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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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  • 0
    Hi mapj, thanks for the response.


    Sorry if I wasn't clear, I was trying to add some context.


    So we have a 50kA supply, 100m away, feeding our Control Cabinet; after 100m the resistance of the cable results in a 10kA short circuit rating at the point of installation.


    A 36kA MCCB is fitted in the Control Cabinet (smaller couldn't be acquired due to the manufacturer's limited kA range for the design current rating).

    *A 10kA MCB, fed from the MCCB above, would be okay to be used in the control cabinet due to the resultant short circuit rating being 10kA; despite the MCCB above being rated for 36kA.


    So the above statement* is true based on your comment?


    I don't know how the outgoing supply of the transformer is protected, but yeah I'd agree with you, I'd imagine there'd be some pretty big fuses!


    Thanks
Reply
  • 0
    Hi mapj, thanks for the response.


    Sorry if I wasn't clear, I was trying to add some context.


    So we have a 50kA supply, 100m away, feeding our Control Cabinet; after 100m the resistance of the cable results in a 10kA short circuit rating at the point of installation.


    A 36kA MCCB is fitted in the Control Cabinet (smaller couldn't be acquired due to the manufacturer's limited kA range for the design current rating).

    *A 10kA MCB, fed from the MCCB above, would be okay to be used in the control cabinet due to the resultant short circuit rating being 10kA; despite the MCCB above being rated for 36kA.


    So the above statement* is true based on your comment?


    I don't know how the outgoing supply of the transformer is protected, but yeah I'd agree with you, I'd imagine there'd be some pretty big fuses!


    Thanks
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