What is an earth pit?

Walk round most schools, churches and commercial buildings with a lightning conductor and, at the foot of each down conductor, you will usually find a small square or round lid set into the ground. Underneath is an earth pit, sometimes called an inspection pit or earth inspection chamber. It is the only part of the earth system you can easily see.

What is inside an earth pit

A typical pit is a short plastic, concrete or clay chamber around 300 mm across, set flush with the ground. Inside you will find:

• The top of the earth rod. A copper-bonded steel rod driven vertically into the ground, often in sections joined end to end so it can go deeper.
• A clamp. The connector that joins the rod to the conductor coming down from the building.
• The conductor. Usually copper tape or cable running from the test clamp on the wall, under the ground, to the rod.

Some sites have several rods in one pit, or rods joined to a buried ring conductor that runs right round the building.

Why the pit matters

During the annual lightning protection test, the tester measures how easily current flows from each electrode into the soil. BS EN 62305 recommends an overall earth resistance of 10 ohms or less, as Historic England’s guidance notes. The pit lets the tester check the connection for corrosion and damage, and gives a fixed point to measure from year after year.

If a pit cannot be found, the tester can still measure from the test clamp on the wall, but cannot see the condition of the connection below ground. Over time, a corroded clamp hidden under soil is one of the most common causes of a rising earth reading.

Lost pits

Pits go missing more often than you would think. Common causes:

A good tester will trace lost earths using the down conductors and a cable locator, and mark them on the drawing. If a pit is buried under hard surfacing, it may need to be dug out and a new chamber fitted. Ask for this to be priced separately on the report, because it can take time.

Electrical earth pits are different

Not every pit on a site belongs to the lightning protection system. Substations, generators, telecoms masts and some older buildings with their own electrical earth have separate pits. They may be bonded to the lightning earths below ground, but they are tested by electricians as part of the electrical installation. If you are not sure which is which, ask the tester to label each pit on the drawing.

Earth pit or earth rod?

The terms are used loosely. Strictly, the rod (or electrode) is the metal in the ground, and the pit is the chamber that gives access to its top. A system can have rods without pits, especially older ones, where the connection was simply buried.

How deep does an earth rod go?

It depends on the ground. Rods come in sections of around 1.2 m and are driven until the earth reading is low enough. In damp clay a few sections may do; in sand, chalk or rock it can take much longer, or need several rods spaced apart and joined together. BS 7430, the British code of practice for earthing, sets out how electrodes are installed and measured.

Looking after earth pits

• Keep lids visible and clear of soil, leaves and stored materials.
• Tell contractors doing ground works where the pits are before they start.
• Replace broken lids promptly so people cannot trip and the connection stays dry.
• Keep the drawing showing each pit with the test certificates.
• If a reading rises sharply after ground works nearby, ask for the connection to be checked.

What a tester records at the pit

On a good report, each pit has its own line: its reference on the drawing, the reading with the test clamp open, the condition of the connection, and whether the lid and chamber are intact. Some testers add a photograph of each pit. Over several years this gives you a picture of which earths are stable and which are drifting, so money goes on the electrodes that need it rather than on new rods round the whole building.

When to call a specialist

If your last report showed a high earth reading, an earth that could not be found, or a corroded connection, a specialist can investigate and put it right. Our page on earth electrode testing explains the measurement methods and the usual fixes. You can also see typical prices on the lightning protection cost page.

Ready to get your earths checked? Use the form below.