Structure and Operations of Earth Leakage Circuit Breakers

Outline of structure

• Earth leakage tripping device which trips the circuit breaker according to short circuit current

• Switching mechanism that opens and closes the contacts

• Earth leakage indication device which is interlocked with the overcurrent protection device and indicates that it has operated owing to ground fault

• Overcurrent trip device which trips the circuit breaker according to overload or ground fault current

• Arc extinguishing device which extinguishes arc generated when current is interrupted

• Terminals for connecting wires and conductors

• Contacts which open and close the circuit

• Test button for checking that the breaker operates upon occurrence of ground fault

• Molded case in which the above parts are contained compactly Fig. 7. 1 shows an example of arrangement of the above parts. The earth leakage trip device is a current operation type device which detects directly the ground fault current and consists of a primary winding of each phase on the core, a zero-phase current transformer (hereinafter, referred to as ZCT) for detecting zero-phase current (ground fault current), an electronic circuit for amplifying the ZCT output and an electromagnetic device for tripping the circuit breaker. The operating principle is explained below. See the input/ output of ZCT on the circuit shown in Fig. 7. 2. If the circuit is sound, the magnetic fluxes generated by the forward current and return current cancel each other, and voltage is not induced on the secondary winding. However, if a ground fault occurs, the forward current IA is divided into current I g which returns to the transformer through the earth and currents IB and IC which return through the ZCT. Therefore, the vector sum of the currents which pass through the ZCT is IA + IB + IC = I g, and a magnetic flux Ig depending on fg is generated on the core of ZCT, thereby inducing voltage on the secondary winding. This signal enters the gate circuit of the thyristor on the electronic circuit ti drive the thyristor, and the electromagnetic device connected with the thyristor in series operates to trip the circuit breaker. In the case of 3-phase, if the circuit is sound even under unbalanced loading under which the currents of the phases are not equal to one another, the sum of currents of the phases is IA + IB + IC = 0, and ELCB will not malfunction. (Fig. 7. 3)

Fig. 7. 1 Construction of ELCB

Fig. 7. 2 Circuit configuration of ELCB (TT system)

Fig. 7. 3 Unbalanced loads

The ELCB contains following devices

Arc extinguishing device

The arc generated at breaking is extinguished. Mitsubishi unique techniques, such as the opposing grid which enhances the arc resistance, are used.

Trip button (PUSH TO TRIP)

The arc generated at the circuit breaker can be mechanically tripped from the outside to check the operation of the alarm switch and the reset by the external operation handle.

Overcurrent tripping device

Since the proven mechanism of Mitsubishi MCCB is used, overcurrent can be surely detected.

Switching mechanism

Since the proven mechanism of The same reliable quick-make, quick-break mechanism as that of Mitsubishi MCCB. (Some models with frame size of 125A and below do not have the quick-make, quick-break mechanism.)

Window frame

Since the handle, trip button, test button, earth leakage indication button and rating plate are collected in one position, a window.

Sensitivity switching device

With this device, the rated sensitivity current can be easily changed. Some models can switch in 3 stages, 100, 200 and 500 mA, and others in 2 stages, 100 and 500 mA.

Earth leakage indication device

Target type with indication button. The device is automatically reset by the handle.

Test button

The button has excellent durability and performance for start-up inspection.

Earth leakage tripping device

• The most important part, the semiconductor circuit, is of an electronic type using highly reliable ICs.

• Two filters for removing higher harmonics and surge components are provided to prevent unnecessary operations.

• The ZCT can detect minute current without fail. It is magnetically shielded and will not operate unnecessarily under the influence of motor starting current, etc.