I see the Fluke 1662 MFT quotes Accuracy 45 Hz – 66 Hz 0.8% + 3digits. I suspect similar from other manufacturers are also available.

(That said: I will say in answer to your question, having had the misfortune in the past of having to design an AC meter for a specialist market, yes it is very difficult and expensive to guarantee that accuracy! And yes to be high accuracy it has to be slow to average out the rubbish - it's the irony that you can have fast and inaccurate or slow and accurate, which absolutely is a pain with voltages that are jumping around.) 

On your original point, it does feel a bit as if you're in a vicious circle here if you have to prove what the voltage swings are before the DNO will prove what the voltage swings are... 

Hi Alan 

Just a suggestion - Could you hire a voltage logger? and gather the voltage values (max/min etc.) over a period of a week or so. If there is anything untoward you have the data to prove to the DNO. 

Kind Regards,

CPC 

errata - I thought I had posted this a few hours ago, but it sat here unloved, given other answers that have come in , it is now slightly out of order, but may still be informative .

Portable Multi function testers  are a compromise, as there is neither the space nor the budget to incorporate  all the bits for a lab grade instrument, that also has to be battery powered, and rugged enough for regular van  transport, and work on a cold day or a hot one.

Consider a 'good' lab bench meter may be one of these, but skip to pages 7(DC) and 8 (AC) to see how the accuracy may vary with temperature and over one year after cal.

And the price tag is not that inexpensive - expect to pay over £1k for one of those.
PDF
A really 'precision'  meter is a few times that times that...

https://www.keysight.com/us/en/product/34470A/digital-multimeter-7-5-digit-truevolt-dmm.html

a snip at 4k

example from Le Croy  Idare not ask the price

But additional accuracy is harder than it looks.

Mike

Could you argue that typically a meter will read either high or low?  After all if it is say 5 volts - or more - out, then it will likely be in the same direction for the low and high voltages; it is not going to be reading low at the lower limit and high at the upper limit so as to give the DNO grounds to ignore it. Is it?

Clive

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.

Are we taking about accuracy (230 V means 230 V, not 235 V) or precision (230 V means 229 to 231 V, or 225 to 235 V), or a combination of the two?

As AncientMariner points out, if it is inaccurate, the MFT will be high or low all the time. If it is imprecise, the measurement could vary from reading to reading, but 15% seems to be well above what a user may expect.

I would repeat the measurements with an electronic multimeter and if doubt remains, I would get out my AVOmeter.

I suggest that the DNO's approach is illogical. They can only dismiss the readings if they take some themselves.

Is a 15% swing feasible? What variable loads could account for it?

Do you have a recent calibration certificate for your meter that you can use to quantify the inaccuracy of the readings, or alternatively have the meter calibrated anyway?

true - most of the error of any given meter and set up will be 'systematic', - by which I mean a feature of the system, like a gun that always shoots low, plus a much smaller random element that varies over time . There may also be trends, like the error changes in a repeatable way as the battery droops or the heating comes on .
Mike

Alan B said:

+/- 2% plus 4 digits, so effectively +/-5V,

If it's a 3-digit display, 500 V range, the accuracy in volts for FSD is given by [(%) + (digit value)], so full-scale deflection accuracy in volts is actually +/- [(2*500/100)+(4)] = +/- 14 V. For a reading of 250 V, the accuracy in volts is +/- 9 V.

It is similar in accuracy overall to the Megger MFT1700 series on voltage measuring range ... ±3% ± 1 V ±2 digits would give +/- 10.5 V.

For analysis of the supply, other instruments, such as a Power Quality Analyzer, are far more appropriate.

It's also worth remembering that the laboratory (intrinsic) accuracy of MFTs are much better than the quoted service accuracy in the field ... the more you pay for instruments, the more accurate they will be in different conditions in the field.