For large buildings and facilities that house sensitive loads, a resistance value should be of ____

Question

For large buildings and facilities that house sensitive loads, a resistance value should be of  _____________ Ω.

Examoogle · Accepted Answer

10 $\Omega$

Explanation: For large buildings, commercial facilities, and sites housing sensitive electronic equipment, the National Electrical Code (NEC) and IEEE standards recommend a grounding resistance of $10\,\Omega$ or less. This low resistance ensures that fault currents are dissipated into the earth rapidly, preventing potential damage to sensitive hardware.

Working Principle: The grounding system acts as a low-impedance path to the earth for transient surges and fault currents. According to Ohm's Law ($V = I 	imes R$), maintaining a low $R$ limits the potential rise ($V$) at the equipment chassis during a surge, thereby protecting sensitive electronic logic components from breakdown voltages.

Key Formulas: $V_{rise} = I_{fault} \cdot R_{ground}$ — Represents the voltage rise at the equipment frame during a ground fault.; $R = ho \frac{L}{A}$ — Resistance of a grounding electrode where $ho$ is soil resistivity.

Key Points: A resistance of $10\,\Omega$ is the industry standard for commercial and sensitive infrastructure.; Soil resistivity plays a significant role in achieving the desired $10\,\Omega$ target.; Sensitive loads require lower resistance to prevent electromagnetic interference and potential difference issues.; Periodic testing is mandatory to ensure the ground electrode impedance remains within limits.

Advantages: Minimizes step and touch potential for personnel safety; Provides stable reference voltage for sensitive electronic logic

Disadvantages: Achieving low resistance in high-resistivity soil is expensive; Grounding electrodes are subject to long-term corrosion

Applications: Data centers and server rooms; Large commercial office complexes; Medical diagnostic equipment facilities

Additional Info: General residential ground resistance is often recommended at $25\,\Omega$ where soil conditions allow.. Option B (15), C (5), and D (20) are incorrect because $10\,\Omega$ serves as the specific benchmark for sensitive commercial loads per standard engineering practice.

Answer: A is correct — The standard grounding resistance recommended for large facilities and sensitive electronic loads is $10\,\Omega$.

Exam Tip: Always remember that in electrical protection, lower grounding resistance ($R$) is inversely proportional to the safety margin for sensitive electronics during a surge.

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