In this article we will discuss about:- 1. Purpose of Installing Earth Leakage Circuit Breaker 2. Types of Earth Leakage Circuit Breaker or Earth Leakage Trip 3. Use.
Purpose of Installing Earth Leakage Circuit Breaker:
i. Use of Earth Leakage Circuit Breaker:
According to Indian Standard it is not necessary to use an earth leakage circuit breaker at a place where the earth impedance does not exceed 5 ohms with normal soil and 8 ohms with dry and rocky soil. But where the condition of earth is such that this low value of earth impedance is not available and the fuse wire does not melt or the circuit breaker does not fall open before the leakage current rises to a dangerously high value, an earth leakage circuit breaker is to be installed with each earth electrode in a wiring system in order to have effective earthing.
An earth leakage circuit breaker is to be installed keeping in mind the following arrangement for this purpose:
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Earth leakage circuit breaker (full name is ‘voltage operated earth leakage circuit breaker’) should remain connected between consumer’s earth terminal and a suitable earth electrode. Insulated wires must be used for its connection. The electrode with which it is connected must remain outside the resistance area of any other earth electrode, specially where an extra electrode is used for a wiring system.
If separate earthing arrangement is provided for different sections of a wiring system and if each earthing arrangement has its own earth leakage circuit breaker, care must be taken to see that the resistance area of one does not overlap with that of other.
In order to obtain low earth impedance either earth electrode is buried very deep into the soil or several electrodes are placed in the same earth pit and joined together.
ii. Earth Leakage Trip or Circuit Breaker:
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The purpose of installing such device (earth leakage trip) in an earthing circuit is that, when leakage current is high, this current flows through the relay of this device and attracts its armature. As a result the metal lever or switch blade falls open and completes the connection of another circuit through which the current starts flowing.
The current opens the circuit breaker of that circuit in which the current is leaking. Hence, the leakage of current is stopped and the possibility of any accident is removed. The coil or relay of leakage circuit breaker remains connected in series with consumer’s earthing circuit.
Types of Earth Leakage Circuit Breaker or Earth Leakage Trip:
Earth leakage circuit breaker is available in three different types:
a. Single-Pole,
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b. Double-Pole and
c. Four-Pole.
Where supply is given by 3-phase, 3-wire or 4-wire, a four-pole trip is used. For an ordinary house connected to low voltage supply line, a single-pole trip is sufficient.
Earth leakage circuit breaker has a relay coil. This coil is connected in series with the earthing wire of heater, cooker etc. and is separately earthed. Whenever there is any leakage of current in any appliance, the leakage current flows into the earth through the relay coil.
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As a result the coil is excited and acts as an electro-magnet which opens a contact inside. The main current of the circuit then flows through this contact. Therefore if the earth wire of a house is connected with this trip coil which is separately earthed, as soon as the leakage current rises to 30 mA at any part of the circuit, at once the flow of current through the circuit is stopped.
For a house this type of device is made in such a way that a current of 15 to 30 amperes can flow continuously through it. The device works effectively for both a.c. as well as d.c. supply. When the potential difference across the relay coil rises up to 20 to 30 volts, at once it starts working. Relay coil should be connected to its separate earth electrode by means of insulated wires.
This separate earth electrode must be at least 1.83 metres (6 ft) away from the main earth electrode of the house. The device has insulated cover over which a push switch is provided. The working of the relay can be tested by operating this switch. When the knob of the switch is pressed, the current flows from the line into the relay coil and the circuit opens. The connection of the push switch with relay coil has been shown in fig. 180.
Use of Earth Leakage Trip with Electric Cooker:
In a house where electric cooker is used, an earth leakage trip is essential for the sake of safety. If there is V.I.R. or P.V.C. or C.T.S. wiring in the house, the relay coil of the earth leakage trip should be connected to iron frame of the cooker by means of insulated wire and separately earthed.
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When there is lead covered wiring or conduit wiring, the connecting wire for the relay coil is tapped from a suitable bonding point of the earthing circuit. For the protection of a cooker or any other fitting, its metal frame is connected to relay coil of the trip. But for the protection of the house as a whole, -the earth leakage trip is installed near the main distribution board, and the connecting wire for the relay coil is tapped from main earthing lead of the house at a suitable point and drawn up to a separate earth electrode by means of insulated wires.
In a big house where total current consumption exceeds 30 amperes, a separate earth leakage circuit breaker is to be installed near each sub-distribution board, and the earth wire of this board may be connected to relay coil of the leakage trip. Here the conduit or the lead sheath of the wiring system of entire house should not remain bonded together. The entire wiring system should be divided into different sections, and each section should have its own separate bonding system. A separate earth leakage trip is provided for each section, and this leakage trip is connected to separate earth electrode by insulated wires.
If one desires, an earth leakage circuit breaker may be used with an appliance operated through a 3-pin plug. In this case 3-pin plug and the leakage trip should be mounted on the same block. This is shown in fig. 179.
The connection of a push switch is shown in fig. 180. When the knob of the push switch is pressed, its contact touches the terminal of a resistance. This causes the current drawn by the apparatus to flow directly into the test resistance from the upper terminal on the right-hand side and then through the relay coil. As a result the relay becomes a strong electro-magnet which opens the circuit.
