ELCB Earth Leakage Circuit Breaker| Ground Fault Protection on Induction Motor:
The ground fault protection is used to protect the induction motor from ground fault and the protection is performed by ELCB (earth leakage circuit breaker). Ground fault in the induction motor occurs due to the insulation failure either motor or cable, cable touches on the earth point and other external condition. When occurs the leakage current or fault current starts flowing from line to ground and finally reaches the source. It creates a earth fault. In induction motors, these faults are relatively frequent and hence protection is required against these. These protection can be done with the help of Earth leakage circuit breaker.
ELCB Working Principle:
Consider an example of a person touches live power supply with his figure unfortunately. Here the leakage current starts flow from live line to the same person’s body and ground (refer the dotted line in the figure). Even though the person weared electrical safety material, the person will receive a severe shock. To protect the person, the live power supply should be cut. This can be done with the help of ELCB which will typically trip in around 25 ms if current exceeds its preset value. Now a day the ELCB comes below low range up to 50 mA of leakage current for home appliances. it has more advantage than mcb, mpcb, and mccb.
ELCB consists of a small current transformer surrounding live phase wire and neutral wire. The output other secondary winding of current transformer is connected to the relay circuit. The relay circuit is connected to the circuit breaker. Under normal conditions, the current flow in the phase wire is equal to neutral wire, hence the total current induced in the current transformer is zero. Typically, there will be no production of flux in the core and no induced emf. Due to zero current the trip does not give trip signal to the circuit breaker.
Consider above case, a man touches a live wire. The fault current starts flow through the person body. Now the current flows from live phase wire is greater than the neutral current. These unbalance current is sensed by the current transformer and which gives the current to the relay unit. If the CT secondary is greater than preset value in the relay unit. The relay unit initiates the trip signal to the circuit breaker.
Note: C.T. works with low value of current, the core must be very permeable at low flux densities.
Zero sequence current transformer (ZSCT) protection (ELCB Working Principle)
In case of three phase circuits, single ring shaped core of magnetic material, encircles the conductor of all three phases as shown in below Figure. A secondary of the CT is connected to relay circuit. Under normal condition, the component of magnetic fluxes due to fields of three conductors are balanced and secondary carries negligible current Core of C T Secondary. During faulty condition, the current in the three phase become unbalance and current is induced in the secondary of the CT . The secondary circuit is already connected with the relay and which trip the circuit breaker if the fault current exceeds the pickup current of the relay. This method to provide earth fault protection is called core balance type protection or zero sequence current transformer (ZSCT) protection.
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