Douglas Krantz - Technical Writer - Describing How It Works

Shouldn't the Ground Rods be the Same Resistance?

The ground rods driven into the earth are each measured separately, and it's the soil that is being measured for resistance, not the rod.

Shouldn't the Ground Rods be the Same Resistance?

Greetings Douglas,

I put down two eight-foot grounds, separated by 16 feet. They are connected with large diameter braided copper cable.

At one rod, I get a reading of 120 ohms, and at the second rod, I get a reading of 32 ohms.

Shouldn't they both read the same ohms?

Thank you, SH

I assume that the grounding-rods are installed according to the National Electrical Code (NEC) guidelines, and that the connecting cable (bonding wire) is installed according to those guidelines. The guidelines also give an exact method for measuring the ground resistance.

The grounding-rods themselves are large electrical conductors. The resistance of each rod, from one end to the other is essentially zero ohms; for all practical purposes, your ohmmeter will say zero ohms for each rod. They are connected using a large diameter braided copper cable, which your meter will also show as zero ohms.

When measuring resistance, however, it's not the resistance of the grounding-rod or bonding wire that's being measured, it's the resistance of the surrounding soil in the ground that's being measured.

Measurement Method

When measuring ground resistance, there's the 120-ohm grounding-rod, there's the 32-ohm grounding-rod, and there's a measurement grounding-rod, or likely several measuring grounding rods. There's also a chunk of soil in the earth between each grounding-rod.

When considering what is being measured, there are three resistors in series: the contact between one grounding-rod and its surrounding soil, the soil in the earth between the grounding-rods, and the contact between the other grounding-rod and its surrounding soil.

However, it's really the resistance of this soil in the earth, or ground, that's important. Even though the soil looks to be the same all over the surface you're standing on, between each grounding-rod and the measurement grounding-rod, the chunks of soil are different.

When measuring the grounding-rod resistance, it's the contact-soil-contact resistance that's being measured. However, so that the grounding-rods don't electrically interfere with each other during the tests, the bonding wire is temporarily disconnected from both grounding-rods. That disconnection makes each grounding-rod independent.

Reason to Measure in the First Place

If all types of soil in the earth had close to zero ohms of resistance, there'd be no reason to measure ground rod resistance. That's not the case. Different soil types have different resistance. Dry desert sand has a very high resistance; wet salt marsh muck has very low resistance. Most soil types have resistance that's somewhere in-between.

When the measurements are performed, the measurements are used to make sure that the grounding-rod, or combined grounding-rods, have a low enough resistance to the earth to make a good electrical ground connection.

Why are the Grounding-Rod Resistance Measurements Different?

The reason that the resistance measurements are different for each of the two grounding-rods is that the soil, under the surface, is different for each of them.

Parallel Resistors

Once they are connected together with the braded copper bonding wire, the two grounding-rods and the surrounding soil can be considered to be electrical resistors in parallel. For your installation, the combined total resistance for the two, bonded-together, grounding-rods is a little under 32 ohms.

Douglas Krantz
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