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Discuss 16th ed. Tt install...only 1 RCD in the Electrical Forum area at ElectriciansForums.net

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Been to a job today, rewire Immersion cct to suit new cylinder pos.
Did Zs got 12 ohm....looked a little closer...
Immersion fed from b16 breaker from main switch sub board.
Sub board fed via 16mm T&E submain from MCB on non RCD side of DB1. DB1 supplied direct from cutout, typical 16th split board, main switch 1 half 30mA the other. Earthing system TT.
Essentially half of this install, including cct I worked on, has no fault protection! Other than this, everything else looks properly done, am I missing something? My proposal is to replace the main switch with a 100mA RCD type, but question is S type or not? The RCD would protect final ccts as well as the distribution cct to sub board, and there is no 30mA RCD on sub board. So S type would not comply on disconnect times for finals, but a standard 100mA wouldn't necesarily provide selectivity! Looking for a simple but compliant solution please!
 
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telectrix

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can you replace the 16A MCB for the immersion with a RCBO?
 

Wilko

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Hi - most likely the “cables in walls” reg will mean a 30mA rcd (non delayed) is now required for the circuit you’ve modified. I’d discuss with the Customer and try to fix the rest of them while you’re at it. Another thought is many old TT places now have PME available from DNO too.
 
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Yep I could 30mA the final cct for immersion. But Zsdb at sub board is 12 Ohms....so an earth fault on T&E sub main would not trip any protective devices.
This, and the fact all the other non RCBO/non RCD side circuits are left unprotected bothers me.
On this occassion we haven't added any buried runs of T&E, all cabling is surface in plant room, so don't believe I'm required to install additional protection, but do think the installation should be protected against earth fault. Will ask UKPN about PME as might be simplest, but everywhere in this village seems to be TT, so may not be available.
 

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