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Household Electrical Guide: Electrical basics, Protection against Errors and Oversights

Posted in Burner, Cable, Cameras, Game Consoles, Homeplug, Household Appliances, Optical Fiber, Powerline, Wi-Fi on July 4, 2008

A basic understanding of electricity is essential, even for those who do not intend to carry out any repairs or servicing of appliances. Ignorance is no protection against your own or someone else’s errors and oversights, whether with repairs, servicing or installation.

Household Electricity supply

Power is supplied to an electricity substation at a very high voltage — 400,000 volts — in three-phase form. There it is converted, via a transformer, to 230 volt single-phase for distribution to our homes. In normal circumstances, current flows from the live supply of the substation’s transformer, through the electrical appliances used in the house and back via the neutral conductor (cable) to the substation transformer’s neutral pole (a closed loop).

The neutral terminal of the transformer is, in turn, connected to the ground: earth — meaning in this case, the general mass of the earth. It is usual to use the armoured sheath of the electricity supply authority’s cable to provide a low-impedance continuous link back to the supply transformer’s start point. There are various types of earthing: connection to the armoured sheath of the authority’s supply cable; own earth rod; transformer earth rod via the general mass of the earth; or the increasingly popular neutral conductor of the authority’s supply cable. The last type is often called PME — protective multiple earthing— or TN-C-S system.

Appliances RenovationThe earth loop path is a safety device intended for protection if there is an earth fault. In this event, it is designed to encourage current to flow to enable the protective devices within the consumer unit — fuse, miniature circuit breaker (MCB) or residual current device (RCD) — to operate in order to isolate the supply to the circuit. Failure to cause the protective device to operate means that the appliance remains live and anyone touching it would receive an electric shock. Electricity always takes the route of least resistance, so a person standing on the ground and touching a live appliance may provide a low-resistance alternative earth path, resulting in a severe shock or worse. The resistance of the earth loop path must, therefore, be low enough to allow sufficient fault current to flow to operate the protective fuse, MCB or RCD.

Variations in Electricity supply systems

In countries other than the United Kingdom supply systems may differ from the typical household supply system described here. Various earthing systems may be encountered, one of the most popular being the PME system whereby neutral and earth are bonded (linked) at the supply point to the property. In fact, the choice of which supply system and ultimately which earth system your property has is a matter for your supply authority. However, the requirement of a sound earth path is common to all domestic systems.

Electricity Earth loop impedance

The term used for testing earthing performance is earth loop impedance. It involves checking whether the current flow is impeded and by how much. This test requires a specialized, professional meter giving resistance figures in ohms. The maximum reading recommended by the Institute of Electrical Engineers (IEE) is 1.1 ohms for a domestic earth path, unless a Type 1 miniature circuit breaker (MCB) is in circuit, in which case a 2 ohm maximum is permissible.

A true and correct test cannot be carried out using an ordinary low voltage meter because a fault can exist that allows the low voltage of, say, 9 volts to pass easily (e.g. just one tiny strand of wire poorly connected) but would break down and go high-resistance or open-circuit if a working voltage of 230v at 13 amps was applied. Although low voltage testing will be able to provide an indication of earth path, it cannot indicate the quality. The professional earth loop impedance meter gives a clearer indication of earth quality in more realistic conditions.

What is an Electricity earth fault?

An earth fault is the condition where electricity flows to earth when, in normal circumstances, it should not do so. There are two ways in which this may happen: direct and indirect.

  • Direct: when contact is made directly with the current carrying conductor which is designed to carry that current.
  • Indirect: touching a part of an appliance, that would not normally carry current but is doing so due to a fault.

The consumer Electrical unit

This is where the power supply into a house is split into separate circuits. It contains a main isolation switch or combined RCD which is used to isolate (remove power from) all the circuits in the house. It also contains various fuse-carriers for cartridge or rewirable fuses or an MCB. Each circuit has its own rating fuse or MCB and no other rating should be used.

Electricity Fuses

An ordinary fuse is a safety device: a weak link designed to break at a pre-set rating. If a circuit is overloaded or a short circuit occurs, the resulting overload will cause the fuse to melt and cut the supply of power. In fact, unless a direct short circuit occurs, the overload on the fuse may not be enough to cause it to ‘blow’ because it has a degree of leeway over its rating value. It, therefore, offers only a very basic level of safety and will not afford any personal safety as the time taken to break is usually too long.

Two types of fuses are available: rewirable and cartridge. Both have drawbacks and are not very ‘user friendly’. The rewirable type is awkward to rewire and the cartridge type, although easier to renew, is often difficult to obtain.

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