Overcurrent Protection Questions and Answers

Answer: C

The plug setting of an over current relay is a percentage of the rated secondary current of the current transformer. A CT with a ratio of 400/8 has a rated secondary current of 8 A.

\[ \begin{aligned} \text{Pick up current} \\ &= 12.5\% \times 8\ \text{A} = 0.125 \times 8 \\ &= 1\ \text{A} \end{aligned} \]

This is the current in the relay coil at which it picks up; the primary current that causes it is \(1 \times 50 = 50\) A, because the CT ratio is 50. The values 5 A, 2.5 A and 0.5 A do not match 12.5% of 8 A. The answer is 1 A, option C.

Answer: D

The common protective schemes in a power system are over current protection, distance (impedance) protection, differential protection, carrier-current (pilot) protection and earth fault protection. Each responds to an electrical quantity such as current, impedance or a signal sent over the line. Humidity is a condition of the air and not an electrical quantity that a relay measures, so humidity protection is not a type of protective scheme. The answer is option D.

Answer: A

The key gives 0.128 s. For an extremely inverse over current relay the operating time is inversely proportional to the square of the plug setting multiplier, \(t = \dfrac{80\,\text{TMS}}{PSM^2 - 1}\) in the standard IEC form. With the fault current of 25 A and the time multiplier setting of 100 per cent, the plug setting multiplier depends on a relay setting that the item does not print, so the stated data do not let the value be derived independently. The explanation follows the key, option A, and the answer is of the order of a fraction of a second, which is typical for an extremely inverse curve at a high multiple of the setting.

Answer: A

Relay co-ordination refers to the timing of protection relays.

In a network, several relays see the same fault current. Co-ordination sets their current and time settings so that the relay nearest the fault trips first and isolates the smallest part of the system, while relays further back wait a grading interval, typically 0.3 to 0.5 s, and act only as back-up if it fails.

It has nothing to do with balancing load or placing current transformers. Mal-operation of relays is what good co-ordination prevents, not what the term means.