Earth electrode testing

Submarine ieiecomthunications cables operate at less than 1 A but sub-. marine power cables may operate at several hundreds Of amperes and will require larger spacing between the earth, electrodes and nearby burled or immersed structures. IfpOssible, testsShould be made with trial earth elfec. trOdes at both ends of. the submarine cable and on Off tests made before bringmg the power supply bii the system into permanent Use., ... 

Earth electrodes connected to the earth terminal of an electric fence controller must be separate from the earthing system of any other circuit and should be situated outside the resistance area of any electrode used for protective earthing. The electric fence controller and the fence wire must be installed so that they do not come into contact with any power, telephone or radio systems, including poles. Agricultural and horticultural installations should be tested and inspected in accordance with Part 6 of the Wiring Regulations every three years. 

Disconnect the installation protective equipotential bonding from the earth electrode to ensure that the test current passes only through the earth electrode. 

Using a line earth loop impedance tester, test between the incoming line conductor and the earth electrode. 

The IT supply system is the same as a TT system (a TT installation is one in which the earth is derived from a local earth electrode rather than from the suppher s supply cable - see Chapter 10) but either without a source earth or with earthing through a high impedance. It is not legal for public supplies but may be used in private installations. The system is monitored to provide visual or audible warning of an earth fault. The applications include continuous processing plants and medical intensive care apparatus where the supply needs to be maintained even if an earth fault should occur. It is occasionally used to supply test facilities as it affords protection against direct shocks. As described for electrical separation , which is a form of IT system, a direct shock hazard arises only if there is an earth fault, and an... 

If the supply company provides an SNE (separate neutral and earth) service from its LV network, it should permit the use of its own earth terminal for the site earth and ought to be able to estimate the earth loop impedance at the intake to enable the designer to proceed with the calculations. If the service is CNE (combined neutral and earth) the supply company cannot legally permit the use of its CNE terminal for earthing and the contractor will have to provide a site earth electrode and terminal, in which case the value of Zg cannot be determined without site testing. 

In lEE Guidance Note No. 3, an alternative test method is given for RCD-protected TT installations. This consists of connecting an earth loop impedance tester between the earth electrode and the phase conductor of the installation, at the intake. Before doing this test, all equipotential bonding... 

Test the earth electrode resistance where the installation incorporates an earth electrode as a part of the earthing system. 

The basic corrosion instrumentation requirement involves the measurement of potential difference. Currents are measured as the potential across a resistor (R ) as shown in Fig. 1.2, where the potential difference is again determined with an operational amplifier. More sophisticated measurements such as polarisation characteristics and zero resistance ammetry involve the use of potentiostats which again use operational amplifiers in a differential mode. The potentiostat is an instrument for maintaining the potential of an electrode under test at a fixed potential compared with a reference cell, and the basic circuit is similar to that for potential measurement with the earth return circuit broken to an auxiliary electrode in the electrochemical cell. Such a circuit would maintain the potential of the test electrode at the reference cell potential. This potential may be varied by inserting a variable potential source (V ) in the input circuit as shown in Fig. 1.3. The actual cell potential (V ) and the current required to polarise the test electrode to this potential may be measured using the basic circuits shown in Figs. 1.1 and 1.2 respectively. 

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