Examination Practice Questions And

Illinois Substation Operator Licensing Examination Practice Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary purpose of a substation in an electrical power

system?

• To convert mechanical energy to electrical energy
• To transform voltage levels and distribute power
• To generate electricity from renewable sources
• To store electrical energy for long-term use

Rationale: Substations step voltage up or down and route power for

distribution and transmission; they do not generate or store energy.

• Which device is used to interrupt fault currents and isolate portions of

a substation?

• Current transformer 1 / 4

• Surge arrester
• Circuit breaker
• Potential transformer

Rationale: Circuit breakers interrupt current flow during faults and

are used to isolate equipment.

• A protective relay that compares current entering and leaving a

protected zone and trips when there's an imbalance is called:

• Overcurrent relay
• Differential relay
• Distance relay
• Frequency relay

Rationale: Differential relays detect differences between currents on

either side of equipment, indicating internal faults.

• What is the main function of a transformer tap changer?
• Measure transformer oil temperature
• Adjust transformer turns ratio to regulate output voltage
• Protect against lightning
• Switch grounding configuration

Rationale: Tap changers vary the turns ratio to maintain desired

secondary voltage under varying load conditions.

5. In substations, a CT (current transformer) is primarily used to:

• Step up voltage for transmission
• Insulate personnel from high voltage 2 / 4

• Provide scaled-down current for protection and metering
• Absorb transient overvoltages

Rationale: CTs produce proportional lower-level currents for relays

and meters while isolating high voltage circuits.

• Which grounding system is typically used to limit touch and step

potentials around a substation?

• Isolated neutral
• Ungrounded system
• Grid/mesh grounding system
• Floating grounding

Rationale: A grid or mesh grounding system spreads fault current

and reduces dangerous potentials at the surface.

7. The National Electrical Safety Code (NESC) primarily addresses:

• Building plumbing standards
• Safety requirements for utility electric supply and communication

lines

• Consumer appliance safety
• Food handling in utilities

Rationale: NESC sets safety standards for transmission/distribution

systems and associated work practices.

• Which relay type acts based on impedance (voltage/current) and is

used for transmission line protection?

• Differential relay 3 / 4

• Distance (impedance) relay
• Overcurrent relay
• Directional relay

Rationale: Distance relays measure impedance to detect fault

location on lines and operate based on V/I ratios.

• Arc flash boundary is the distance from an arc source within which a

person could receive a second-degree burn. Which standard provides arc flash guidance and labeling requirements?

A. NESC

• NFPA 70E (Standard for Electrical Safety in the Workplace)
• IEEE 1584 only
• OSHA plumbing standards

Rationale: NFPA 70E provides workplace electrical safety practices,

including arc flash risk assessment and labeling guidance.

• What is the purpose of a surge arrester in a substation?
• To control load flow
• To protect equipment from overvoltages (lightning, switching)
• To monitor harmonic distortion
• To filter power factor

Rationale: Surge arresters divert transient overvoltages to ground,

protecting insulation and equipment.

• When performing switching operations in a substation, the most

important step is:

• / 4