Question 1

What is the ideal resistance to ground value for electrical systems?

Accepted Answer

Ideally, a ground should have zero ohms resistance.  However, practical standards, such as IEEE Std 80, recommend ground resistance below 5 ohms for substations to limit touch and step voltages during faults.  The National Electrical Code (NEC) requires that "man-made" electrodes shall not exceed 25 ohms.

Question 2

Which standards provide guidance on acceptable resistance to ground values?

Accepted Answer

Several key standards offer guidance on acceptable resistance to ground values.  IEEE Std 80 recommends values below 5 ohms for substations.  NFPA 70E and NEC 250.56 also provide requirements, with the NEC stating that man-made electrodes should not exceed 25 ohms.

Question 3

What are the common methods used to measure resistance to ground?

Accepted Answer

Common methods for measuring resistance to ground include the fall-of-potential method, which determines the resistance of the earth between the grounding electrode and a voltage test point.  The soil resistivity measurement method, using four stakes, is typically employed before installing an earthing arrangement.  These methods are often guided by IEEE standards for accurate measurement.

What is the Resistance-to-Ground?