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Earth electrode testing: method E1 and the geometryWhere does the spike go?

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Earth electrode testing: method E1 and the geometry

Guidance Note 3 · 2.6.13 · pp.78–79. On a TT system the DNO provides no earth, so the electrode has to be proved on its own. Method E1 is the most accurate — and the most time consuming.

✋E1 is a DEAD test. You disconnect the earthing conductor, so the main switch must be isolated and locked off. Leave the earthing conductor connected and every circuit in the board gives you a parallel path that ruins the reading.

The layout, with the current spike 12 m from the electrode as the example:

Current spike 12 m away

Distance to the current spike  = 12 m
Voltage spike, first position   = halfway = 6 m

Then move the voltage spike 10% of the electrode-to-current-spike distance each way:
  10% of 12 m = 1.2 m  ->  readings 2 and 3, at 4.8 m and 7.2 m
The book's example moves the spike 3 m because its current spike is 30 m away: 10% of 30 m. Same rule — scale the 10% to your electrode-to-current-spike distance, not the diagram's.
You need the ground for it. The book's example puts the current spike 30 m from the electrode, which is why TT tends to be rural.
The kit. Not earth rods — T-bars, knocked in just far enough to clamp onto. The longest (red) lead goes to the current spike, the blue to the voltage spike, the green straight onto the electrode. It is expensive; do not buy it unless you really do electrode testing.
ground electrode 1.2 m deep voltage spike current spike ± 1.2 m — readings 2 and 3 6 m — halfway 12 m — electrode to current spike 10% of 12 m = 1.2 m scale it to YOUR distance
A dead test — the earthing conductor comes off, so the main switch is isolated and locked. The book’s example has the current spike 30 m away, so it moves 3 m; scale the 10% to the distance you actually have.

Averaging the three readings, and the accuracy rule

Three readings, repositioned each time, then averaged.

The worked example

Reading 1 = 100 Ω
Reading 2 = 101 Ω
Reading 3 = 99.5 Ω

Total     = 300.5 Ω
Average   = 300.5 ÷ 3 = 100.17 Ω

The book does not accept readings that differ by more than 5 per cent.

Is it acceptable?

Guidance Note 3 · p.79: the largest deviation of any reading
from the average, as a % of the average.
Reading furthest from the average = 101 Ω
101 − 100.17 = 0.83 Ω
0.83 ÷ 100.17 = 0.008, so 0.8%
Accuracy ≈ 1.2 × 0.8% = about 1%
Readings differing by more than 5% are not accepted — this is well inside.
If you are outside 5%, change the test, not the result. Extend the distance between the electrode and the current spike, or drive the spikes in deeper. A different method, such as E3, is the other option, though it may not be as accurate.

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