Fusing for TT supplies - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

Fusing for TT supplies

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

  • 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.

  • Load end RCBO's, upstream BS88-2 in combination with RCD (30mA)

    Two tiers of 30mA RCDs sounds distinctly odd (if I've understood what you're proposing correctly). Normally the upstream RCD would be at least 100mA or 300mA time-delayed (S-type) (and design to avoid the need for additional protection by 30mA RCD for the distribution circuits - e.g. choose wiring systems with a concentric c.p.c. rather than soft sheathed cables for routes embedded in walls etc.) For decent earth fault currents (e.g. >5x IΔn) you should get disconnection within 0.15s which is usually fast enough to protect the cables for currents too low for the fuse.

       - Andy,

  • Thanks Andy, This problem was the BS88's don't trigger with a Zs =20 ohms. Therefore the upstream solution incorporates 300mA time delayed (S type)  RCD's. By removing the BS88's this has corrected the design. Many thanks for you help