Hi
I am trying to calculate a cable size from a 1000kva transformer to a switchboard using single core XLPE/non-magnetic armour. The cables are laid direct in the ground spaced. Protection is via a 1600A ACB.
The issues I have:
1) there is no option on Hevacomp installation and arrangement for single core SWA cables laid in a trench. How do I calculate the required cable size?
2) Whenever I use ACB's on hevacomp I get short circuit failures due to unlimited energy I2t. I think this may be down to the fact that I2t data is missing from the Hevacomp data base? Is this correct? How do I clear this fault/error?
Hi MarkFor instances where we have standard cable ranges (not the manufacturer specific ones) the data is taken directly from the regulations. For XLPE/SWA cables (for example table 4J1A in BS 7671) there is no information for Reference Method D (direct in ground). With regards the ACB - you are generating a high fault current that is causing the device to fail in less than .01 of a second. This would usually bring in to play the I2t check, but ACB's don't carry I2t data, hence the unlimited energy fail. The device needs to fail above .01 of a second. You may be able to adjust the parameters of the device to stop the unlimited energy fail.Regards,
Duncan Brown
Answer Verified By: Mark Cramp
Thanks Duncan.
I switched off the instantaneous pick up and tweaked a few settings to clear the I2t issue.
Regarding the cable sizing I am wondering if we use clipped direct as worse case and use a cable factor of 0.9 to make allowance for underground de-rating. I could use the insulation de-rating option and add 0.9 in that box. I'll check with the IET tomorrow I think. Get their opinion on how to correctly size the cables.
Any other views are welcome !!
In theory that sounds like a solution but as I'm not an electrical engineer, I would certainly recommend getting peer advice.