Earth electrodes in parallel

I came across an article giving guidance to hams about antenna / station grounding, presumably for lightning protection.

The question is, what is the ground resistance improvement of one electrode over the two shown above. Let’s ignore the issue of earthing conductor size and deal only with the issue of parallel electrodes.

We don’t know the soil type, and we need to guess the spacing… it appears to be one house brick which is 9″ or 225mm in a lot of the world, perhaps that applies to the pic.

By way of an example, let’s make some assumptions that are likely to apply in lots of practical implementations.

Clay soils are common and a good wet clay soil may have soil resistivity around say 50Ωm.

If we assume that the ground is continuous and homogenous in all directions (although it isn’t quite due to the building footings), the resistance of a single electrode of say 12.7mm (1/2″) driven 1.5m (5′) is about 60% of the soil resistivity, or say 30Ω for our example scenario.

With two electrodes spaced so close, the simple formulas traditionally used don’t work, there is essentially no significant improvement in combined resistance.

If the electrodes were twice that distance apart (0.45m), 30% of the electrode length, the combined resistance is just 20% lower.

If the electrodes were 1.5m apart, 100% of the electrode length, the combined resistance is 40% lower… just over half the resistance of a single electrode.

You might jump to the conclusion that the OM has totally wasted his time, but that would be unfair. He is unlikely to have achieved a significant reduction in combined resistance, but he has provided some redundancy to protect against very local corrosion / failure of one of the electrodes.

If the electrodes were say 2m apart, the combined resistance under our assumptions of wet clay would be around 15Ω. That could have been achieved by a single rod driven to 3.6m in the same soil type.

In a practical application, the soil is not uniform, for clay soils the upper soil is drier than the deeper clay, and you might be lucky to achieve double the resistance or 30Ω for two 1.5m electrodes spaced 2m.

An advantage of the single deeper electrode (3.6m) is that it is less affected by surface dryness as more of the electrode is immersed in wet clay and so a practical 3.6m electrode might achieve a resistance better than 20Ω.

The discussion gives some guidance about effects that can be expected, but part of installing and commissioning a ground system is to measure the earth resistance. In my own case, a 2.4m long 19mm copper clad steel rod driven into quite wet clay consistently measures 12Ω (it is measured from time to time as part of station maintenance). If another identical electrode was driven, it needs to be 3m separated to achieve halving of combined resistance. A broad Rule of Thumb is minimum spacing is equal to driven length but it does depend on soil resistivity.