Earth rod: which type, size and clamp to use

What an earth rod does

An earth rod is the most common form of earth electrode: the buried metal that connects the main earthing terminal of a building to the soil. The UK wiring regulations, BS 7671, also accept tapes, plates and the welded reinforcement of concrete — the steel rebar in a foundation — as electrodes, according to the IET’s guide to TT earthing.

For rod earthing, depth decides most of the result. The IET names installation depth as the main factor in a rod’s resistance to earth and advises driving it deep enough to reach the water table, so that it still works when the topsoil dries out.

Types and sizes of earth rod

ABB’s Furse earth electrode catalogue sells three kinds of earthing rod:

Types and sizes of earth rod

Type	Build	Diameters	Lengths	Where the catalogue places it
Copper-bonded steel	high-tensile low-carbon steel core, 99.9% copper bonded on, at least 250 microns	12.7, 14.2, 17.2 mm (1/2, 5/8, 3/4 inch nominal)	1,200–3,000 mm	general earthing; can be driven deep with a power hammer
Solid copper	copper throughout	15, 20, 25 mm	1,200–3,000 mm	very aggressive soils, such as ground with a high salt content
Stainless steel	stainless throughout	16, 20 mm	1,200–3,000 mm	where other buried metals nearby would cause galvanic corrosion

Weights give a sense of scale: a 14.2 mm copper-bonded rod 1,200 mm long weighs 1.53 kg, and a 17.2 mm rod 2,400 mm long weighs 4.35 kg, per the same catalogue.

Copper earth rod or 16 mm stainless

A copper earth rod in shop listings can mean either a copper-bonded steel rod or a solid copper one. Furse’s copper-bonded rods are not sheathed — the copper is bonded to the steel — and the high-tensile core is what lets them take power-hammer driving. The catalogue keeps solid copper for salty or chemically harsh ground.

A “16 mm earth rod” may be a true 16 mm rod or a 5/8 inch one, since 5/8 inch is 15.9 mm (our calculation: 0.625 × 25.4). Check the catalogue size before buying clamps: the Furse 5/8 inch copper-bonded rod measures 14.2 mm, while its stainless rod is a true 16 mm.

Couplings, driving heads and earth rod clamps

Rods are joined end to end to go deeper. Furse makes its couplings from a high-copper alloy rather than commercial brass, and its driving studs and spikes from hardened steel; two 1,200 mm rods with one coupling make an 8 ft (2,440 mm) electrode.

The cable from the building is fixed to the top of the rod by a mechanical earth rod clamp, a compression fitting or exothermic welding, the catalogue says. Buy the earth rod and clamp as a pair sized to the same diameter, and leave the joint reachable — Furse sells inspection pits for this, with lids rated 1,200 kg (concrete) and 5,000 kg (polymer). On a TT supply, the earthing system that earths the steel armour of an armoured cable through its gland ends at this rod.

Resistance: the 200 Ω mark

Note 2 to Table 41.5 of BS 7671, quoted by the IET, says that an electrode resistance above 200 ohms may not be stable. On a TT supply, where the rod is the installation’s only earth, the IET adds that an RCD must provide shock protection. These are UK figures, given as a reference point rather than as Tanzanian requirements.

Where the reading stays high, Furse offers bentonite backfill — a sodium-activated clay that swells when wet and holds moisture around the rod — and a conductive concrete mix that widens the electrode’s contact with the soil.