Ra maximum = touch-voltage limit / IΔn. 50 V is the usual limit. Use 25 V where the location needs the lower figure. Enter a measured Ra to see the touch voltage.
This is the usual RCD check for a TT supply. It assumes the residual current flows for long enough to produce that touch voltage, and that the electrode is the earth path. Confirm the arrangement in BS 7671 for the location.
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