Can Rcd Compensate For High Zs

[spkelectrical]

hi currently doing remedial works on a commercial office. I have a lighting circuit on a 6 amp mcb type B within a split load board covered by a 63amp 30ma Rcd . I have taken a zs reading of 750 ohms from a light which is too high . The existing installation is taking its earth from metal conduits embedded in the concrete ceiling.
My question is do I ignore the reading as there is a rcd in place or run a earth wire to the lights. The lights are metal Flaurecents ?

 

[Richard Burns]

The metal conduit should provide earth continuity, at a low resistance, to the lighting circuit, if you are getting a value of 750 ohms then there is a problem with the continuity of the conduit and this should be addressed.

It may be as simple as one loose joint that could be determined from Zs readings along the circuit.
A R2 reading for sections of the metal conduit could also be a useful guide.
Leaving this situation may mean that as the resistance gets higher (if the conduit deteriorates) there could easily be a point where the protection provided by the RCD is no longer effective, so resolution now is the answer.

 

[spkelectrical]

Thank you for the reply. I think the problem is that some idiot in the past has put a plastic board in and forgot to run a earth back to the board that the mcb is in from where ever the conduit enters the cupboard.

 

[Richard Burns]

Connect a temporary lead from the conduit end to the consumer unit earth bar and see if you have an adequate Zs for the circuit breaker, if so then install a permanent earth of suitable csa (probably 4mm² minimum as it is an unprotected cpc) to complete the earthing.
Though if the customer allows it may be wise to check other circuits if they are all earthed via the conduit and are disconnected and consider making up a manifold for all circuits and a single cpc back to the board.