Earth leakage circuit breakers

An earth leakage circuit breaker (ELCB) is a double pole mains switch which automatically trips (switches off) when there is sufficient leakage of current from a live wire or earth terminal to earth. It does not normally operate, however, when a circuit is overloaded or if it develops a short circuit (when the live and neutral wires are in contact with each other). Some models do also serve as excess current devices, but these are not normally used in the home.

The ELCB is used as an alternative or back-up device for earthing; in other words it is used where earthing is likely to be poor or ineffective. It is fitted with a tripping coil which is energized by current leaking to earth through it or in some part of the installation, producing an out-of-balance current in the circuit breaker. When the tripping coil is energized, an electro-magnet lifts a latch and releases the switch mechanism which is operated by a powerful spring. On release, the switch contacts open and the circuits and/or faulty apparatus are isolated from the mains electricity.

The current required to energize the trip coil is a tiny portion of the current flowing through the circuit breakers under normal conditions. By cutting off the current to a circuit when there is an earth leakage, the ELCB does the work of a fuse — but with much less current and with greater speed. For example, a 30amp rewireable fuse (the largest in most homes) requires 60amps to blow it; a 30amp cartridge fuse requires 45amps and a 30amp miniature circuit breaker (MCB) requires 37amps. The large currents required to cut off the supply when there is an earth leakage put great strain on the circuit wiring and a very good earthing system is needed if the fuse is to blow or the MCB is to operate. But the ELCB needs only lamp (and often less) to operate, so the fuses and any MCBs remain intact since the ELCB does the work for them.

How it works

An earth leakage current returns to the electricity supply system, usually at the substation which might be some distance from the house. But instead of returning through the neutral conductor, as current does in its normal state, it has to follow an alternative path. Originally the faulty current left the house through the mains water pipe, through the mains water network and onto the substation. Because water authorities are now using insulated pipes, the mains water system may no longer be used as the sole means of earthing in new or existing installations. Now the metallic sheathing of the electricity company’s cable provides a continuous metallic path for the earth leakage current back to the substation. In some areas the electricity company also offers another system known as protective multiple earthing (PME). This takes advantage of the fact that the neutral pole of the mains electricity is solidly connected to earth at the substation and gives a first class earthing system.

Earthing terminal

The electricity company sometimes provides a terminal in the house for earth connection, for which it may make a small charge. Where the Board is unable to provide an earth terminal — either because the metallic sheathing is not continuous or because PME has not been adopted in the area — the consumer must find an alternative, since he is responsible for earthing.

For direct connection of an earthing circuit to earth, the impedance (AC equivalent of DC resistance) of the earth connection must not exceed 4 ohms to enable it to carry the 60amp or more current which may result from a leakage. Earth terminals provided by the electricity company meet these requirements; but alternative systems provided by the consumer are unlikely to do so. For example, simply installing an electrode in the ground to provide an earth terminal would result in fuses failing to blow should an earth leakage fault occur; the installation would be dangerous with a high fire and electrocution risk. The solution here would be to install an earth leakage circuit breaker in conjunction with an earth electrode.

Installing earth electrode

The copper or copper-sheathed electrode must be a minimum of 1200mm (or 48in) long and it should be driven vertically into firm soil so the clamp terminal is just above ground level. Firm soil is essential to provide good electrical conductivity between the electrode rod and earth; the moisture content of the subsoil will further improve conductivity. When deciding on a suitable position for the electrode, bear in mind the concrete foundations of a house wall are probably not more than 300mm (or 12in) below ground level and they protrude between 75 and 100mm (3 and 4in) beyond the faces of the wall. If your consumer unit is situated in a cellar, you can drill a hole — larger than the electrode — in the wall, drive the electrode horizontally through this into the subsoil and seal around it with mastic; or drill a hole in the floor and insert the rod, but in either case beware of the presence of a damp proof membrane which would be punctured by this process and allow damp to penetrate. Connection to the terminal clamp should be with 2.5sq mm single core green/yellow PVC-insulated cable; at the connection point fix an indelible label stating: ‘Safety Electrical Earth — Do Not Remove’.

Types of ELCB

There are two main types of ELCB installed in the home: one is voltage-operated and the other current-operated. The current-operated ELCB should be your first choice, but if the product of the operating current and the impedance of the earth loop (as measured on an impedance tester by the electricity company or an electrical contractor) exceeds 40, you must use the voltage-operated type.

Voltage-operated

This is the cheapest ELCB and is simply a double pole circuit breaker with trip coil and tripping mechanism. The circuit breaker section has four main terminals — a conventional double pole mains switch and two subsidiary terminals. The mains leads from the meter are connected to one pair of live (L) and neutral (N) terminals; the other pair are load terminals which feed the installation and are normally connected by two leads to the mains terminals of the consumer unit. The subsidiary terminals are marked ‘F’ and SE’. The F terminal is connected to the earth terminal on the consumer unit; the E terminal is connected to the earth clamp terminal of the earth electrode in the soil. Any leakage of current to the earth conductor in the main circuit passes through the tripping coil of the ELCB, energizing the electro-magnet which trips the circuit breaker. About 40 volts is needed to operate the tripping device and the current required to produce this voltage is very small.

Since the leakage current in the earth conductor must flow through the tripping coil to operate the release mechanism, this type of ELCB is not wholly reliable. For example, if fault current flows to earth via another path, such as a gas or water pipe (called a parallel earth path), it bypasses the ELCB and the ELCB does not trip. If this parallel earth path is satisfactory, sufficient current will flow to blow the fuse. If not, the gas or water pipework indoors will remain live and be dangerous.

Warning

It is not a good idea to install two or more voltage-operated ELCBs to protect different sections of the installation, unless the respective earthing rods are at least 2.5m (or 8ft) apart and are 2.5m (or 8ft) away from buried water or gas pipes. Current-operated This has been developed to overcome the disadvantage of the voltage-operated ELCB. Although similar in style to the voltage type, its tripping coil is not energized directly by earth leakage current and has no earth conductors connected to it. There are four terminals – L and N mains terminals and L and N load terminals. The earthing lead from the consumer unit in this case goes direct to the earth electrode.

This type of ELCB works on the current balance principle – that current flowing into a circuit is equal to the current flowing out. For example, if 10amps flow into a circuit through the positive or live wire, you can expect 10amps to return via the negative or neutral wire. However, if a live wire touches earthed metal, some of the return current is diverted from the main circuit into the earth circuit; this causes an out-of-balance current in the main conductor. At the instant of the fault, the live pole of the mains switch will be carrying a lot of current and the neutral pole less, since some will be leaking to earth. By having an out-of-balance current sensing device and by connecting this to a tripping coil, you have a current-operated ELCB. No matter which path the earth leakage current takes, the sensing device will detect the out-of-balance in the mains lead and operate the tripping coil. The normal current-operated ELCB of 60- 100amps switch rating needs about tamp of out-ofbalance current to operate, which means the earthing system needs to carry only that amount of current for the ELCB to work.

High sensitivity ELCB

An ordinary ELCB gives no protection from direct electric shock, which is generally caused by touching a live wire or contact when also touching earthed metal or standing on the ground. With a normal current-operated circuit breaker this is because the amount of current required for it to operate is more than that required to electrocute someone; with a voltage-operated type it is because the current flowing to earth via another path (through the human body) does not flow through the ELCB trip coil. There is, however, a current-operated high sensitivity ELCB which will protect you against electrocution if you touch a live wire while in contact with the ground either directly or via earthed metal.

  • It must be emphasized, however, that neither this ELCB nor any other device installed in the home will prevent you being electrocuted if you come into contact with both live and neutral poles of the 240V supply, even if you are standing on an insulated surface or wearing rubber boots in the garden. But if you are in contact with the ground at the same time, there is just a chance the high sensitivity ELCB will trip before you are electrocuted.
  • The high sensitivity ELCB, which trips at 30 milliamps in a fraction of a second, is especially useful for selective circuits such as those used for power tools, hedgecutters, mowers and other appliances where shock risk is fairly high. It should not be inserted in the main house circuits because, if condensation or some other small earth leakage current causes it to trip, it will cut off the power to the whole house. This is called nuisance tripping and is not only inconvenient, but it could also cause a serious accident.
  • A consumer unit is now available in which an ELCB protects some socket outlets and a conventional mains switch controls other circuits including lighting. Where a circuit is installed to supply a few socket outlets, it is best to use fittings of a different colour for the protected ones, so only these are used with high shock risk appliances.
  • A high sensitivity ELCB is available for plugging into a 13amp socket, from which you can run high risk power tools. Similar to an adaptor but larger, it is a useful safety device for the home.
  • High sensitivity ELCBs are fitted into some special consumer units in place of the usual mains switch. Although these give additional personal protection, however, they cannot be expected to prevent electrocution when handling live wires.
  • The ELCB should be tested regularly using the test button.

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