Question 1

What is an acceptable earth ground resistance value?

Accepted Answer

The acceptable earth resistance value depends on the application.  IEEE 80 and IEC 60364 define limits by use: less than 1 ohm for substations and critical infrastructure, 5 ohms for commercial buildings, and 25 ohms for residential installations.  Telecommunications sites (Motorola R56, AT&T) typically require 5 ohms or less.  When resistance exceeds the target, engineers use parallel ground rods, chemical ground enhancement, or deep-driven electrodes to reduce it.

Question 2

Is 5 ohms or 25 ohms the correct earth resistance target?

Accepted Answer

Both values are correct for different applications.  The NEC (NFPA 70) specifies 25 ohms as the maximum for a single ground rod before a supplemental electrode is required.  IEEE 80 recommends less than 1 ohm for substations.  Telecom standards such as Motorola R56 and AT&T 7510 require 5 ohms or less.  Always consult the applicable standard for your specific installation type.

Question 3

How do you measure earth resistance?

Accepted Answer

Earth resistance is most commonly measured using the fall-of-potential (three-point) method per IEEE Std. 81, or the Wenner four-pin method for soil resistivity.  A ground resistance tester (earth tester) injects a known current and measures the resulting voltage drop.  Clamp-on testers can measure resistance without disconnecting the electrode, but are less accurate in complex grounding systems.