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Backup Protection for Short Circuit, and sizing Downstream devices.

CSkinner
In a particular install, we have 50kA supply available from a Distribution Board (it's very close to the site transformer, hence the large kA rating).

We are designing a Low Voltage Control Cabinet that will be 100m away from this point, suspected to run in 120mm² PVC cable (calcs pending).

I've had a go at calculated the resultant kA at the point of installation and got about 10kA.


The Main Protective Device inside the panel, is rated at 36kA (due the manufacturer's limited kA range for larger current MCCBs).


I understand the concept of Backup protection, and cascading the kA rating of switches within an installation; but in this case if I have an MCB, in the cabinet, feeding devices in the field will I need to rate that breaker at 25kA, and subsequently the outgoing cable (inline with using the Main switch as Backup protection, and the idea of cascading kA ratings), or can I get away with a 10kA rating as per the resultant kA at the point of install; despite the upstream device being 36kA?


Thanks
Parents
  • 0
    I hope I have understood you correctly.

    100m down the line from a 50KA PSSC, the resistance of the cable limits the short circuit current to 10kA ?

    For reasons of fitting in the hole in the box, a 36kA part will be used, well that is OK,  just over sized a bit.

    and some parts fed by that are rated to break only 10kA - well that is also OK.


    Actually if you put a fuse of a few hundred amps in the way you could probably use 6kA parts after the fuse, as it would limit the duration of any high current fault, and therefore the ability to melt contacts and so on (for really bad events, fuses are faster and more positive than breakers, and can be used to limit let through energy)


    If however you were thinking of putting 10kA parts at the transformer end of the cable, please don't ! How is that end protected by the way ?

    There I would expect some bosky 'death or glory' fuses just in case a digger gets the cable one day.


Reply
  • 0
    I hope I have understood you correctly.

    100m down the line from a 50KA PSSC, the resistance of the cable limits the short circuit current to 10kA ?

    For reasons of fitting in the hole in the box, a 36kA part will be used, well that is OK,  just over sized a bit.

    and some parts fed by that are rated to break only 10kA - well that is also OK.


    Actually if you put a fuse of a few hundred amps in the way you could probably use 6kA parts after the fuse, as it would limit the duration of any high current fault, and therefore the ability to melt contacts and so on (for really bad events, fuses are faster and more positive than breakers, and can be used to limit let through energy)


    If however you were thinking of putting 10kA parts at the transformer end of the cable, please don't ! How is that end protected by the way ?

    There I would expect some bosky 'death or glory' fuses just in case a digger gets the cable one day.


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