Joined: 25 January 2011
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.
Joined: 17 May 2012
As an estimate, doubling the Ph-N figure to get the 3 phase figure is OK, but in practice it is more complex than that. For this to work, you need to have a single full size neutral to match the phase cables all the way back to the transformer and the neutral will need to be as close to the phase cables as they are to each other (particularly for large cables). There will also be some error associated with the upstream network and type of transformer but hopefully that will be small.
A larger problem is the cable temperature at the time you measured 8.1kA. Maximum PSCC is usually calculated with all cables at 20 degrees celcius. If you measured with cable temp higher than this you will need to increase your measured value accordingly.
As for whether you can use the Ph-N value to size the breaker instead of the 3 phase value - I am not sure. I always size to the 3 phase level, but you probably don't need to (I haven't found it in a regulation). The only way you could exceed this with single phase breakers is if you have a fault across neighbouring breakers on separate phases - unlikely, but might be possible.
So if you go with a PSSC of say 18kA, you can usually use 10kA breakers if you have a partnering fault current limiting breaker upstream or you could use a hrc fuse.
If you are using C60H breaker in the board, they can be cascaded with a a variety of NSX MCCBs upstream. E.g a NSX160B with a TM-160D trip unit will back up the full range of C60H devices up to 25kA with full discrimination. Obviously need to check the PSCC at the feed board that holds the NSX160B is 25kA or less - if not try a NSX160F(36kA) it produces the same result.
If you want to go with a BS88 fuse, a 160A unit will protect a C60H up to 50kA, but the discrimination limit is much lower than the previous suggestion (only 3kA).