short circuit ratings

Comment on uno (Electrical) Apr 29, 2004 marked ///\\\
Hoever I would like to bring about some change with increasing technological developments . Only one thing struck me. Why do you say that it is good to know that no fuse is being used?
///A fuse can be current limiting. Such fuse can reduce the short circuit current, e.g. from 100,000A to about 15,000A (see Ferraz-Shawmut fuse AD2 or AD6). If such a fuse is prudently engineered and designed in the power distribution system, it will significantly reduce the short circuit current level downstream. Consequently, the downstream circuit breaker short circuit interrupting rating will be much lower, and so will be bracing of buses in panelboards, MCCs, switchboards, switchgear, etc.\\\

RE: Circuit Breakers with different short circuit ratings

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.

RE: Circuit Breakers with different short circuit ratings

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.

RE: Circuit Breakers with different short circuit ratings

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 www.schneider-electric.com/cahier_technique/en/home_en.htm  ;



 

RE: Circuit Breakers with different short circuit ratings

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.

RE: Circuit Breakers with different short circuit ratings

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.

RE: Circuit Breakers with different short circuit ratings

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?

RE: Circuit Breakers with different short circuit ratings

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.

RE: Circuit Breakers with different short circuit ratings

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.

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

Start each new day with a smile.

Get it over with.

RE: Circuit Breakers with different short circuit ratings

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.

RE: Circuit Breakers with different short circuit ratings

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.

RE: Circuit Breakers with different short circuit ratings

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

RE: Circuit Breakers with different short circuit ratings

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

RE: Circuit Breakers with different short circuit ratings

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.\\\

RE: Circuit Breakers with different short circuit ratings

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.

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