RCDD Question #134: Real Exam Question with Answer & Explanation

The correct answer is B. Oversized neutral and grounding conductors. Switching power supplies are non-linear loads that produce harmonic currents, which accumulate on the neutral conductor rather than canceling out, requiring oversized neutral and grounding conductors to prevent overheating.

Question

You are designing a large equipment room (ER) that will house many servers, switches, backup storage drives, and other communications equipment including peripheral devices. To prevent problems from harmonic currents that can be created by switching power supplies, you should arrange for:

Options

AIsolated grounding
BOversized neutral and grounding conductors
CSurge protection on power supply
DA 3-phase feeder supply separate from the building supply
EPower filtering at the service panel

Explanation

Switching power supplies are non-linear loads that produce harmonic currents, which accumulate on the neutral conductor rather than canceling out, requiring oversized neutral and grounding conductors to prevent overheating.

Common mistakes.

A. Isolated grounding reduces noise coupling and ground-loop interference but does not address the overcurrent hazard on neutral conductors caused by harmonic accumulation.
C. Surge protection on power supplies mitigates transient voltage spikes; it does not reduce or handle sustained harmonic current loading on the neutral conductor.
D. A separate 3-phase feeder isolates the ER load from the building distribution but does not resolve the internal harmonic current buildup on the neutral conductor within the ER.
E. Power filtering at the service panel can attenuate incoming harmonics but is not the standard primary countermeasure for harmonic-related neutral overloading inside an equipment room.

Concept tested. Neutral conductor oversizing for harmonic current from switching power supplies

Reference. https://www.nfpa.org/codes-and-standards/nfpa-70-standard-for-the-national-electrical-code

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