An Earth Leakage Circuit Breaker (ELCB) is a safety device used in electrical installations with high earth impedance to prevent shock.
These dangers can be drastically reduced by the use of an ELCB or Residual-current device (RCD).
The ELCB makes such installations much safer by cutting the power if these dangerous conditions occur. This approach to electrical safety is called EEBAD. In Britain EEBAD domestic installations became standard in the 1950s.
In non-technical terms if a person touches something, typically a metal part on faulty electrical equipment, which is at a significant voltage relative to the earth, electrical current will flow through him/her to the earth. The current that flows is too small to trip an electrical fuse which could disconnect the electricity supply, but can be enough to kill. An ELCB detects even a small current to earth (Earth Leakage) and disconnects the equipment (Circuit Breaker).
Early ELCBs responded to sinewave fault currents, but not to rectified fault current. Over time, filtering against nuisance trips has also improved. Early ELCBs thus offer a little less safety and higher risk of nuisance trip. The ability to distinguish between a fault condition and non-risk conditions is called discrimination.
Voltage-operated ELCBs were introduced in the early 20th century, and provided a major advance in safety for mains electrical supplies with inadequate earth impedance. v-ELCBs have been in widespread use since then, and many are still in operation.
When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated are called Residual-current devices.
The ELCB detects fault currents from live (hot) to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB's sense coil, it will switch off the power, and remain off until manually reset. An ELCB however, does not sense fault currents from live to any other earthed body.
ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, therefore they have less nuisance trips. (This does not mean they always do, as practical performance depends on installation details and the discrimination enhancing filtering in the ELCB.) Therefore by electrically separating cable armour from cable CPC, an ELCB can be arranged to protect against cable damage only, and not trip on faults in downline installations.
ELCBs have some disadvantages:
When this occurs, fault current may pass to earth without being sensed by the ELCB. Despite this, perhaps counterintuitively, the operation of the ELCB is not compromised. The purpose of the ELCB is to prevent earthed metalwork rising to a dangerous voltage during fault conditions, and the ELCB continues to do this just the same, the ELCB will still cut the power at the same CPC voltage level. (The difference is that higher fault current is then needed to reach this voltage.)
When an installation has two connections to earth, a nearby high current lightning strike will cause a voltage gradient in the soil, presenting the ELCB sense coil with enough voltage to cause it to trip.
If the installation's earth rod is placed close to the earth rod of a neighbouring building, a high earth leakage current in the other building can raise the local ground potential and cause a voltage difference across the two earths, again tripping the ELCB. Close earth rods are unsuitable for ELCB use for this reason, but in real life such installations are sometimes encountered.
Both RCDs and ELCBs are prone to nuisance trips from normal harmless earth leakage to some degree. On one hand ELCBs are on average older, and hence tend to have less well developed filtering against nuisance trips, and on the other hand ELCBs are inherently immune to some of the causes of false trips RCDs suffer, and are generally less sensitive than RCDs. In practice RCD nuisance trips are much more common.
Another cause of nuisance tripping is due to accumulated or burden currents caused by items with lowered insulation resistance. This may occur due to older equipment, or equipment with heating elements, or even wiring in buildings in the tropics where prolonged damp and rain conditions can cause the insulation resistance to lower due to moisture tracking. If there is a 30 mA protective device in use anf there is a 10mA burden from various sources then the unit will trip at 20 mA. The individual items may each be electrically safe but a large number of small burden currents accumulates and reduces the tripping level. This was more a problem in past installations where multiple circuits were protected by a single ELCB.
Heating elements of the tubular form are filled with a very fine power that can absorb moisture if the element has not be used for some time. In the tropics, this may occur, for example if a clothes drier has not been used for a year or a large water boiler used for coffee etc has been in storage. In such cases, if the unit is allowed to power up without RCD protection then it will normally dry out and successfully pass inspection. This type of problem can be seen even with brand new equipment.
With any mechanical device, failures occur, and ELCBs should ideally be tested periodically to ensure they still work.
If either of the earth wires become disconnected from the ELCB, it will no longer trip and the installation will often no longer be properly earthed.