Mains voltage variation

With overhead mains supply I’ve found it prudent to use a UPS on things like our dect phone/router/etc… Anyway the UPS I’ve had in place for the last 5 or 6 years packed up… kept on switching off altogether :frowning: So I got a new one and this new model has a display telling you things like the mains voltage etc…

Anyway yesterday the mains voltage coming into the house was 241V… just checked again and it’s now showing 253V! Actually I’ve just remembered I’ve got another mains tester in the garage which I’ll plug in next to the UPS to see if it agrees. I’ll keep my eye on it but should one expect the voltage of the mains to their house to fluctuate like this?

UK voltage range is 240 +6% -10%, i.eCan range from 216-254.4. I believe that may be a statutory requirement, though I am not certain.

And yes it can and when I have ckecked commonly does vary, sometimes high, sometimes low, which will depend on demand and other things like supply switching.

I haven’t seen it jump around while looking at it, and that might be concerning unless an obvious high load switching on or off - in a home that might be something with a heating element especially an electric shower, though one benefit of a dedicated spur is that it would tend to be unaffected by domestic things unless tge load pulls the supply voltage down, which could happen depending on capacity if the supply.

I also have a mains multiplug unit I have certain kit connected to that has a voltage input and load output LCD display on the front of it. In line with comments above, the mains in the UK is regulated to be within the tolerance of mainland European high voltage supplies as the networks are shared across borders and have been for some time.
Mine fluctuates and usually falls within a range around 235-242v which is by design and entirely normal. If it is going beyond the expected ranges then that may suggest an issue with the local substation supply and would warrant a call to your supplier, same would probably apply if you saw it changing a lot and often, mine doesn’t change frequently, rather will vary day to day within a small range.
If your incoming mains supply is running on the high side as you indicate you might find the power supplies in your Naim kit humming a bit more than usual, just something to keep an eye on and if you had any concerns I’m sure a call to your supplier to have it checked and verified at source would remove any uncertainties, the values you specify are not abnormal even if they are on the upper end of expected normal tolerance, I doubt if you checked your UPS display over a period of time you’d find it sat exactly on 240v that often anyway, mine certainly doesn’t at least.

Still at 253V… tbh I wouldn’t like to hassle the local generating board… not on the say so of some made-in-china el-cheapo UPS. If it’s still high tomorrow I’ll get a proper DVM out and check it… damn just remembered the DVM is at ‘the other’ house that we’re supposed to be doing up but that’s sat there doing nowt for 6 weeks :frowning:

I’ve been told that if people report concerns thay happily put a monitor on for a bit to see what’s happening - but during Lockdown might be a bit different.

There is also the possibility that your new UPS digital meter just reads high. Has the UPS meter been calibrated. Also consider what tolerance the UPS meter operates to - it may be designed to only have an accuracy of say ± 10%. The UPS supplier may be able to help here.

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If you are close to the local step down transformer you will have a higher incoming voltage than houses down the street away from it. The voltage range seen is due to voltage drop along the wires, quite normal.

We had a power cut here just after 4pm yesterday, which lasted for around 45 minutes. According to the message on the power networks phone line, it was due to a sub-station “fuse” blowing. I guess with the rotten weather and so many people at home, there were too many kettles being switched simultaneously for tea time?

I’m going to dig a DVM out and compare. My gut feeling is the UPS meter is a bit rubbish… heck what can you expect for fifty quid! :wink:

There was an interesting item on TV a couple of days ago, I think during the One Show (early evening BBC TV in UK) where they filmed at a “secret” power system control centre. The on-duty manager explained the difficulty they are having keeping the balance between supply and demand when much of the usual industrial and commercial demand is turned off. I suspect the voltage going up a bit towards the top of the range is something to do with that, as they are trying to keep the frequency accurate at 50 Hz as their top worry.

But @MikeB probably could explain more…



The EU in its wisdom decided to harmonise UK 240 and European 220 to the EU standard of 230V +10 & -6%
So allowing anywhere between 216 & 240V


I found our electricity meter ( no point calling it a smart meter as the comms module has failed…) will report voltage (amongst many other parameters). Saves me getting the DVM out.

Do you want to try that maths again? :wink:

Its quite simple, as of Jan 1995 the EEC altered the regulatory voltage limits in both Europe & UK. Nothing actually changed, Europe is still something around 220v & UK is 240v
However officially the UK nominal voltage is 230 volts
The permitted range limits are +10 % / -6 % giving an actual voltage between 216.2 & 253volts

Yes… I know. :slight_smile:

Gosh that must be one darn butch fuse! Something like inch thick rebar…

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The big question is, would Richard’s music sound better if they replaced the fuse with a Synergistic Research Blue fairy dust, sorry grahene coated one?


btw todays voltage is 249 by the UPS display… yes, I know, I need to stop faffing about and get the DVM out! :slight_smile:

I can’t believe that Richard voluntarily referred to a fuse (or even a ‘fuse’) on the forum. Lockdown’s clearly getting to the best of us.



Don’t worry Mark, I’m sure they replaced it with exactly the correct type and rating…