Thursday, 30 October 2014

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         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 passing 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 currentspassing from live to any other earthed body.

There are two types of ELCBs:
1. Voltage Earth Leakage Circuit Breaker (voltage-ELCB)
2. Current Earth Leakage Current Earth Leakage Circuit Breaker (Current-ELCB).
Voltage-ELCBs were first introduced about sixty years ago and Current-ELCB was first introduced about forty years ago. For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBs because it was a simpler name to remember. But the use of a common name for two different devices gave rise to considerable confusion in the electrical industry.
If the wrong type was used on an installation, the level of protection given could be substantially less than that intended.
To ignore this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current.

Voltage Base ELCB

  •  Voltage-ELCB is a voltage operated circuit breaker. The device will function when the Current passes through the ELCB. Voltage-ELCB contains relay Coil which it being connected to the metallic load body at one end and it is connected to ground wire at the other end.
  • If the voltage of the Equipment body is rise (by touching phase to metal part or failure ofinsulation of equipment) which could cause the difference between earth and load body voltage, the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body passes the relay loop and to earth. When voltage on the equipment metallic body rose to the danger level which exceed to 50Volt, the flowing current through relay loop could move the relay contact by disconnecting the supply current to avoid from any danger electric shock.
  • 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.
  • These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over 50 volts.
  • These devices are no longer used due to its drawbacks like if the fault is between live and a circuit earth, they will disconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal water pipe), they will NOT disconnect, as the voltage on the circuit earth will not change. Even if the fault is between live and a circuit earth, parallel earth paths created via gas or water pipes can result in the ELCB being bypassed. Most of the fault current will flow via the gas or water pipes, since a single earth stake will inevitably have a much higher impedance than hundreds of meters of metal service pipes buried in the ground.
    • The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device. They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to the installation and no personal protection at all. You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum.

Current-operated ELCB (RCB)

  • 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.
  • The supply coil, the neutral coil and the search coil all wound on a common transformer core.
  • On a healthy circuit the same current passes through the phase coil, the load and return back through the neutral coil. Both the phase and the neutral coils are wound in such a way that they will produce an opposing magnetic flux. With the same current passing through both coils, their magnetic effect will cancel out under a healthy circuit condition.
  • In a situation when there is fault or a leakage to earth in the load circuit, or anywhere between the load circuit and the output connection of the RCB circuit, the current returning through the neutral coil has been reduced. Then the magnetic flux inside the transformer core is not balanced anymore. The total sum of the opposing magnetic flux is no longer zero. This net remaining flux is what we call a residual flux.
  • The periodically changing residual flux inside the transformer core crosses path with the winding of the search coil. This action produces an electromotive force (e.m.f.) across the search coil. An electromotive force is actually an alternating voltage. The induced voltage across the search coil produces a current inside the wiring of the trip circuit. It is this current that operates the trip coil of the circuit breaker. Since the trip current is driven by the residual magnetic flux (the resulting flux, the net effect between both fluxes) between the phase and the neutral coils, it is called the residual current devise.
  • With a circuit breaker incorporated as part of the circuit, the assembled system is called residual current circuit breaker (RCCB) or residual current devise (RCD). The incoming current has to pass through the circuit breaker first before going to the phase coil. The return neutral path passes through the second circuit breaker pole. During tripping when a fault is detected, both the phase and neutral connection is isolated.

A residual current device (RCD), or residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the electriccurrent is not balanced between the phase ("hot") conductor  and the neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions; RCDs are designed to disconnect quickly enough to mitigate the harm caused by such shocks.

  • Phase (line) and Neutral both wires connected through RCD.
  • It trips the circuit when there is earth fault current.
  • The amount of current flows through the phase (line) should return through neutral .
  • It detects by RCD. any mismatch between two currents flowing through phase and neutral detect by RCD and trip the circuit within 30Miliseconed.
  • If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD (because u mite not be able to get enough fault current to trip a MCB)
  • The most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps)
  • A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small
  • RCDs are an extremely effective form of shock protection
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