Earth Leakage Circuit Breaker

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  • Topic: Residual-current device, Earthing system, Electricity distribution
  • Pages : 5 (1421 words )
  • Download(s) : 141
  • Published : November 25, 2011
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An Earth Leakage Circuit Breaker (ELCB) (now referred to as residual current circuit breaker) is a safety device used in electrical installations with high earth impedance to prevent shock. History ELCBs were mainly used on TT earthing systems. Nowadays, ELCBs have been mostly replaced by residual-current devices (RCDs). However many ELCBs are still in use. Early ELCBs responded to sine wave 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. ELCB manufacturers include: Legrand, Havells, ABB, Siemens AG, Areva T&D, Camsco, Telemecanique, Orion Italia, Crabtree, MEM.

[edit] Types

There are two types of ELCB:
• voltage operated and,
• current operated.

[edit] Voltage-operated

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 but are no longer installed in new construction. A voltage-operated ELCB detects a rise in potential between the protected interconnected metalwork (equipment frames, conduits, enclosures) and a distant isolated earth reference electrode. They operate at a detected potential of around 50 volts to open a main breaker and isolate the supply from the protected premises. [2] A voltage-operated ELCB has a second terminal for connecting to the remote reference earth connection. The earth circuit is modified when an ELCB is used; the connection to the earth rod is passed through the ELCB by connecting to its two earth terminals. One terminal goes to the installation earth CPC (circuit [[Power system protection |protective]] conductor, aka earth wire), and the other to the earth rod (or sometimes other type of earth connection). Disadvantages of the voltage-operated ELCB are the requirement for a second connection, and the possibility that any additional connection to earth on the protected system can disable the detector.

[edit] Current-operated

Current-operated ELCBs are generally known as Residual-current devices (RCD). These also protect against earth leakage. Both circuit conductors (supply and return) are run through a sensing coil; any imbalance of the currents means the magnetic field does not perfectly cancel. The device detects the imbalance and trips the contact. When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated are called residual-current devices. However, some companies use the term ELCB to distinguish high sensitivity current operated 3 phase devices that trip in the milliamp range from traditional 3 phase ground fault devices that operate at much higher currents (traditional gf devices are insensitive due to the error inherently associated with the summation of currents from multiple current transformers).

[edit] Connection

[edit] Operation

An ELCB is a specialised type of latching relay that has a building's incoming mains power connected through its switching contacts so that the ELCB disconnects the power in an earth leakage (unsafe) condition. The ELCB detects fault currents from live 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. A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.

[edit] Advantages

ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, and therefore have fewer 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...
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