R ≤ 50V/Ia - where Ia is the fault current necessary to open the associated protective device within 5s. (Reg 415.2.2)

Picking the right device(s) to consider can be a bit tricky - it's more about thinking about the circuits that could impose a voltage on the exposed- and extraneous-conductive-parts within the bathroom rather than necessarily just the bathroom circuits. E.g. don't overlook things like an immersion heater in a bedroom that shares metallic hot water pipework with the bathroom taps.

   - Andy.

Hi Gary, what work is being done to this bathroom? 

The supplementary bonding conductor must be no smaller than the smallest cpc, so that could be 1 mm². Appendix I of OSG shows that about 14 m of  1 mm² have a resistance of 0.25 Ω. Could the bonding be that long?

The alternative path is cpc back to the CU, from there to MET, main bonding to pipe work.

With the CU adjacent to the bathroom, I do not think that you would be able to measure the resistance sufficiently accurately to determine whether or not there is any supplementary bonding. Either way, 0.25 Ω seems a little high.

Could you fit an RCBO in the existing board, or externally just for that circuit?

I was doing an EICR 

I suspect it's the cpc. The CU is only about 5 to 6m away. The circuit is the bathroom and a bedroom, the bedroom is farther away. I had an r1 + r2 of .80Ω in the bedroom.  It's a first floor flat.  

That's a good point, i didn't think of that although the board is probably 20 years old. An old proteus thing.

I presume the shaver skt is fed by the lights, and the exposed front panel screws are the only earthed part that in the event of a fault might not clear in the required 0.4 seconds ?

0.25 ohms is still a touch under 1kA. What devices do you have in the CU that will not promptly trip at that sort of current, or even at 100A, if the answer is 'none' then the bonding is adequate.

I suspect bonding present but maybe a bit corroded, and that if a real fault with bit of oomf came along it would re-weld into a lower impedance state.

As there is no RCD do you have a high current loop test on your meter ? - it may give a lower reading. Or if there is a very whispy connection that would not stand a real fault, it will blow it open cct, but at least you know that it was inadequate,  before it fails to save someone.

Mike.

Thanks Mike.  As far as i can see there is nothing exposed, not even screws. The cover hiding the terminals is behind the lamp and that screw goes into the plastic. The front diffuser clips on.The ends are all plastic.    Yes i can do a high current loop test.  

Cheers

Thanks Andy