Correct Answer - Option 4 : Drop out ratio
Drop Out Ratio:
It is the ratio of drop-out current to the pickup current of the relay.
\(DR = \frac{{{I_d}}}{{{I_p}}}\)
Where DR is drop out ratio
Id is drop out current
Ip is pickup current
Drop out current:
The maximum current at which the relay has to release to its de-energized position.
Pickup Current:
- It is the minimum current in the relay coil at which the relay starts to operate.
- Current in the relay is less than the pick-up value, the relay does not operate.
Circuit Breaker Rating:
Under fault conditions, a circuit breaker is required to perform the following three duties :
- It must be capable to open the faulty circuit and breaking the fault current.
- It must be capable of being closed onto the fault.
- It must be capable of carrying fault current for a short time
Considering above the duties of a circuit breaker, the rating of the circuit breaker is classified as follows:
Breaking capacity:
It is the RMS value of current that a circuit breaker is capable of breaking at a given recovery voltage and under specified conditions (e.g., power factor, rate of rising of restriking voltage).
Making capacity:
- The circuit breaker has the property of closing or making the circuit under shortcircuit conditions.
- The capacity of a breaker to make the current depends upon its ability to withstand and close successfully against the effects of electromagnetic forces.
- These electromagnetic forces are proportional to the square of maximum instantaneous current on closing.
- Making capacity is given in terms of the maximum value of current.
- The maximum value of current (including d.c. component) during the first cycle of current wave after the closure of the circuit breaker is known as making capacity.
- Making capacity = 2·55 × Symmetrical breaking capacity
Short-time rating:
It is the period for which the circuit breaker is able to carry fault current while remaining closed.
