Backup Protection for Short Circuit, and sizing Downstream devices.

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.

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

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

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".

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.