May I just point out that the meters are set to test L-N as it's an Ipf test, so it is a high current loop test that cannot trip a RCD and there are only red and green test leads, not any blue leads in the photos. 

Good spot. I was not familiar with the history of these type of testers, but apparently when they were launched in 2011, there were a few cases of confusing readings and then a firmware tweak, whatever that entails

https://www.test-meter.co.uk/blog/megger-mft1700-series-overview-of-multifunction-installation-testers

It may be interesting to compare firmware versions if that is possible,

Also the official accuracy is (from here)  L-N/L-L tests ±5% ±5 digits

Now it seems the spread observed is larger than that, but before rushing off, I'd be looking at the quality of the connections to the test leads - a very thin film of oxide on the banana plugs is all that is needed to add a handful of unwanted milli-ohms, When new these shrouded 4mm connections can  manage 0.5- 1milliohm per mated pair, but it rises with time as the condition of the mating surfaces deteriorates.

Mike.

Good spot. I was not familiar with the history of these type of testers, but apparently when they were launched in 2011, there were a few cases of confusing readings and then a firmware tweak, whatever that entails

https://www.test-meter.co.uk/blog/megger-mft1700-series-overview-of-multifunction-installation-testers

It may be interesting to compare firmware versions if that is possible,

Also the official accuracy is (from here)  L-N/L-L tests ±5% ±5 digits

Now it seems the spread observed is larger than that, but before rushing off, I'd be looking at the quality of the connections to the test leads - a very thin film of oxide on the banana plugs is all that is needed to add a handful of unwanted milli-ohms, When new these shrouded 4mm connections can  manage 0.5- 1milliohm per mated pair, but it rises with time as the condition of the mating surfaces deteriorates.

Mike.

mapj1 said:

Now it seems the spread observed is larger than that, but before rushing off, I'd be looking at the quality of the connections to the test leads - a very thin film of oxide on the banana plugs is all that is needed to add a handful of unwanted milli-ohms, When new these shrouded 4mm connections can  manage 0.5- 1milliohm per mated pair, but it rises with time as the condition of the mating surfaces deteriorates.

During my training, we were encouraged by one of the tutors to oscillate the plugs three or four times in order to abrade any corrosion. I have no idea whether it works or not.

Notwithstanding annual calibrations, let's not forget the use of a check box or even monthly testing on a particular socket (in the workshop, kitchen, or office). If the plug connexion is deteriorating, it should be evident.