Fusing for TT supplies

Question

An interesting design issues has cropped up. The DNO has provided a 100A TT supply. Would the fusing be different from the BS 1361 fusing for TNC-S arrangement? Also when designing downstream electrical designs I'm having difficulty with excessive energy let throughs. Load end RCBO's, upstream BS88-2 in combination with RCD (30mA). Any suggestions welcome

mapj1 · Accepted Answer

Fusing for L-N faults or shorts remains unchanged on TT relative to TN supplies. What differs (of course) is that the Zs is is nearly always too high to blow that fuse, or indeed any fuse at all in many cases so RCD/ earth fault relay protection at or near the origin is required. Can you clarify the problem of excessive energy let through ? usually TT is associated with the opposite problem and fault currents that do not trouble 2.5mm let alone switchgear.. 
 What are your measured Zs (with and without bonding in place) and your L-N loop PSSC ? 
 regards Mike.

Fusing for TT supplies

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