Electrical Equipment & Maintenance Flashcards

What is the primary reason for performing a core ground test on a power transformer?

Answer: Detect multiple core ground points that cause circulating currents and overheating

A transformer core should be grounded at only one point; multiple ground paths create a closed loop where circulating currents cause localized overheating.

When a soft starter is used with an induction motor, what happens to the starting torque compared to across-the-line starting?

Answer: Starting torque is reduced because torque varies with the square of voltage

Motor torque is proportional to V², so reducing voltage to 50% reduces starting torque to 25% of full-voltage torque.

In a battery backup system (UPS), what is indicated by excessive equalization charging voltage causing electrolyte to bubble vigorously?

Answer: Overcharging is occurring, causing water electrolysis and gassing

Excessive charge voltage electrolyzes water in the electrolyte into hydrogen and oxygen gases, causing vigorous bubbling and water loss.

During scheduled maintenance, a technician finds that an air circuit breaker's trip time during testing is significantly longer than specified. The MOST likely cause is:

Answer: Worn or sluggish trip mechanism components requiring lubrication or replacement

Slow trip times indicate mechanical wear, corrosion, or dried lubrication in the trip mechanism that prevents rapid contact separation.

What is the purpose of installing surge protective devices (SPDs) on motor control circuits?

Answer: Protect sensitive control electronics from voltage transients and lightning surges

SPDs clamp voltage transients from switching operations and lightning to safe levels, protecting PLCs, drives, and control components from damage.

A technician measures rotor bar currents using a clamp-on ammeter around individual rotor bars on a wound-rotor motor. Unequal readings between bars indicate:

Answer: Cracked or broken rotor bars causing unequal current distribution

Broken or cracked rotor bars have higher resistance, forcing adjacent bars to carry more current, resulting in unequal readings and increased vibration.

When de-energizing high-voltage switchgear for maintenance, what is the correct sequence after opening the main breaker?

Answer: Lock out/tag out, verify absence of voltage, install grounds

NFPA 70E and OSHA procedures require LOTO to be applied before testing, then voltage absence is verified, and finally protective grounds are installed.