For a circuit-breaker the disconnecting current is taken as 5, 10 or 20 times the rating for types B, C and D. Maximum Zs is then U0 × 0.95 divided by that current. The pass line on this page is 10% lower again, so a pass is inside a margin. Fuse curves are not stored here: type the disconnecting current, or type the maximum Zs from your book.
This is a cautious screen, not a copy of the BS 7671 or On-Site Guide tables. The numbers are rounded so a pass sits inside a margin. If the result is close, or if it fails, check the current tables before you decide. Do not put this figure on a certificate in place of the published value.
These tools are quick guides for a ballpark figure. They are not certified design or test software, and a result on this page is not a design, a certificate, or a substitute for the current edition of BS 7671. Check the working and cross-reference it before you apply it to a real installation. Thanet Electrical Ltd does not accept liability for loss or cost arising from reliance on these figures, except where the law does not allow that liability to be excluded, including death or personal injury caused by negligence.
All tools