Ground leakage

These are protection CTs and are used for ground leakage protection. They are also a form of summation CTs, where the phasor sum of the three phase currents is measured. The phasor difference, if any, is the measure of a ground leakage in the circuit. They are discussed in Section 21.5. 

Figure 20.2 An ungrounded or isolated neutral system (circuit completing through the ground leakage capacitances)...

X, j = Ground leakage capacitive reactance Zg = Additional impedance (fl, Xc or )... 

On a healthy system the ground leakage capacitances find no return path and therefore do not activate. They do so when there is a ground fault as illustrated in Figures 20.3 and 20,4... 

Principle of operation of a ground leakage circuit breaker (application of a core-balanced CT) 21/680... 

Figure 21.1 as in lEC 60479-1 illustrates the various zones of ground leakage current versus duration of fault and its effect on a human body. 

Prevention from ground leakage as such is important, as it causes corrosion through electrolysis and may damage the insulation of wires due to ageing. 

Figure 21.2(b) Two-pole ground leakage circuit breakers (ELCBs or GLCBs)... 

For domestic light and power distribution this practice is common. Modern industrial installations can now adopt a fuseless system and use MCCBs and provide a more sensitive ground leakage protection scheme for more safety and reliability. 

The leakage current at hazardous locations such as refineries, petrochemical plants and mines should not exceed 15% of the rated current of the circuit or 5 A, whichever is greater. Table 21.3 indicates the maximum permissible ground leakage currents for such areas at 15% of the rated current and the recommended maximum ground loop impedances. 

The use of core-balanced CTs is quite common for such applications. They are specially designed to detect the ground leakage current of a circuit. This ground leakage is then used to trip the faulty circuit through a ground... 

Table 21.3 Maximum impedances of ground loop lor protection by ground leakage relays in hazardous areas...

Current rating of circuit Amp Maximum permissible ground leakage current C Amp Recommended maximum ground loop impedance on a 240 V phase to ground circuit 240... 

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