When carrying out preliminary electrical checks what does a resistance to earth test check?

Ground resistance measurements are important to insure the resistance of the grounding system meets specifications.

We use various equipment by AEMC Instruments. Go to Ground Resistance Testers listed on the left menu.


National Electric Code requires a maximum 25 ohms or less. This value has been determined to be the maximum allowed for a safe system, as a minimum. A simple instrument for basic installations is the clamp on unit.

Many times specifications of an particular installation require much less, such as data centers, telephone and cell phone equipment, transmitter systems, substations, and certain process equipment.

Often,even though we install a grounding system as per specifications, part of the specs is to measure the resistance to insure the system designed is working properly and meets or exceeds specs.

The condition of the soil is the wildcard. If specs cannot be met, then more ground rods would be needed,as there is nothing that can be done to the soil.

We recently were hired to do acceptance testing of a grounding system for a cell site headend installed by another contractor using the Three Point Fall of Potential method.

It Had four 50 foot deep ground rods at four corners of a 200 foot by 100 foot pattern, made by Lyncole XIT Grounding - XIT Grounding System, Electrical Grounding, Ground Resistance Meters.

The specs were no more that 3 ohms allowed, and found the system to be at 2.8 ohms. The system passed and turned over to the owner.

There is a great deal of information on the web, Google for "ground resistance".