How to test earth electrode resistance with an MFT

Helpful video reference. Megger's "How to do Earth Electrode RESISTANCE Test! Megger MFT X1 Tutorial" demonstrates the full fall-of-potential procedure using a Megger MFT. Megger is a UK test equipment manufacturer whose instruments are standard issue on UK installations. The video is technically accurate and specific to UK practice.

1. Understand what you are measuring

Earth electrode resistance (Ra) is the resistance between the surface of the earth rod and remote earth (the general mass of earth far enough away to be unaffected by the current injected during the test). It is not the same as the earth fault loop impedance you measure with the MFT in Zs mode during a standard test sequence.

The relevant regulation is BS 7671 Regulation 411.5.3, which states:

Ra × I∆n ≤ 50 V

Where Ra is the electrode resistance in ohms and I∆n is the rated residual current of the RCD in amps. For a standard 30mA (0.03A) RCD protecting a socket circuit, this means Ra must be 50 ÷ 0.03 = 1,667Ω or less. That sounds generous, but a reading of 1,500Ω gives almost no margin. Most electricians and inspection guides use 200Ω as a practical target, with values below 100Ω considered good.

In sandy or chalky soils like much of east Kent, electrode resistance can be higher than in clay-heavy ground, particularly in dry summers. This is worth factoring into the test timing.

2. Gather your equipment

You need:

• A multifunction tester with a dedicated earth electrode resistance test function. This is usually a separate 3-terminal socket on the MFT, marked E (electrode), P (potential) and C (current), or similar. Not all MFTs include this - check before heading to site. Common models that do include it: Megger MFT1741, Megger MFT1731, Fluke 1664FC, Kewtech KT64DL, Martindale MFTE20.
• Two steel auxiliary test spikes, 200mm or longer. These come with most MFT earth test accessory kits.
• Long test leads for the electrode test, usually green (earth/electrode), red (current C2) and yellow (potential P2). The leads need to be long enough to reach the spike positions - 30m each is common.
• A steel measuring tape, 25m or longer.
• A hammer for driving the spikes.

3. Temporarily disconnect the electrode from the installation

Isolate the installation at the consumer unit and test dead. Then disconnect the earthing conductor from the earth electrode terminal. Many earth electrodes have a clamp or bolted connection at the top of the rod where it enters the ground or an inspection pit - this is where the conductor disconnects.

Keep the disconnected earthing conductor safe and out of the way. The installation has no earth path now. Do not restore power until the conductor is reconnected after the test.

4. Drive the auxiliary current and potential spikes

The standard UK method for positioning auxiliary spikes follows the 62% rule from BS 7430:

1. Measure or estimate the depth of the earth electrode (D). For a standard 1.2m copper-clad steel rod, D = 1.2m.
2. Drive the current spike (C2) in a straight line away from the electrode at a distance of at least 10 × D. For a 1.2m rod, that is at least 12m away. In practice, 25m to 30m gives better results in typical UK soil conditions.
3. Drive the potential spike (P2) in a straight line between the electrode and C2, at 62% of the distance to C2. So if C2 is 25m away, P2 goes at 25 × 0.62 = 15.5m from the electrode.

All three points (the electrode, P2 and C2) should be in a straight line. If site constraints force a triangle arrangement, the error increases - note this in your records.

5. Set up the MFT and run the measurement

Connect the test leads to the MFT terminals:

• Green lead: from the E terminal on the MFT to the earth electrode (or the disconnected earthing conductor connected to it).
• Yellow lead: from the P terminal on the MFT to the potential spike P2.
• Red lead: from the C terminal on the MFT to the current spike C2.

Select the earth electrode resistance function on the MFT. Press the test button and hold. The MFT injects a small AC test current (typically around 25mA at a frequency not synchronised with mains, to avoid interference) between the electrode and C2, and measures the resulting voltage between the electrode and P2. It divides voltage by current to calculate resistance and displays the result in ohms.

Read the result and note it down.

6. Verify with spike transpositions (the 62% check)

A single reading can be affected by underground metalwork, services or resistivity variations in the soil. The 62% check confirms the result is valid:

1. Move P2 5% of the electrode-to-C2 distance closer to the electrode. Re-test and note the reading.
2. Move P2 5% further from the original 62% position (now 67% of the electrode-to-C2 distance). Re-test and note the reading.

If all three readings are within approximately 10% of each other, the 62% method has given a valid result and the middle reading (at the 62% position) is your Ra value. If they vary more than that, the spikes need to be spaced further apart - the resistance zones of the electrode and C2 are overlapping. Increase the electrode-to-C2 distance and repeat.

7. Reconnect and record the result

Reconnect the earthing conductor to the earth electrode and tighten the clamp securely. Restore power at the consumer unit.

Record the following in your test results:

• Measured Ra value in ohms.
• Earth electrode depth and type (e.g. 1.2m copper-clad steel rod).
• Spike spacings used (electrode to P2, electrode to C2).
• Soil conditions at time of test (wet/dry, soil type if known).
• Confirmation that Ra × I∆n ≤ 50V for the RCD installed.
• Date of test and MFT serial number.

If Ra exceeds the calculated limit for the installed RCD, the electrode is inadequate. Options are: driving the rod deeper, driving an additional electrode and connecting them in parallel, or treating the soil with bentonite clay to lower resistivity. If in doubt, call an electrician to advise.

When to call us

Earth electrode testing is part of every EICR on a TT installation and every initial verification on a new outbuilding supply. Richard tests and certifies earthing arrangements across Sandwich and east Kent, and can advise on improving a poor result.