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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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  • Former Community Member
    0 Former Community Member
    Hi CSkinner,


    There will be few steps to look check prior to deciding the kA rating for the LV control cabinet that you are installing.
    1. Do check whether the 50kA distribution will have prospective fault current about 50kA? A quick check can be done by MVA method based on the Transformer Impedance rating and the cable size & distance to the distribution board.

      A good example of this method can be found here: https://www.jmpangseah.com/wp-content/uploads/2003/01/chapter-5.pdf


      Sometimes, the prospective fault current on the distribution panel can be much smaller than selected fault current rating on the ACB/ MCCB.

       

    • Next, based on this, you can continue with estimating the prospective fault current on the control panel with the same method.

      For the case that the prospective fault current on the LV control cabinet is <36kA, there will be no issue with installing 36kA Main Protective Device. 

       

    • On the circuit breaker selection inside the LV control cabinet, it is as simple as checking the cascading table provided by the manufacturer. 

      Suitability in between 10kA circuit breaker with 36kA main protective device will only be tested in the lab by the manufacturer, a good practice will be referring to the cascading table and using the same breaker brand inside the control cabinet.


    Good luck with it. Stay safe.
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  • Former Community Member
    0 Former Community Member
    Hi CSkinner,


    There will be few steps to look check prior to deciding the kA rating for the LV control cabinet that you are installing.
    1. Do check whether the 50kA distribution will have prospective fault current about 50kA? A quick check can be done by MVA method based on the Transformer Impedance rating and the cable size & distance to the distribution board.

      A good example of this method can be found here: https://www.jmpangseah.com/wp-content/uploads/2003/01/chapter-5.pdf


      Sometimes, the prospective fault current on the distribution panel can be much smaller than selected fault current rating on the ACB/ MCCB.

       

    • Next, based on this, you can continue with estimating the prospective fault current on the control panel with the same method.

      For the case that the prospective fault current on the LV control cabinet is <36kA, there will be no issue with installing 36kA Main Protective Device. 

       

    • On the circuit breaker selection inside the LV control cabinet, it is as simple as checking the cascading table provided by the manufacturer. 

      Suitability in between 10kA circuit breaker with 36kA main protective device will only be tested in the lab by the manufacturer, a good practice will be referring to the cascading table and using the same breaker brand inside the control cabinet.


    Good luck with it. Stay safe.
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