RCBO Max Earth Loop Impedance Readings

KiltedSparky · May 18, 2009

Hi all,

This is my first post on this forum, so please be easy on me.

We currently have new Merlin Gerin consumer units installed within Aberdeen College, and looking at table 41.3 b the eli readings should be 0.72 minus rule of thumb. This fails most of the existing circuits. When i contacted manufacturer they said to forget tables and work out readings by deviding 50 by MA present at RCBO. Can someone verify if this is correct as in regs book it gives you a table seperatly for a TT system which works with manufacturers method but not for TN system

Many thanks for advice in advance , as my next step is contacting NICEIC technical

Spudnik · May 18, 2009

There is general thought that if your loop imp results are under 1667 and the circuit is 30mA RCD protected then its fine on any system.

I, however would prefer to get them as low as possible regardless of the book.

KiltedSparky · May 18, 2009

Thats what i thought Jason but my Qualifying manager has put doubts in my mind

Gnasher · May 18, 2009

Dont't forget that you are talking about RCD part of the RCBO and the same rules apply as a regular RCD.

Guest123 · May 18, 2009

Hi Kilted, welcome along.

As they are existing circuits that fail then yes an RCBO can be fitted as additional protection and this then technically does increase the max ZS value to 1667 ohms, overuling table 41.3 in ol' red.

The current trend that seems to be rife at the moment is that Zs no longer matters as "we can just bung it on an RCD" this I feel is not the case. In my opinion fitting an RCD to a circuit that doesn't technically need it because the Zs is too high should be a last resort and we should always do our best to get our readings within limits.