When installing an earth electrode it may not be easy to obtain an acceptably low value of earth electrode resistance, RA. This article looks at installations forming part of a TT system, the installation of the earth electrode and the use of multiple electrodes to achieve an acceptably low value of RA.
What is a TT System?
A TT system is defined in BS 7671 as a system having one point of the source of energy directly earthed, the exposed conductive-parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the source, see figure 1.
An example of the layout of an installation forming part of a TT system in a dwelling is shown in figure 2.
The need for a TT System
There are occasions when the electrical supplier will not provide a means of earthing with a new electrical supply to a building. This is quite acceptable, as regulation 24 (4) the Electricity, Safety, Quality and Continuity Regulations (ESQCR) states that if the supplier can reasonably conclude that it is appropriate, for reasons of safety, then a means of earthing could be refused for a new connection.
An example of this could be a house situated in the countryside which receives its electrical supply from an overhead line. The overhead supply consists simply of a phase and a neutral conductor. The supplier may state that, for safety reasons, a means of earthing for the consumer’s installation will not be provided.
The owner of the installation must then provide their own means of earthing. In this instance, an earth electrode, or electrodes, could be installed to provide the means of earthing and, hence, the installation becomes part of a TT system.
The legal requirements
The Electricity at Work Regulations 1989 require that earthing, or other suitable precautions are taken, to prevent danger arising when any conductor (other than a circuit conductor) which may reasonably foreseeably become charged as a result of either the use of a system, or a fault in a system, becomes so charged.
The reference to “other suitable precautions” means that some installations, or parts of installations, may operate without a connection to Earth and use another method as a means of protection against electric shock. However, in a dwelling, it is unlikely that any other method of protection would be used throughout the installation, therefore, to meet the requirements of the law, an electrical installation in a dwelling must be provided with a means of earthing.
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