Live wires

日期:2017-08-01 05:26:04 作者:韦婵 阅读:

By Barry Fox A COMMON method of earthing electricity mains could have lethal consequences, according to a former professor of engineering. He wants Britain’s wiring regulations to be overhauled. Peter Fellgett, emeritus professor of cybernetics at Reading University, stumbled across what he believes is a dangerous anomaly in the rules after narrowly avoiding a shock from the casing of his washing machine, which he found to be carrying a potentially deadly 180 volts—almost full mains voltage. His electricity company quickly fixed the problem, but on investigating its cause, he discovered that the wiring to his home near Bodmin, Cornwall, was earthed by a metal stake driven into the ground near the house. Because there is often rock near the surface in Cornwall, the stake had not gone in deep enough and so did not provide a good earth. In addition, a joint in the neutral wire between the substation and Fellgett’s house had corroded. This led to the neutral and earth wires in his house carrying 180 volts. Fellgett’s area uses an earthing method known as Protective Multiple Earthing (PME), one of three ways to supply electricity to homes. The others are Separate Earth (SE) and Independent Earth (IE). In PME, the neutral wire is connected to earth by two stakes: one near the substation and one near each home. The earth wire in the house is connected to the neutral wire—which means that the earth wire, and exposed metalwork, are connected to the live wire through the circuitry of appliances (see Diagram). This does not matter if there is a good neutral return path. But if the neutral wire is broken, and the ground path is poor—as in Fellgett’s case—metalwork in the house can carry near-mains voltage. In an SE system, three wires travel from the local supply to the home. The disadvantage is that if the earth wire is broken, any fault on home equipment can apply mains voltage to earthed metalwork. The same risk applies to an IE system, where wiring is earthed by connecting it to a metal water pipe. If plastic piping is placed anywhere along the route, metalwork and pipes in the house can become live. Fellgett says Britain should ban neutral wire joints that might corrode, and enforce SE connections in regions with poor earths. But he has yet to convince the Institution of Electrical Engineers, which advises the government on supply standards. The IEE says it is “unable” to publish his warnings. And its international wiring committee says the risks of PME are “relatively unusual” and that, anyway, it is the responsibility of electricians to make each installation safe. Says Fellgett: “Rarity is no excuse for ignoring necessary precautions. Nor is it adequate to rely on the installer to assess all the circumstances…A hazard has been identified both theoretically and by actual example. The regulations need to be amended.” But David Start, head of engineering at Britain’s Electricity Association, says: “We only hear of a few incidents a year, so far none of them fatal.” Although the US does not use PME, Start says that countries like Germany, Australia and New Zealand use it—and Norway, with its rocky geology,