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GATE 2014 EE (EE1) – Question 53

Power Systems · Principles of over-current, differential, directional and distance protection; Circuit breakers · 2 marks · Numerical answer

IDMT line-protection relays and loads are shown. Their characteristic is $t_{op}=0.14\,\mathrm{TMS}/(\mathrm{PSM}^{0.02}-1)$, with time and plug setting multipliers TMS,PSM. Maximum and minimum fault currents at B are 2000 A and 500 A. Relay R_B has TMS = 0.1 and plug setting 5 A. Find its operating time (s).

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Correct answer: 0.21 to 0.23

Explanation

The official result assumes relay RB’s downstream100Aload sets a100/5CT ratio and uses the maximum2000Afault. Its secondary fault current is100A, so PSM=100/5=20. Substitute TMS=0.1:t=0.14×0.1/(20^0.02−1)=**0.226736s**. The CT ratio and choice of maximum fault are not explicit; using the500Aminimum with the same CT would give about0.428s.