ACB I2t values / Single core swa cable sizing in trench

Question

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?

Duncan Brown · Accepted Answer

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,

ACB I2t values / Single core swa cable sizing in trench

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