Why is the accuracy of multifunction testers so low

when you measure NE, do so between supply N relative to a test electrode in the garden - which could be as simple as a garden fork and a  croc clip or some random old (non insulated of course) screwdriver  or similar for voltage readings - its not an electrode to carry massive fault currents after all. Measuring NE between network N and E tells you very little if there is any PME type wiring on the network it tends to hide such problems, as CPC and N then move together. True terra-firma earth does not move and then the full Neutral voltage lift is evident..
I'd be slightly surprised if you are more than 500m from your 11kV to LV transformer, but occasionally it happens.  The location of the 33kV 11KV step-down means your 11kV line is about 5km long, which is long but not very long for an 11kV line.

The one metre per volt rule for economic line length is a rule of thumb, not hard and fast but it is a good start as to what sort of distances to expect or consider 'unusually long'.

Mike.

I shall put my money on a network problem rather than a metering one.

That said, I fully appreciate GK's comment about the type of meter which is required. The DNOs must have them.

The real question must be where the burden of proof lies. Does a customer have to show that a supply is problematic, or does the DNO have to show that it is sound? At first glance, ESQCR 2002 is unhelpful.

Thanks for all the input.

On meter accuracy the KT65DL measures voltage to  4 digits, example 230.5, so my max error is 4.6V + 0.4V, total 5V.

Interesting to see that the meter linked above is much less accurate for AC voltages than DC, I guess it's difficult to compensate for voltage drops across diodes, assuming meters use a bridge rectifier.

Meter appears to be more accurate than that in real life, the error becomes a concern when we are on edge of acceptable, if I am seeing 216V, is it actually 216 or 214 for example.
So the voltage variations I was seeing were occurring around about 5.30 in the evening. It's in a village that is fairly long and strung out, with a mixture of overhead and underground lines. Looking at the SSEN network map, the substation is about 4km away, marked as 33/11KV, which I assume is input and output. Also states fully constrained by thermal on the 33KV input line, no constraint down stream. There is also a solar farm covering a few hectares presumably on that substation. Looks like the substation can take a 50% reverse feed.
I think the transformer for the village is about 1km away and the house in question is close to the end of line; with a couple of houses another 0.5 km outside the village, may be on a different feed. Main part of the village is spread out over about 1Km. Haven't managed to find a diagram of the local network

Customer thinks he has actually solved his charging issues with a new 12V battery in the car, but he was reporting issues with both cars, so not sure. I am also seeing quite big swings in my house, 5 or 6 houses down from them. Now looking at this out of interest as much as anything else and wondering if there is actually a bit of an issue with the neutral. I am going to take some measurements of neutral voltage over the weekend to see what I get, will use a different meter as the KT65DL doesn't measure voltages below 25V.

mapj1 said:

the house lights dim when the kettle goes on..

maybe a bit less diagnostic these days than it used to be - as filament lamps are replaced by electronically driven LEDs or CFLs.

A high current loop test can sometimes be revealing too.

   - Andy.

If you really are seeing swing of 40V, and there is not 100A of load being keyed on and off at the house between the two states, then there are a number of possible nasty things happening - but some we can eliminate with a bit of local knowledge.
If you need more evidence to get the DNO out of bed, then perhaps look if there is any evidence for  some of these issues.

(and while one may argue the toss about the absolute meter accuracy,  a swing of that magnitude is really notable, even if both voltages would read higher or lower on a better instrument.) An L-N PSSC test may be revealing - have you done that ? Is this a 3 phase supply - I presume not but if it is, then looking at the other phases may  be instructive.

Voltage drop at the transformer - this should be no more than about 5% if the substation or transformer is not horribly overloaded, but it could be,  or there may be a burnt  contact on one of the LV side fuses.

Pole transformers where the paint has burnt off, or the oil has dripped are getting rarer, but are still seen occasionally and are a clear overload marker.

Voltage drop on the line from transformer to the house - is the house at the end of a long thin street with the tranformer at the other end, and what loads (houses or businesses) does it share with.  (sharing such a supply with a welding shop can be bouncy, or somewhere with big loads like kilns or industrial ovens)

Normally the DNO network designs for no more than about 10% voltage drop. But then the loads sort of grow...

poor network neutral - prohibited by law, but not so uncommon in older aluminium clad buried mains. Varying N-E voltages and large currents in bonded services like water mains and things are a symptom.

Faulty cut out or cut out fuse - smell of burning, dripping tar or plastic.

then the only one that is not a DNO issue,

Fault on load side - loose contacts scratchy main switch etc, all likely to be obvious by the heat produced, and the fact the house lights dim when the kettle goes on...
test the volts  as near origin as is sensible.

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.

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

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?

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?

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.