Let thru current and the point to point fault method

[Tomatoeyes55]

Hello all,


I have a situation where I have a 5k rated contactor who fault is calculated at 9k using the point to point method. I have an availiable fault at the service of 40k and the panel where the contactor is being used is fed with 400A class "RK1" fuses which limit the current to 10k.

My question is can I use the 10k to do my point to point calculation or does this just mean that the fault anywhere down that line can not be greater than 10k?

It is a 480V 3 phase system.

 

[jim dungar]

You may not use the current limiting effect of the fuse unless the down stream contactor is 'series rated' with that fuse.

In reality, you need to consider the let through current of the fuse, the peak current of the fuse, and its operating time. These would be compared to the corresponding withstand values of the contactor. Remember, in determining these values, you must use the fault current (you said 9kA) that will be flowing through the fuse not the amount available at its line side terminals.