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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
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    Agree with OMS.


    The rating you need is the fault current the breaker may have to interrupt. The fact that the fault level upstream at the Distribution Board is 50kA means that a CB installed in the DB may have to interrupt 50kA (e.g. if there is a fault in the cable at the DB end). However a CB in the panel where the fault level is only 10kA will only be able at worst to see a fault of 10kA. This is why specifications, standards, etc. all refer to "suitable for the prospective fault level at their point of installation".
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  • 0
    Agree with OMS.


    The rating you need is the fault current the breaker may have to interrupt. The fact that the fault level upstream at the Distribution Board is 50kA means that a CB installed in the DB may have to interrupt 50kA (e.g. if there is a fault in the cable at the DB end). However a CB in the panel where the fault level is only 10kA will only be able at worst to see a fault of 10kA. This is why specifications, standards, etc. all refer to "suitable for the prospective fault level at their point of installation".
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