Discrimination between breakers 1

[PKJENNY]

When I do a fault discrimination analysis, if I have a final circuit of 32A 1ph MCB with an immediate 400A 3ph upstream MCCB, do I still need to detail check the discrimination, or, from the ratings of the upstream and downstream breakers I can already told discrimination is achieved?

 

[che12345]

Dear Mr. PKJENNY
Q. When I do a fault discrimination analysis, if I have a final circuit of 32A 1ph MCB with an immediate 400A 3ph upstream MCCB, do I still need to detail check the discrimination, or, from the ratings of the upstream and downstream breakers I can already told discrimination is achieved?
A. With 400A up-stream and 32A down-stream would achieve discrimination on "over-load". But NOT necessary covered the "short-circuit kA " level of the branch feeder.
FYI: The difference between "over-load" and "short-circuit" :
a) "over-load" is on a " healthy " circuit when the current increases exceeding 32A to say up to 400A. Discrimination is achieved i.e. the 32A is going to trip while the 400A remained closed.
b) "short-circuit" is when there is a "fault" where the short-circuit current can reach up to say 20...50kA... Under this circumstance, a "full discrimination may be achieved up to say 10kA i.e. 32A opens while 400A remain closed. But, [discrimination failed] at say 30kA i.e. both the 32A and the 400A trip open.
c) most MCCB and MCB manufacturers publish discrimination tables showing full or no discrimination up to certain kA.
Attention: The tables are NOT suitable for breakers of different manufacturers, unless specifically stated.
Che Kuan Yau (Singapore)

 

[PKJENNY]

Understood Thx. I saw a few discrimination studies that ignored the final circuit MCBs in the calculation, but just calculate up to the upstream of the final circuits. I'm not sure why is that so I guess it may be up to a certain ratio of upstream to downstream rating would imply discrimination even under short circuit condition?

 

[che12345]

Dear Mr. PKJENNY

Q. " I saw a few discrimination studies that ignored the final circuit MCBs in the calculation, but just calculate up to the upstream of the final circuits. ... I guess it may be up to a certain ratio of upstream to downstream rating would imply discrimination even under short circuit condition? "
A1. The full-discrimination level (kA) between Up-stream MCCB and down-stream MCB is almost? impossible to be analysed by "calculation" on paper, due to numerous factors. The Tables published by the manufacturer is through actual (lengthy and costly) laboratory tests. There is no known substitution.
A2. The current ratio between the up-stream and the down-stream would NOT imply full discrimination under short circuit condition. The published Table is the only sure way to know the full-discrimination kA level between known current ratings of the up-steam MCCB and down-steam MCB.
Che Kuan Yau (Singapore)

 

[PKJENNY]

Thx Che Kuan. Therefore you think the reason why the that final circuit MCB is not included in the calculation is mainly becoz it is already checked against the manufacturer's table?

 

[che12345]

Dear Mr. PKJENNY

Q. " Therefore you think the reason why the that final circuit MCB is not included in the calculation is mainly becoz it is already checked against the manufacturer's table? "
A1. As an outsider, we do not know on whether the panel builder had checked against the manufacturer's
Table. You would have to request from the panel builder for their official statement on what had they done or why had they not done so.
A2. FYI: You can do it by yourself with the known brand/manufacturer, based on the model, current rating of the MCCB and MCB. These are openly published Tables. You may request them from the manufacturer. These are NOT trade secret!
A3. The Tables published by manufacturers are specific. These are valid for [their product only], between a specific [model and the current rating]. It is NOT an (universal Table) that is based on the [ratio of the current ratings]. These Tables are NOT valid between other models and current ratings, of other manufacturers.
Che Kuan Yau (Singapore)

 

[LionelHutz]

From what I've seen, you won't get full discrimination unless the upstream breakers have adjustable delay time on the instantaneous trip. Your breakers may instantaneous trip at say 500A, 2kA and 5kA which appears to be coordinated, but they will all instantaneously trip when the fault current is 7.3kA.