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Achieving acceptable Zs on a final circuit.

Sorry, brain fog….existing three phase final circuit protected by a 63a type c mcb. Zs is well in excess of the tabulated value. Is it acceptable practice to put a BS88 fuse after the mcb (next to the db) to increase the maximum permitted Zs?

Parents
  • Yes, but it will not make any significant difference. You still need to trip within 0.4 s i.a.w. 411.3.2.2 (i) and Table 41.1. Tables 41.2 and 41.3 show that you will be able to increase the Max Zs from 0.35 Ω to all of 0.36 Ω.

  • depends on how much  lower rating is the proposed fuse - but it has to be quite a bit smaller than the MCB to allow the higher Zs.

    Or fit an RCD of course...

Reply
  • depends on how much  lower rating is the proposed fuse - but it has to be quite a bit smaller than the MCB to allow the higher Zs.

    Or fit an RCD of course...

Children
  • Or, perhaps, apply supplementary bonding. Check for thermal compliance.