ACB I2t values / Single core swa cable sizing in trench

RE: ACB I2t values / Single core swa cable sizing in trench

Duncan Brown — Fri, 02 Nov 2018 09:52:44 GMT

In theory that sounds like a solution but as I'm not an electrical engineer, I would certainly recommend getting peer advice.

RE: ACB I2t values / Single core swa cable sizing in trench

Mark Cramp — Thu, 01 Nov 2018 16:41:40 GMT

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 !!

RE: ACB I2t values / Single core swa cable sizing in trench

Duncan Brown — Thu, 01 Nov 2018 16:03:32 GMT

Hi Mark

For 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,