Circuit Breaker Selection

It depends on your local electrical regulations, and is likely to be something along the lines of FLC of all loads on the feeder plus 1.5 times the FLC of the largest motor.

To be clear, can you give an example? what is the specific parameter to look in a CB for this purpose? (Icu, Ics, Icw, etc..)

The nominal rating of a CB should be somewhat larger than the current it will normally handle, and will allow for the brief starting current of motors. The formula I gave is based on the United States National Electrical Code; your local regulations may differ somewhat.

In addition to the nominal rating, a CB or fuse has a maximum interrupting rating, such as 10kA, 22kA, 50kA, etc. In the event of a large fault, the CB/fuse must be able to successfully interrupt this large current, which is different from the nominal rating.

You can search the Web or IEC literature for the meanings of I_cu, I_cs, I_cw, etc.... Many of these numbers will be fairly close to each other.

That was perfectly on topic. What worthless idiot voted otherwise?

It was off topic because you gave the answer. You were supposted to do the whole homework page for him. -- JHF PS GA!

The next step is to get a copy of British Standard 7671, and digest and apply the contents.

Rated operational current to be calculated properly considering the type of load (continuous, fluctuating,periodical,duty-AC1,2,3 etc and S1,S2,S3 etc)

Vide IEc 60947-2, for circuit breakers, the rated operational current (In) should be equal to the rated uniterrupted current (Iu) which again should be equal to the rated free air thermal current (Ith). The Standard does not specify anything about Utilisation Categories like AC1, AC2, AC3, etc. for circuit breakers. They are applicable for contactors only.

The number of times/frequency of switching on/off,time interval between them,duration, reversing,loads like elevator(lift) increases the current/heat generated in the CB,which will influence its rated current.

The frequency of swithing, the electrical life and the mechanical life are given as Standard for the given breaker in the manufacturers' catalogues. And, the CB current rating does not vary much on these parameters.

For reversing loads like elevators, contactors are normally employed with AC-4 Utilisation Category ratings. It is highly unlikely that one would go for a circuit breaker for reversing motor applications.

Am I correct to say, that the CB ratings selection is based upon these circuit parameters:

a) Maximum and minimum momentary (first cycle) single- and three-phase short-circuit current.

b) Maximum and minimum interrupting duty (1.5 cycles to 8 cycles) three-phase short circuit current.

c) Maximum and minimum 30-cycle three-phase short-circuit current.

d) Maximum and minimum ground-fault current.

Is it the Icu (Rated Ultimate Short Circuit Breaking Current) or the Ics (Rated Service Short Circuit Breaking Current) of the CB that should be higher than the max momentary three-phase short circuit ?

And the rated operational current and voltage of the CB (Ie & Ue) should be higher than the rated operational current and voltage of the circuit within which the CB is to be placed? please correct me if i'm wrong..

Pl. note my comments itemwise:

a) This is to be checked with the rated short circuit making capacity or short circuitmaking current of the CB. The capacity of the CB shall be more than actual, obviously.

b) This is to be checked with the rated short circuit breaking capacity (either ultimate or service) of the CB. Again, the CB capacity shall be more than actual.

c) This is to be checked with the rated short time withstand current of the CB. The CB has to withstand carrying the maximum aniticipated short circuit current for the maximum anticipated duration.

d) If the source is a transformer with the LV system neutral solidly grounded, then the single-line-to-ground fault current would be equal to the initial symmetrical three phase short circuit current, which however, the CB is designed to break. In any other method of earthing, the earth fault current would only be lesser; hence, the CB would very well break such ground fault currents comfortably.

In case the source is a generator, then maybe, if the fault is very close to the generator and if the neutral is solidly grounded, the single-line-to-ground fault current may exceed the three phase fault current. A detailed analysis is needed in such cases.

e) The duty cycle for Icu is O-t-CO and for Ics is O-t-CO-t-CO; as can be seen if you want more life then have Ics more than the three phase short circuit current. Else, have Icu more. Choice is yours.

Hope the above is clear.

Thank you for the clear explanation. Do you have any reference for this, so that i can cite it based on your reference...?

IEC 60947-2

Thanks again for your help.