Thirty years ago Europe tried to harmonize electric plugs, but the attempt was abandoned. Six years ago, an international standard was produced by the International Electrotechnical Commission (IEC) in Geneva. The aim was to agree on one universal plug and socket, operating at 250 volts in Europe and 125 volts in the USA and Japan. The standard is entirely voluntary, and so far only one country has promised to adopt it. The countries in the EEC/EU and EFTA have also agreed on this in principle; but the transition phases are 20-30 years and the costs enormous. One sticking point arises from the condition that no country benefits from harmonization.
Despite the single market, almost every country in the EC has differences in its domestic electricity supply or sockets sufficient to prevent all but the most basic appliances (such as hair driers) from confident use throughout the region. For example, Dutch washing machines cannot be used everywhere in Europe and telephone answering machines can play havoc if there is a tiny difference in frequency from the country of manufacture.