Could an unfused R.C.D. protected spur, from a complient ring final circuit, supplying a single outdoor socket via 1.5mm2 6242Y cable of max. length 300mm through a brick wall from an indoor socket outlet be considered compliant?
Would the swift operation of the R.C.D. limit the time the fault current flows, on a L to E fault, thus reducing the heating effect due to energy let through on the 1.0mm2 C.P.C? The R.C.D. should open within 2 cycles shouldn't it?
The maths is easy enough - the best we're guaranteed for a 30mA RCD is that it opens within 40ms at significant fault currents - 5x IΔn or above (or 250mA for some makes) - so you can calculate the energy let-though as just I²t - so for say 600A fault and 0.04s we'd have 600² x 0.04 = 14,400A²s. Then compare that with the withstand of the cable - k²S² - k for a PVC cable is 115 and so for a 1mm² conductor it comes out at 13,225 A²s.
Would the swift operation of the R.C.D. limit the time the fault current flows, on a L to E fault, thus reducing the heating effect due to energy let through on the 1.0mm2 C.P.C? The R.C.D. should open within 2 cycles shouldn't it?
The maths is easy enough - the best we're guaranteed for a 30mA RCD is that it opens within 40ms at significant fault currents - 5x IΔn or above (or 250mA for some makes) - so you can calculate the energy let-though as just I²t - so for say 600A fault and 0.04s we'd have 600² x 0.04 = 14,400A²s. Then compare that with the withstand of the cable - k²S² - k for a PVC cable is 115 and so for a 1mm² conductor it comes out at 13,225 A²s.
