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?
In theory that sounds like a solution but as I'm not an electrical engineer, I would certainly recommend getting peer advice.
Duncan Brown
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 !!
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,
Answer Verified By: Mark Cramp