Let's say we did…and keep everyone happy!
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Your Electricity Board by law must deliver 230 volts +10% - 6% (ie. between 216.2 volts and 253 volts), and to maintain the frequency at 50Hz ± 1% (ie. between 49Hz and 51Hz) over a 24 hour period.
The EU decided, in its wisdom, to harmonise the UK standard mains voltage of 240v AC and the European standard of 220v AC, at 230v AC. Fine in theory but the costs of replacing all the supply equipment to deliver 230v was uneconomic (there being no advantage whatever in changing, other than ‘harmonisation’). So to avoid accusations of failure to harmonise, they simply fiddled with the legal voltage limits, nothing actually changed!.
The law now states 230v +10% -6%, thereby allowing the European 220v system to stay at 220v and UK to stay at 240v, yet both appear to be harmonised!
Homes are supplied by a transformer at a sub-station (serving 10-100 homes) and is what's called single phase (one Live, one Neutral). The substation is supplied with three phase 11,000 volts and distributes the load to customers by balancing the demand across the three phases. As new consumers are added to a phase or the existing consumers use more power, that phase gets overloaded and the voltage and frequency sag below the permitted limit, particularly at times of high demand like meal times and TV programme breaks. The effect of 20 million kettles being switched on at the same time can stretch the National Grid supply to breaking point! This is why we often need a Variac for coffee roasting
The voltage of the ‘11,000 volt system’ is automatically regulated at the Primary substations to about 11,200 volts, and is maintained at this value over a wide range of system loadings. Most of the voltage problems come from the 240 volt domestic system which becomes overloaded over time e.g. at peak hours in the evening when everyones cooking, or at Xmas, when my mains voltage was really low almost all the time.. Sometimes though, low voltage problems can be caused by poor internal house wiring, or other high current drain devices on the same circuit or ring….so it might be worth getting it checked, especially if you have an old house. At the other end of the scale, don’t ignore the possibility of over voltage, it can cause roasting problems and affect the longevity of your roaster.
This handy mains chart from Wikipedia, is the best one I have found and actually shows the former (and in most cases actual) voltages of the so called "harmonised" European countries, Important if your buying a roaster to use in those countries. There are inaccuracies however, specifically Ireleand which I don't believe was a former 220V country. It is true that average voltages tend to be 239 in NI and 235 in Souther Ireleand
This link explains all about voltage harmonisation
230V what really happened
We are now in 2014 and the permissible limits for Voltage variation in the UK and Ireland are a voltage range of 207 Volts to 253 Volts. This is in accordance with European Standard EN50160.