Electricity + Control October 2017

EARTHING + LIGHTNING PROTECTION

within close proximity of one another. In power systems a typical example is the power sub- station Meshed electrodes are usually rectangular in shape and is constructed in such a way that the size of the rectangle corresponds to the size of the object area. In this way the earthing arrangement ensures a fa- vourable and approximately uniform, surface earth potential distribution throughout the object area. A significant advantage of the meshed elec- trodes is a favourable potential distribution in the earth’s surface. During an earth fault meshed elec- trodes increase the surface area that experiences a voltage rise as the result of current flow to the earth electrode. Although an equipotential exists over the area of the mesh, there is a potential gra- dient at the periphery of the electrode. Due to the mesh extending beyond any metal structure by more than one meter it is unlikely that there would be any touch potential during fault conditions, however, dangerous step voltages could occur. In order to avoid this phenomenon, the outer el- ements of the meshed earth electrode should be placed at a greater depth than the rest of the grid. A disadvantage of the meshed earth electrodes is that, due to the large area, which is covered by the grid system, it is not practical to bury the earth- ing system very deep, hence this type of system is more susceptible to changes in soil moisture content. Improved stability of resistance can be achieved by including a number of long vertical rods in the mesh. The resistance to earth of meshed electrodes is approximated to be proportional to the resistance of a plate electrode buried into the soil. Natural Earth electrodes Metallic pipes (e.g. water pipes) embedded in the ground as well as foundation earth electrodes, are sometimes also called ‘natural’ earth electrode systems.

of wire-mesh reinforcement thus ensuring ade- quate corrosion protection for the electrode. The electrode should be fastened to the reinforcement mesh with wire strands at intervals of not more than 2 m over the electrode length. It is not nec- essary to make a sound electrical connection at each point because the main electrical connection is via the concrete. If the foundation is constructed as separate panels connected to each other with expansion joints, the earth electrodes of each pan- el should be galvanically connected each other. These connections must be flexible and must be located so that they remain accessible for meas- urement and maintenance purposes. Using armored cable as earth An armored cable’s metal shield can be used as a means of providing leakage to earth with efficien- cy similar to that of a strip earth electrode. These types of installations are mainly used as earth electrodes in power electrical substations supplied with cable lines as well as other infrastructure net- works. Additionally, it can be connected with the foundation earth electrode of the substation build- ing. In other cases an additional earth electrode system may need to be installed in order to satisfy the fault current values. The impedance to earth of a single cable line depends on the cable length and the electric resistivity of soil. • Designed as a grid of strip or rods placed hori- zontally in the ground. Strip is normally the pre- ferred material as it has a larger surface for a given cross section and is considered to have superior performance at higher frequencies due to a slightly higher capacitance when in- stalled in soil • Mainly used in earthing arrangements where multiple devices in a specific area are installed Meshed electrodes Meshed electrodes are:

Evert Swanepoel

+27 (0) 11 824 3916 Evert.swanepoel@copperalliance.org.za

34 Electricity + Control

OCTOBER 2017