Calculation of Max EFLI readings for type C MCCB'S

[sheeni]

When calculating the max earth fault loop impedance of a 3 phase MCCB that you use the nominal voltage ie in this case it is on a 60Hz system (120 / 208V ) and a 350A MCCB : therefore 208 / (350 x 10 (C type)) = 0.05 ohms
Therefore Zs is required to be equal or lower than this in order for the MCCB to disconnect in the correct time under fault condition??

 

[Engineer54]

With MCCBs you need the manufactures data that came with the MCCB if new. Or if existing, contact the manufacturer (or website) for that units specific requirement. You will find that, for a given MCCB current rating and even type, max Zs can be very different from one manufacturer to another, and even between model types of the same manufacturer... They don't have the same type testing as you find with MCBs eg, B, C, D,

 

[sheeni]

Cheers for that, i am looking into that at the moment, problem being that these american style breakers have got bugger all information on them so making it near impossible, i don't believe that there all type C's but that is what our instruction is to class them. If we have could determine the type then i'm sure that 50% of the faults i now have would rectify themselves with the new allowable max Z's

 

[Engineer54]

If were talking MCB's, All American MCB's will normally have a model number on it, you then go to the manufactures website and hopefully find a time/current graph!! Most American MCB's will be more towards a European ''D'' type rather than a 'C' type, and i jest not ...lol!!! Absolutely crazy curves on most of there MCB's!!...

Errr, why are you using American equipment anyway?? What are the MCB's manufacturer??

 

[sheeni]

Thanks for your advice, i have now found all relevant information and these range from x3 up to x23 so have eliminated a lot of the potentially high Zs readings.

 

[Engineer54]

Thanks for your advice, i have now found all relevant information and these range from x3 up to x23 so have eliminated a lot of the potentially high Zs readings.

I told you they would have crazy T/C curves didn't i!!! ...lol!!

 

[sheeni]

Madness !!