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Backup Protection for Short Circuit, and sizing Downstream devices.

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 Former Community Member over 4 years ago

There are three things you can do:

1 - Look at the feeder end of the MCC supply - what is the protective device at that position, does it offer a cut off characteristic that you can use

2 - Take account of the 100m run and the reduction in fault level at the MCC (which you've done, but could also include point 1 above)

3 - From 2 above, you now have a "new" prospective fault level at the MCC - say 10kA. So work out what the fault reduction would be in terms of the cut off characteristic of your 35kA MCCB - it has to be 10kA or less. From that value, select the MCB ratings for the outgoing circuits.

So basically, what you say is true at the moment and 10kA MCB's will be fine. (Whether you make that the ultimate rating and accept the MCB may be knackered but open safely, or the operational rating so the MCB can be re used is up to you.

Basically, you need your MCC to be a 10kA rated assembly for stated time - so say 0.1 seconds - which is basically what you'd get from a simple distribution board (in fact a simple distribution board would have a higher rating)

Regards

OMS
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0 Former Community Member over 4 years ago

There are three things you can do:

1 - Look at the feeder end of the MCC supply - what is the protective device at that position, does it offer a cut off characteristic that you can use

2 - Take account of the 100m run and the reduction in fault level at the MCC (which you've done, but could also include point 1 above)

3 - From 2 above, you now have a "new" prospective fault level at the MCC - say 10kA. So work out what the fault reduction would be in terms of the cut off characteristic of your 35kA MCCB - it has to be 10kA or less. From that value, select the MCB ratings for the outgoing circuits.

So basically, what you say is true at the moment and 10kA MCB's will be fine. (Whether you make that the ultimate rating and accept the MCB may be knackered but open safely, or the operational rating so the MCB can be re used is up to you.

Basically, you need your MCC to be a 10kA rated assembly for stated time - so say 0.1 seconds - which is basically what you'd get from a simple distribution board (in fact a simple distribution board would have a higher rating)

Regards

OMS
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Backup Protection for Short Circuit, and sizing Downstream devices.

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