I don’t remove anything. In a specific scenario, I encountered a metallic staircase within an installation where it was possible to come into contact with the staircase while simultaneously touching an exposed part of the installation. To assess the situation, I conducted a test using an R2 wandering lead between the MET (croc lead the Main Earth Terminal) and the staircase. No disconnections were carried out during the test, and the measured resistance was less than 22 kΩ. Based on this assessment, I made the decision to install main equipotential bonding. 

I'd agree - assuming that this is all about limiting shock currents to 10mA or less, when someone holding a semi-live thing that is faulty then touches the mystery object, even for a very wet (low resistance) human, it is most unlikely that if someone is getting a shock, they will have time to run round and disconnect the bonding to the gas main or whatever to lower the shock current a  bit and then reconnect themselves ! It  has to be measured with the wiring in the normal operational state whatever that is.

The assumption is that the semi-live faulty thing is at or near mid-rail voltage as for a perfect low impedance fault and mains supply  with equal resistance in L and E cores, then the exposed case of the thing is at half mains voltage, until something trips.

Note also that the battery powered meter lead test can give results that are a bit at odds when compared to a 'zs' style mains derived test- things like concrete floors that are slightly humid have a non-linear resistance becoming more conductive at higher voltages than ohms law constant resistance would predict.

Mike

Hi Mike, I use a Megger MFT instrument set to the ohms measurement. would it be considered sufficiently accurate for this test? 

I'd not be too worried unless the reading was really borderline - the whole thing is a bit 'measure with micrometer, mark in chalk , cut with axe' anyway as you are getting a snapshot reading of some leakage resistances that may depend on when the floor was last washed, recent weather or if the heating has been on yet this year

I'm just cautioning that a battery powered measurement  made with a few volts DC may not read the same as an AC mains test, generally the DC test reads higher, even on the same day and with the same conditions.
But the fact we are looking for 22k ohms, or 10mA rather than the RCD limit of 30mA means we have a bit of safety margin in term of how bad the largest unmitigated shock might be.

Mike.

It's more an issue if you've got either supplementary bonding - which must be disconnected to avoid parallel paths - or if you've got e.g. a class I appliance mounted on the extraneous conductive part such as e.g. a gas boiler is connected to the gas pipes and any copper water pipes.

You might think an external metal staircase doesn't have parallel paths. But then some external metal staircases have galvanised conduit running on them to feed an outside light.

Sometimes it might not even be practical to split up the parallel paths. One problem with conduit as a CPC is that dismantling for testing isn't as trivial as a wire into a MET. So you've got to work with what you've got and find the most reasonable way to be assured of the integrity of the bonding.

It's more an issue if you've got either supplementary bonding - which must be disconnected to avoid parallel paths - or if you've got e.g. a class I appliance mounted on the extraneous conductive part such as e.g. a gas boiler is connected to the gas pipes and any copper water pipes.

You might think an external metal staircase doesn't have parallel paths. But then some external metal staircases have galvanised conduit running on them to feed an outside light.

Sometimes it might not even be practical to split up the parallel paths. One problem with conduit as a CPC is that dismantling for testing isn't as trivial as a wire into a MET. So you've got to work with what you've got and find the most reasonable way to be assured of the integrity of the bonding.