Medium-Voltage Power System Components

James R. White, Shermco IndustriesColumns, Summer 2021 Columns, Tech Quiz

The Tech Quiz questions for this issue concern medium-voltage power system components.


What is medium-voltage? Great question, as there are varying answers from NETA, NEMA, NFPA, and IEEE. NETA’s Standards Review Council has defined and classified medium-voltage within the NETA standards as “a class of nominal system voltages greater than 1,000 volts and less than 100,000 volts,” which is in accordance with NEMA C37.84.1, American National Standard for Electrical Power Systems and Equipment – Voltage Ratings (60 Hertz).

1. Corona causes tracking and discoloration of medium-voltage bus insulation and ionizes the surrounding air. Which of the following is most likely to cause corona?

a. Moisture on the insulation surface

b. Dirt on the insulation surface

c. Oil on the insulation surface

d. Small air gaps between the energized bus and the insulation

e. No air gaps between the energized bus and the insulation

2. Metal-enclosed, medium-voltage interrupter switches should be:

a. Stored energy operated

b. Motor operated

c. Closing coil operated

d. Non-loadbreak

e. Hook stick operated

3. Secondary control wiring on a medium-voltage vacuum circuit breaker is NOT used for:

a. Monitoring breaker position

b. Load current

c. Close operation

d. Shunt trip operation

e. Auxiliary devices

4. The red indicating lamp on a medium-voltage switchboard indicates that a circuit breaker is closed and that the dc system is on. What else may the lamp indicate?

a. Nothing

b. The circuit breaker closing coil circuit is complete, and the breaker is ready to open if a relay operates.

c. The circuit breaker trip coil circuit is complete if the light is connected in parallel with the tripping contacts.

d. The DC voltage is too low.

e. The last trip was caused by a relay operation.

5. What type of trip device is commonly used to trip open a medium-voltage circuit breaker where a battery source is not available?

a. Capacitor

b. Series

c. Shunt

d. Flux-shifter

e. AC coil



James (Jim) R. White, Vice President of Training Services, has worked for Shermco Industries Inc. since 2001. He is a NFPA Certified Electrical Safety Compliance Professional and a NETA Level 4 Senior Technician. Jim is NETA’s principal member on NFPA Technical Committee NFPA 70E®, Electrical Safety in the Workplace®, NETA’s principal representative on National Electrical Code® Code-Making Panel (CMP) 13, and represents NETA on ASTM International Technical Committee F18, Electrical Protective Equipment for Workers. Jim is Shermco Industries’ principal member on NFPA Technical Committee for NFPA 70B, Recommended Practice for Electrical Equipment Maintenance and represents AWEA on the ANSI/ISEA Standard 203 Secondary Single-Use Flame Resistant Protective Clothing for Use Over Primary Flame Resistant Protective Clothing. An IEEE Senior Member, Jim received the IEEE/IAS/PCIC Electrical Safety Excellence Award in 2011 and NETA’s Outstanding Achievement Award in 2013. Jim was Chairman of the IEEE Electrical Safety Workshop in 2008 and is currently Vice-Chair for the IEEE IAS/PCIC Safety Subcommittee.