NETA - InterNational Electrical Testing Association Certified Technicians Protective Relay Applications Questions and Answers

Question 1

A technician is testing a bus differential (87B) protection scheme. The fundamental principle of operation for this relay is based on:

Accepted Answer

Kirchhoff's Current Law, where the vector sum of currents entering and leaving the bus should be zero.

Answer

The impedance of the bus changing during a fault.

Answer

A time-delayed response to overcurrent conditions.

Answer

The detection of negative-sequence voltage during unbalanced faults.

Question 2

During the commissioning of a large industrial motor, a technician must verify the settings for the thermal overload protection (ANSI Device 49). Which of the following conditions is this relay primarily designed to protect against?

Accepted Answer

Sustained overcurrents that cause excessive heating.

Answer

Instantaneous short circuits within the motor windings.

Answer

Phase reversal that would cause the motor to run backwards.

Answer

Ground faults on the motor frame.

Question 3

A distance relay (ANSI Device 21) is most commonly applied for the protection of which of the following power system elements?

Accepted Answer

Transmission lines

Answer

Generators

Answer

Transformers

Answer

Capacitor banks

Question 4

A technician is performing maintenance on a generator's protection panel and encounters a relay designated as 51V. What is the function of this protective device?

Accepted Answer

Inverse time overcurrent protection with a voltage restraint feature.

Answer

Instantaneous overvoltage protection.

Answer

A synchronism check relay that verifies voltage match.

Answer

A directional power relay based on voltage.

Question 5

When testing a differential protection scheme for a power transformer (87T), which of the following is a critical check to ensure the relay remains stable for external faults (through-faults)?

Accepted Answer

Confirming correct CT polarity and ratio mismatch compensation.

Answer

Verifying the instantaneous trip setting is below the maximum fault current.

Answer

Ensuring the time-dial setting allows for coordination with downstream devices.

Answer

Testing the undervoltage blocking function.

Question 6

Which of the following protective functions is NOT typically applied directly to a medium-voltage induction motor?

Accepted Answer

Distance Protection (21)

Answer

Locked Rotor / Excessive Starting Time (51LR / 14)

Answer

Thermal Overload (49)

Answer

Differential Protection (87)