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Earth rods, the deviation calculation, and RA80, 82 and 84 ohms. What's the deviation?

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The question at the end: 80, 82 and 84 ohms. What's the deviation? The answer is in the notes below.

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Earth rods, and the three methods again

The rod itself is a threaded bar driven into the ground with a cover at the top. It can be as long as it needs to be — one metre, two, five — and rods can be coupled together to go deeper.

Soil decides. Hard ground that still needs a TT system may mean driving in several.
MethodIn a sentence
E1The most accurate — and hard graft in difficult soil
E2More accurate if there are a number of earth electrodes; the clamp needs at least one other earth path
E3An earth fault loop impedance test. The easiest, and the one you will do — but it may not be as accurate as a dedicated earth electrode tester
Easiest is not most accurate. E3 being the common choice does not make it better than E1.

The deviation calculation

Guidance Note 3 · p.79. This is the step after it, and it is the part that gets skipped.

Three readings: 64, 62.5 and 65.3 Ω

Step 1 — the average
  64 + 62.5 + 65.3 = 191.8
  191.8 ÷ 3 = 63.93 Ω
✋If you got 191.8, you forgot to divide by three. An average is the total divided by how many readings you took.

Step 2 — the percentage deviation

Find the reading furthest from the average.
Here that is 62.5 (1.43 below), not 65.3 (1.37 above):

  63.93 − 62.5   = 1.43
  1.43 ÷ 63.93  = 0.0224
  0.0224 × 100  = 2.24% → 2.2%

Step 3 — the 1.2 factor

Guidance Note 3: the accuracy of a method E1 measurement is
typically 1.2 × the percentage deviation you calculated.

  2.24% × 1.2 = 2.69% → 2.7%
Which reading will be highest, and why. Test 1 is taken at the centre. Test 2 moves the voltage spike towards the electrode by 10% of the full electrode-to-current-spike distance (current spike at 30 m: move 3 m), so you expect a lower resistance. Test 3 moves it the same distance the other way, so you expect a slightly higher one — which is usually your highest figure.
Three readings 64 62.5 65.3 62.5 is furthest from the average 1 average = (64 + 62.5 + 65.3) ÷ 3 = 191.8 ÷ 3 = 63.93 Ω ✋ if you got 191.8, you forgot to divide by three 2 gap ÷ average = (63.93 − 62.5) ÷ 63.93 = 0.022 → 0.022 × 100, so the spread is 2.2% Limit is 5% — 2.2% passes. Above 5% and the spikes move further out, then you test again.
This is the step after it, and it is the one people skip — the average on its own tells you nothing about whether the three readings agreed.

RA: the electrode result, and where it goes

Method E1 gives you an RA result — unless it has been handed to you as a Ze, in which case take it as RA.

RA = the electrode plus the earthing conductor back to the MET

A second worked deviation, with cleaner numbers

Readings: 80, 82, 84 Ω

Average  = 246 ÷ 3 = 82 Ω
Furthest   = 80 and 84, both 2 away
Deviation  = 2 ÷ 82 = 0.024  →  2.4%

Always use the reading furthest from the average.
Where it goes on the EIC. On page 2, tick earth electrode as the means of earthing and TT as the nature of earthing, name the electrode (earth rod, copper rod), and find the field marked RA or Ze — cross out the one that does not apply and write your value in.

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