Hi Alan

We have run into this kind of issues a couple of times with customers in rural locations (for different makes of EVCP) - but to the credit of our local DNO (National Grid), they would investigate and act when we raised a query such as yours.

I suspect your own MFT may well be more accurate than the numbers the EVCP itself can report, however the readings of the EVCP may help to give you the extra evidence to persuade the DNO to come out and check for themselves with good diagnostic kit, without you having to spend extra time or money. As the EVCP has been tripping out and (I imagine) is connected to the internet, can the supplier help you by providing some data on the tripping events (or even - a time trace of the voltages)? I am not too familiar with the Tesla offering - can the voltage trace even be downloaded from the customer's app / web portal somehow? Armed with such information I would hope the DNO will be more helpful. 

Whatever the absolute precision of the voltages involved, the big variations you noted yourself during the span of only an hour do suggest that the local supply is not very stable and liable to exceed the permitted supply parameters at times - so really the DNO should be dealing with this one. Just to convince yourself it is not your kit playing up, does it produce more stable readings at home / in a built-up area with a presumably more stable supply?

Thanks for your thoughts and ideas. I am reasonably confident that my meter is significantly more accurate than the 2% quoted.

Just frustrating that I can't make an absolute statement that the meter is accurate to say 0.5 percent. Going way back in memory, but I think even an old avo8 analogue meter was accurate to something like 2%. Although when I did a stint of calibration I remember learning how to move solder joints around on wire wound resistors to adjust calibration.

Thanks for your thoughts and ideas. I am reasonably confident that my meter is significantly more accurate than the 2% quoted.

Just frustrating that I can't make an absolute statement that the meter is accurate to say 0.5 percent. Going way back in memory, but I think even an old avo8 analogue meter was accurate to something like 2%. Although when I did a stint of calibration I remember learning how to move solder joints around on wire wound resistors to adjust calibration.