Circuit Breakers Questions and Answers

Answer: A

The primary function of a circuit breaker is to protect the circuit from overloading and faults.

A circuit breaker is a switch that can make and break the circuit under normal conditions and, crucially, interrupt the large currents of an overload or short circuit, isolating the faulty section before the equipment is damaged.

It does not energise the relay; the relay senses the fault and energises the breaker's trip coil. Measuring fault current is done by current transformers and relays, and raising voltage is the job of a transformer.

Answer: B

The asymmetrical breaking current is \(\sqrt{I_{dc}^{2} + \left(\frac{I_{m}}{\sqrt{2}}\right)^{2}}\).

The asymmetrical breaking current is the rms value of the total current at the instant the contacts separate. That current is the sum of a dc component \(I_{dc}\) and an ac component whose rms value is \(I_{m}/\sqrt{2}\). The rms value of a dc plus a sinusoid adds in squares:

\[ \begin{aligned} I_{ac} &= \frac{I_{m}}{\sqrt{2}} \\ I_{asymm} &= \sqrt{I_{dc}^{2} + I_{ac}^{2}} \end{aligned} \]

Adding the two components directly overstates the rms value. Leaving out the square root gives a quantity in amperes squared, and taking the root of the plain sum has the wrong units as well.

Answer: C

The answer is Statement I is false but Statement II is true.

Statement I is false: an air blast breaker's extinguishing power is the same whatever the current. When it interrupts a small current, such as the magnetising current of an unloaded transformer, the arc is cut off before current zero (current chopping), causing high overvoltages. It is suited to interrupting large currents at high voltage quickly, not to low-magnitude currents.

Statement II is true: minimum oil breakers, which use a small quantity of oil only for arc quenching, are used for medium and high voltages up to about the 220 kV level; above that, air blast and SF₆ breakers take over.

Answer: D

The rated breaking capacity equals the product of rated symmetrical breaking current and rated voltage.

The breaking capacity of a circuit breaker is stated in MVA and is based on the rms value of the symmetrical (ac) component of the current it can interrupt at its rated voltage. For a three phase breaker the product carries the factor √3: breaking capacity = √3 × rated voltage × rated symmetrical breaking current.

The other options are less precise or wrong. 'Breaking current' without 'symmetrical' leaves out which component is meant, and the fault voltage is not the basis of the rating. 'Twice the rated current and voltage' describes no standard rating.