Chris Pearson mentioned using an AVOmeter (I now have two).

Twenty four years ago, along with my Robin KTS1620 I bought a Seward Checkbox 16 to check the calibration, checking the resistance measurements and RCD is straightforward, checking the voltage is a bit more airy fairy, as it’s still dependent upon the mains voltage, there is a voltage output on the front of the check box, but it tells you the test results will be significantly different measured with digital and analogue meters.

I just happen to have a modern conventional moving-coil analogue meter, so here’s a comparison of voltage when measured using the check box with both types of meter.

and here is a comparison measuring the mains directly 

Which is in line with what is to be expected having taken note of what it says on the back of the check box.

A Fluke blog, including a video, remember what you are trying to measure is a moving target and it’s an event that needs to last for more than four seconds, so there could be a lot of near misses lasting several seconds.

www.fluke.com/.../what-is-true-rms

And then there's the further complication in this case - I've only just thought about this - that it all depends what the cut-out circuit on the EV charger is working on, which almost certainly won't be true rms!

So actually the only way to know whether it's the voltage swing that's causing the charger to cut out is to have a meter that measures the voltage in the same way (and at the same speed) as the voltage monitoring circuit in the EV charger...which I can well imagine is just a tiny transformer, a diode, and a capacitor...

Yes, both the Open PEN device and tester need to be testing in the same way to get the same results, BS7671 says the measurements should be RMS, but a lot of EVSE has been developed since the relevant regulations were written and implemented, so can comply if they vary from what is written in the book, because there’s an allowance for new technology.

Here is the best article I found on online,  if you have to try and determine the voltage test result taking into account anything after the decimal point,  there's a problem.

I would be working in whole numbers, not fractions and certainly not taking millivolts into account. 

electrical.theiet.org/.../