Electrical Flashcards

A 480V, 3-phase motor draws 18A under full load. The power factor is 0.82 lagging. What is the true power consumed by the motor?

Answer: 14,965 W

True power (watts) = √3 × V × I × PF = 1.732 × 480 × 18 × 0.82 ≈ 14,965 W. The √3 factor applies to balanced 3-phase systems. Multiplying line voltage × line current × √3 gives apparent power (VA), and multiplying by power factor converts to true (real) power in watts.

When megohm-testing a motor winding, a technician gets 0.5 MΩ at 20°C. Using the standard temperature correction formula, what is the corrected resistance at the reference temperature of 40°C?

Answer: Approximately 1.0 MΩ

Insulation resistance roughly doubles for every 10°C decrease in temperature (or halves for every 10°C increase). Going from 20°C to 40°C is a +20°C rise, so the resistance measured at the higher reference temperature would be approximately 0.5 × 2 = 1.0 MΩ. Cold readings appear artificially high; correcting to 40°C normalizes comparisons.

A control circuit uses a normally closed (NC) overload relay contact wired in series with a start coil. During commissioning, the overload relay trips but the motor contactor STILL energizes. What is the MOST likely cause?

Answer: The NC overload contact is wired in parallel rather than in series with the coil

If the NC overload contact is wired in parallel with the coil circuit instead of in series, opening that contact has no effect — current can still reach the coil through the parallel path. This is a wiring error that bypasses the protective function of the overload relay entirely, allowing the contactor to energize even when the overload has tripped.

A technician measures 120V across a load that should see full voltage, but the load operates weakly. Probing reveals 120V at the source terminal and 120V at the load terminal — both measured to the same neutral. What does this most likely indicate?

Answer: An open neutral between the two measurement points

Measuring 120V at both the source and load terminal relative to the SAME neutral, yet having a weak load, is a classic symptom of an open neutral. With an open neutral, the voltmeter completes the circuit through the load itself, giving a valid voltage reading even though insufficient current flows to operate the load properly. The voltage reading is misleading — it does not confirm current flow.

According to NFPA 70E, what is the MINIMUM approach boundary for exposed energized conductors at 480V where an arc-flash hazard study has NOT been performed?

Answer: The 'table method' default of 1.2 cal/cm² PPE at the arc-flash boundary applies, which is 4 feet for 480V equipment

NFPA 70E permits using the arc-flash PPE category table method as an alternative to an incident energy analysis. For 480V switchgear and panelboards under specific conditions, the table assigns PPE Category 1 (minimum 4 cal/cm²) with an arc-flash boundary of 4 feet. Workers must still wear appropriate PPE (arc-rated clothing at the required cal/cm² rating) and observe the restricted and limited approach boundaries.

A delta-wye transformer bank is feeding a non-linear load that causes significant 3rd-harmonic current. Where will the 3rd-harmonic current primarily circulate, and why?

Answer: It will circulate within the delta primary winding and not appear on the primary line conductors

Third harmonics (and all triplen harmonics — 3rd, 9th, 15th…) are zero-sequence currents. In a delta winding, zero-sequence currents circulate inside the delta loop itself rather than flowing out onto the line conductors. This is one of the primary reasons delta-wye transformers are used to isolate triplen harmonics from propagating back to the utility, keeping them trapped in the delta.