In a TN-C-S installation, neutral currents (in the network in general, but also in the installation) can affect Ze between measurements. There's no saying that Ze was 0.31 ohms at the time Zs was measured for any circuit.

Some instruments are better than others at nulling the effects of load currents and 'leakage' currents.

There will also be a small resistance (measured in a few 1/100ths of ohms, but perhaps upto 0.1 ohms in lower rated breakers) attributable to the contact resistances in each RCBO, which will differ from device to device. These resistances are typically less, the higher the breaker rating (or the higher the current of loads connected in use, especially if it's an existing installation) ... but I'm guessing it's anything above 0.08 to 0.1 ohms difference you're questioning (quite rightly)?

In a TN-C-S installation, neutral currents (in the network in general, but also in the installation) can affect Ze between measurements. There's no saying that Ze was 0.31 ohms at the time Zs was measured for any circuit.

Some instruments are better than others at nulling the effects of load currents and 'leakage' currents.

There will also be a small resistance (measured in a few 1/100ths of ohms, but perhaps upto 0.1 ohms in lower rated breakers) attributable to the contact resistances in each RCBO, which will differ from device to device. These resistances are typically less, the higher the breaker rating (or the higher the current of loads connected in use, especially if it's an existing installation) ... but I'm guessing it's anything above 0.08 to 0.1 ohms difference you're questioning (quite rightly)?

gkenyon said:

but I'm guessing it's anything above 0.08 to 0.1 ohms difference you're questioning (quite rightly)?

Thank you Graham (and others) - spot on!

gkenyon said:

Possibly, however if inductance were the principle issue, surely 'lights up' and 'lights down' would have a similar impedance delta?

There are one or two inconsistencies in the information that I have about occupancy of the house, but it seems to have been let.

Remember that existing let properties had to be inspected by 01/04/2021 and by coincidence this one was I&T'd on 06/11/2020 with a recommendation for testing 5 years later.

The first question is whether R1 + R2 was measured, and if so, why? I wouldn't normally expect it for an EICR.

The DBs (there is an off-peak one, but this appears not to have been connected at the time of the EICR) are barely accessible. They are very low down under the stairs, so it would be a case of lying on one's front with head and shoulders in the cupboard. I wouldn't pull the DB apart unless I had to.

The DBs look only a few years old - modern all RCBO at the budget end of the market. When I saw them, I assumed that they had been installed in 2020. Perhaps they were, but then there should be an EIC rather than an EICR.

I don't think that I would have submitted this report. I am happy to accept a few tens of milliohms discrepancy, but I would want to explain more even if it meant checking the calibration of my tester.