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Topic Title: PSCC For Design Purpose
Topic Summary: Help!
Created On: 16 October 2012 12:07 PM
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 16 October 2012 12:07 PM
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Posts: 79
Joined: 25 January 2011

Hi all,

I'm currently designing a 3phase distribution system and need to know what pscc to use.

The pscc has been measured across a single phase to neutral and found to be 8.1kA. In the past i have always doubled this figure as per guidance note 3 to allow for a sychronised phase fault. Therefore on this job i would have a pscc of 16.2kA.

This causes me a problem because i was planning on using schneider 60898 MCB's but they only have a breaking capacity of 10kA.

My problem is that although the site has a 3 phase supply the 60898 MCB's will be on single phase circuits so should the synchronised fault currents be used for these circuits, bare in mind they are in a 3 phase board.

Can anyone firstly confirm that i am correct to double the measured PSCC and secondly suggest a way around the problem.

Thankyou in anticipation of your much needed help.
 09 November 2012 12:02 PM
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Posts: 167
Joined: 06 November 2012

The doubling of PSCC is a quick easy rule of thumb. If you really want to be clever you can calculate the circuit resistance of the PN circuit bases on the SP fault current then use that resistance and recalculate using 400v instead or 230 to give you the P-P fault current - this would probably reduce your fault current a little too.

You may have SP circuits so it is perfectly acceptable to use the SP rating unless you can see any genuine reason there is likely to be a P-P fault. Its worth remembering that you probably have upstream protection provided by the supply authority service BS88 fuses which have 80k protection if there was a fault within your distribution board. If you are supplied directly from the transformer at the main intake then you should probably be using 60946 MCCBs as these have 25Ka fault protection or a main switch fuse with BS88 fuses in.

Hope this helps

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