AC Mains Voltages

['LIVEWIRE?']

Today I checked the voltage in my workshop. It measured 248V. This was at a mains socket on the front of my workbench.

[Takapuna]

I usually leave one of those plug-in power meter devices in the toaster socket. These have an LCD that can be switched between volts/current/peak current/power/peak power etc. Mains voltage varies considerably here and I've seen 248V on occasion when using a Fluke measuring inside radios during repair. This thread has got me thinking however, as I remember the plug-in power meter indicating a very low 224V on Chrismas morning. I've just cross-checked with the Fluke and found the power meter is reading 8V low. Therfore things aren't quite as bad as I first though! It's indicating 234V as I write, which is 242V really...

Phil.

[Heatercathodeshort]

This is interesting. I'm in a very rural part of West Sussex supplied by overhead cables. My transformer is mounted on a pole in the lane and was replaced about four years ago. The original was dated 1949, the year electricity finally made it's way to this area.
The electricity engineer told me the transformer was working ok but was rather inefficient so was replaced with a smaller unit as can be seen in the picture. I took a reading on 21-11-2012 and as you can see it was 251v.
I have just taken another two readings, one from the digital meter and another from the workshop mains monitor that shows just on 250v. The monitor is a very old but highly accurate instrument as can be seen. The digital meter reads 253v! It's never below 248v. Maybe I'm lucky! Regards, John.

[crusher19860138]

Nice pictures of the transformer and of your workshop! 253V? Very nice, can't beat a higher rather than lower voltage!

[Ed_Dinning]

Hi John, looks as if you are at the end of a reasonable 3 phase 11KV line, much better than being at the end of a single phase one. The transformer looks as if it will supply several properties so will have better regulation than some types. The transformer will have internal taps that have been set on the high side, much better than being set for the "politically correct", nominal 230v!

Ed

[Tim]

238 volts just measured here in Bradford on Avon, and 253 everytime I measure it at work. I set all my(and repaired sets) to the 250 volt tapping. I work on the theory that under running heaters isn't as bad as over running. If the voltage drops on the 250 volt tap the performance may suffer slightly, and dial lamps be slightly dimmer.

[mole42uk]

Originally Posted by kalee20

Probably not. Significantly under-run (20% or more) is a different story though. And the same could apply to reasonably modern electronic equipment too.

Most modern electronic equipment (radios, TVs, cd players, computers etc) have a switch-mode power supply which is rated to run from, for example, 90v to 250v AC. This is simply economics of scale - it is cheaper to build a million SMPS that will handle any mains voltage that the are connected to than to provide a separate power supply for each country. But it does mean that the said electronic equipment doesn't really care about the voltage presented to it.

Richard

[mole42uk]

Originally Posted by Heatercathodeshort

This is interesting. I'm in a very rural part of West Sussex supplied by overhead cables. My transformer is mounted on a pole in the lane and was replaced about four years ago. The original was dated 1949, the year electricity finally made it's way to this area.
The electricity engineer told me the transformer was working ok but was rather inefficient so was replaced with a smaller unit as can be seen in the picture. I took a reading on 21-11-2012 and as you can see it was 251v.
I have just taken another two readings, one from the digital meter and another from the workshop mains monitor that shows just on 250v. The monitor is a very old but highly accurate instrument as can be seen. The digital meter reads 253v! It's never below 248v. Maybe I'm lucky! Regards, John.

Looks like a split-phase transformer to me - there are only two phase connections into the HV end.

There is a 3-ph supply connection to the 11kV but it goes down the pole to another location.

Richard

[matthewhouse]

That's a beautiful voltmeter John! Whilst we are comparing to see who has the largest, it's dead on 245V here which is my favourite single phase voltage. A small factory we do regular work in has it's own supply transformer, with an old skool 440V between phases.

[Lucien Nunes]

When measuring mains voltage, unless you use a true RMS meter there's a possibility that waveform distortion will affect the measurement to an extent that depends on the design of the meter. For example, the readings on a moving coil meter (which is average-responding) and a true RMS digital would diverge as the waveform departs from ideal. Distortion is increasing on power networks due to the replacement of resistive loads such as tungsten lamps with electronic alternatives that demand a non-sinusoidal current.

Lucien

[trickie_dickie]

Originally Posted by TrevorG3VLF

I never did understand why dropping the frequency drops the power, certainly fluorescent lights will consume more.

Trevor

It doesn't. With increasing load the frequency will drop as will the voltage and V2/R takes care of the power. With decreasing load the opposite happens.
Check out this site for more info http://www.dynamicdemand.co.uk/grid.htm#

[trickie_dickie]

Last year, I worked for this company http://www.sollatek.com on the shop floor building Professional: Three Phase - 20-3000 Amps per Phase(2mVA) voltage regulators. (They do a simple plug in unit for individual appliances too. Very good.)

In operation, they use an autotransformer with 9 taps at roughly 30v taps selected by a pair of SCRs to power the primary of a Buck transformer whose secondary is in series with the live feed to the Load. All controlled by a microprocessor. Voltage output is programmable. In use by the likes of M&S and Morrisons in their food distribution warehouses to save power, etc.

[John_BS]

Our mains averages 247v rms and has never been observed below 240v. Waveform is not so good: about 50 degrees of top-clipping!

[Sideband]

My true RMS power meter is currently reading 252 volts. During the day it sometimes dips down to 238 but most times hovers around 242. All sets adjusted to 240/250V here!


Rich

[trickie_dickie]

Originally Posted by John_BS

Our mains averages 247v rms and has never been observed below 240v. Waveform is not so good: about 50 degrees of top-clipping!

Probably due to all the rectifiers in the electronics we use today, charging all them capacitors.

[emeritus]

I got a similar waveform when checking my mains recently and assumed that the scope needed attention: it's been some time since it was last used. I will now have to dig a sine wave signal generator out of the loft to check out the scope.

Since the nominal voltage was changed to 230V, our mains supply is now usually 235V. It used to be 240V.

[Vicboduk]

I have just checked the supply within my lab. Firstly with a recently calibrated digital meter and then the proper way... it appears the digital meter is a bit off! +0.06Hz ??

Vic

[broadgage]

As others post, the nominal or declared voltage in the UK is now 230 volts.
The actual voltage supplied varies a fair bit but tends to be nearer to 240 volts most of the time im most places.

Supply company transformers are usually designed for about 250 volts, so as to give a generous margin for voltage drop along cables.

At times of low load, consumers nearest the transformer will be close to the upper limit of 253 volts, whilst at full load the most distant consumers may be close to the lower limit of 217 volts.

Voltage at the point of use might be up to 1% higher if a grid tied solar inverter is back feeding into the supply and causing voltage rise, or up to 5% lower due to voltage drop in the consumers installation.

[Nicklyons2]

Re post #37 - I just have to say those meters are magnificent. They bear a striking (no pun intended) resemblance to those fitted to the mercury arc installation at the Odeon Cinema in Leeds - I wish I could have rescued them before the re-development.

[hannahs radios]

I have checked my voltage here in weymouth and it usually between 239 and 247 so is acceptable all my radios are set to the 240 or 245 tap. I scoped my supply and found that the peakes are clipped i guess its the same everywhere regards hannah