Earth Electrodes

Earth Electrodes

The earth electrode is the component of the grounding system which is in direct contact with the ground and thus provides a means of releasing or collecting any earth leakage currents. In earthed systems it will normally be required to carry quite a large fault current for a short period of time and so will need to have a cross-sectional area large enough to be able to carry this safely. Electrodes must have adequate mechanical and electrical properties to continue to meet the demands on them over a relatively long period of time, during which actual testing or inspection is difficult. The material should have good electrical conductivity and should not corrode in a wide range of soil conditions. Materials used include copper, galvanised steel, stainless steel and cast iron. Copper is generally the preferred material for reasons described later. Aluminium is sometimes used for above ground "bonding", but most of the standards forbid its use as an earth electrode due to the risk of accelerated corrosion. The corrosive product - an oxide layer - is non-conductive, so could reduce the effectiveness of the grounding. The electrode can take a number of forms. These include vertical rods, plates and horizontal conductors, the most common of which are described below.

Rod Electrode:

These are usually of solid copper or copper-clad carbon steel, the letter being used for the large-diameters rods with extensions facilities. These facilities comprise: a thread at each end of the rod to enable a coupler to be used for connection of the next rod; a steel cap to protect the thread from damage when the rod is being driven in; a steel driving tip; and a clamp for the connection of an earth tap or conductor

The choice of length and diameter of such a rod will, as previously mentioned, depend on the soil conditions. For example, a long thick electrode is used for earth with little moisture retention. Generally, a 1 – 2 m rod, 16 mm in diameter, will give a relatively low resistance

Rod electrode

Plates:

There are several types of plate used for grounding purposes, but the only type which is generally considered as an electrode would be solid and of substantial size. Lattice type plates, as illustrated in Figure 4-8 are used for potential grading and would not be expected to pass significant amounts of fault current. They are normally made of copper or steel mesh.

Grounding Plates

Earth Plates (courtesy A N Wallis and Co):

Plate electrodes are of copper or ribbed cast iron. The cast iron plates normally are a minimum of 12 mm thick and either 915 mm or 1,220 mm square. Copper plates are typically 600 mm to 900 mm square and between 1.6 mm and 3 mm thick; Where multiple plates are used, they must be some distance apart to prevent any interaction.

Normally this is a minimum of 2 m, possibly extending to 9 m.