I'm a beginner in electronics, and I've recently got myself a UNI-T UT61E multimeter, which is marked as true RMS. I stuck its probes into a mains socket and I was expecting 230 V (Europe/Poland) but the DMM showed 240 V. Is it possible that my meter is wrong, or is it a consequence of a true RMS measurement?
I don't have any voltage standard nearby to check the meter.
Is it possible that my meter is wrong or is it a consequence of a true RMS measurement?
It shouldn't be an issue. The sine wave purity of most "mains" supplies is usually pretty good and this will be registered correctly on an RMS meter anyway. Cheaper (rectifer type) meters will start to show a discepancy as the distortion increases but this would be hardly noticeable given the "very reasonable" quality of most AC supplies but this doesn't apply to a lot of battery powered (solar) inverters.
It might be a 4.3% error of measurement or it might be that your AC supply is not quite exactly right. Read the data that came with your multimeter to find out.
According to this wiki page and over-voltage or under-voltage is classified at the 110% and 90% level.
The European standard is now 230V +/- 10%. So anything between 217 and 253V is acceptable. Conveniently, that almost exactly matches anything within the old 220V +/-6% previously used by some countries in Europe and the 240V +/- 6% used by others.
Here in the UK, most substation transformers are still wired for 240V. So it's actually quite unusual to see a 230V supply here. I'm currently getting about 243V here.
I think 240 Volts is well within the accepted tolerance - it is only 4.3% greater than 230 Volts.
The AC line voltage will vary over time, depending on loading. The voltage will likely be lower around dinner time, when many people are using electric stoves, and higher mid-day, when everone is out at work, so very little power is used.
You say you're using a UNI-T UT61E meter which, according to its manual, is a 22000 count meter with 1.2% basic accuracy + 10 counts of error on the 750 VAC range, which is the one the meter would use for that measurement. This comes from page 48 of that manual.
Although that meter claims to be a 22000 count meter, it is only that good on the lower ranges. It only has 0.1 V resolution on the 750 V range, which effectively makes it a 7500 count meter on that range.
That resolution spec means the reading could be off by as much as 1 V and still be in-spec by that error source alone. But the bulk of the error comes from the basic accuracy spec, which adds up to another 1.2% of the measurement to the error spec, or 2.88 V in this case. That means your meter would still be in-spec if it gave a value ±3.88 V of the correct value.
Given the 230 V ±10% value from other answers, your wall voltage could be as high as 253 V, while your meter's error could only be as much as 244 V, rounded off. That means your wall power is within its proper range, unless your meter is very badly out-of-spec.
