How to test earth pit resistance, and when to do it

Why the test matters

An earth pit can look perfect and still fail. Conductors corrode at the joint, a clamp loosens, a strip gets cut during later digging, or the soil around the electrode dries out. The only way to know the pit still performs is to measure it, and to compare the result with earlier ones.

What you need

• An earth resistance tester (three- or four-terminal) with its test leads
• Two auxiliary steel spikes
• A measuring tape
• Spanners to disconnect the test link
• A test register or sheet

The fall-of-potential method, step by step

1. Isolate the electrode. Open the test link or disconnect the earthing conductor so you measure only this electrode, not the whole bonded network. Follow your site's safety procedure. On live installations, the electrode may carry current, so take appropriate precautions.
2. Place the current spike (C or H). Drive it in a straight line away from the electrode, far enough that the two "resistance zones" do not overlap. A common rule of thumb is at least five times the size of the earthing system under test. For a single 3 m electrode, that often means 20 to 30 m or more.
3. Place the potential spike (P or S) on the same line at about 62% of the electrode-to-current-spike distance. With the current spike at 30 m, the potential spike goes at about 18.6 m.
4. Connect and measure. Electrode to E, potential spike to P/S, current spike to C/H, as marked on your instrument.
5. Check the reading. Move the potential spike about 10% of the distance closer, then 10% further, and measure again. If the three readings are close, the result is reliable. If they differ a lot, the spikes are too close together. Move the current spike further out and repeat.
6. Reconnect the electrode and record the result.

The 62% position comes from the theory behind the method, described in IEEE 81. It assumes reasonably uniform soil and enough spacing between the electrodes. Pipelines, rebar and other buried metal near the test line can distort readings, so run the test line away from them.

Clamp-on testers

A clamp-on tester measures without spikes or disconnecting the electrode. It works only where the electrode is part of a network with several parallel earth paths, such as a group of interconnected pits or a utility multi-earthed neutral. On a single isolated electrode it gives no meaningful result. Clamp testers are useful for quick routine checks across many pits. Use fall-of-potential for commissioning and for any disputed reading.

What value should you get?

There is no single number for every installation. Required values depend on the type of installation, its voltage level and fault current, and the project specification or inspecting authority. Lightning protection and sensitive equipment often call for lower values than general LV earthing. Take the target from your specification or consultant. If one pit cannot reach it, the usual remedies are:

• going deeper,
• adding electrodes in parallel, spaced apart so their resistance zones do not overlap,
• using an earth enhancement compound in the bore.

When to test

• At commissioning, before the pit is handed over.
• Periodically: at least once a year for most installations, more often on critical sites.
• In the dry season. Readings are highest then, so a pit that passes in May will pass in August. The reverse is not true.
• After any change: excavation nearby, a lightning strike, or a fault event.

Keep a register

For each pit, record:

• pit ID and location,
• date, weather and soil condition (dry or wet),
• instrument, method and spike distances,
• the reading,
• who tested it.

A rising trend over two or three years is the early warning you want. Numbered pit covers and a clear test link make this much easier. Our earth pit covers keep the test point accessible.