Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning the highest RCD rating for TT supplies, is answered by SELECT:
Answer: RCDs provided for fire protection are normally 300mA rating and for shock protection 30mA; this is when using TN-S and TNCS systems, and they may have to be installed in addition to a front line device when using a TT system.
The RCD installed to provide suitable protection would have to be calculated as a product of touch voltage and earth electrode resistance. Normally, however, a 30mA device is used, installed in an insulated enclosure and the earth electrode resistance is measured and verified as no greater than 200ohms.
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