IEC 92-202 Fuse question 1

[robertedlund]

Hi all

I got so much help here before so I post another issue I have.
Chapter 6.4.1 in 92-202, second paragraph.

"Discrimination requires:
1/ only the protective device nearest to the fault shall open the faulty circuit.
2/ the protective devices shall be capable of carrying, without opening, a current not less than the short-circuit current at the point of application for a time corresponding to the opening of the breaker, increased by the time delay required for discrimination."

Ok, number one above is logical. No problem. But number two, I do not get ?!
Number one says that the fuse closest to the fault shall open, but number two says that it shold NOT !
Lets say that we have a normal motor-breaker suppling a motor. Above the motor breaker we have fuses. The question is how do you fullfill both requirements above in that scenario ? I mean the motor-breaker is the "protective device" closest to the motor. But in a short-circuit situation the motor breaker will not break the current in time. The suppling fuses will blow before. Now, is that wrong ???

What is it I am missing ?

regards
Robert

 

[rbulsara]

The requirement no. 1 is for the device closest to the fault. (say motor breaker in our example).

The second requirement is the for the upstream device (the fuses in your example) in sereis with the device in question, which has to wait or hold the short circuit currents without opening untill the downstream device opens. This is the defininiton of 'discrimination'.

From a pure theoritical point of view,the blowing of the fuses before the opening of the downstream brekaer will be 'non-discriminatig' or 'lack of coordination'. However, if the either device takes out the same load, no reliability is lost. Except it may cause some dealy in locating the fault. Perfect 'coordinations' are seldom acheived, some trade off are always there as long as they are justified and understood by the Owner and operators. There are no clear right or wrongs in all the cases.

It is also very difficult or even impossible to acheive coodrination betweeen two device of same or very close rating.

 

[robertedlund]

Thank you so much.

So now I have a new topic to discuss around the coffee table he he he...

Happy New Year to all out there !