reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

• Insulated cable glands for 11 kV cables Where re-

quired, the bond for an insulated cable gland should

consist of an aluminium bar arranged to provide the shorting strap across all glands of the circuit

and the connection to the switchgear earth bar. This aluminium bar should have a minimum cross-

ectional area of 300 mm 2 .

• Insulated cable glands for 3.3 kV and 415 cables

For single core cable circuits, when required, the bond should consist of aluminium bar arranged to provide a shorting strap across all glands of the circuit and also connect to the switchgear earth bar. this aluminium bar should have a minimum cross- , ectional area of 300 mm 2 .

For multicore cable circuits, where the cable armouring is utilised as the earth reiurn circuit for he plant, the earth bonds at both ends of the cable shall use an earth cable of cross-section not less :han half that of the power cable core, with a minimum cross-section of 4 mm 2 .

• rfmtrol and instrumentation insulated cableglands

\‘ here cable gland earth bonds are required they shall be 4 mm 2 stranded copper earth cables.

P'.int bonding arrangements for extraneous metalwork:

▪ Bonding of structural steelwork All major vertical

, teel columns of the structural steelwork shall be bonded together and to the main earth network

"oh a 300 mm 2 cable. The connection to each steel

Lolumn is made via a suitable connecting lug which hould be welded to the appropriate steel columns

under the main steelwork contract. The interconnections between columns is carried on the cladding support steelwork or other adjacent steelwork. Connections to the earth electrodes should be made

dr all locations where the steelwork bonds pass an earth electrode position.

Electrical plant compound fences shall be connected to earth as for station perimeter fences and, in addition, a separate connection shall be made from the fence to the main earth network associated with the plant in the compound. This connection shall be a 10 mm 2 earth cable.

11.6 Testing

11.6.1 Earth resistivity measurement

The method of soil resistivity measurement most commonly used is that derived by Dr. F. Wenner [301,

In practice the method uses four electrodes driven into the ground in a straight line with equidistant spacing 'a' as illustrated in Fig 6.130. To obtain accurate results it . is important that the electrode depth does not exceed one twentieth of the electrode spacing 'a'. The outer two electrodes are used to circulate current and the inner two measure the ground potential difference. This is carried out using an earth tester which has the appropriate four terminals for connection to the electrodes and gives a direct reading in ohms. Assuming the soil to he homogeneous the value of soil resistivity is given by:

Q = IiraR

where R is the resistance measured by the instrument

If the distance 'a' is increased, the current will penetrate deeper into the ground and the measured value of resistivity will be related to this greater depth. To access the resistivity at various depths, a series of measurements are taken with increased electrode spacings

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FIG. 6.130 Measurement of earth resistivity

lbout the central point. A typical set of measurements o Jk en in Table 6.27 and these are shown graphically

Fig 6.131.

The next step is to decide the best way to apply these roults. The first step must be to relate the electrode pacing to the effective depth to which the resistivity

k measured. CPI013 states that the average resistivity measured is for a depth equal to the electrode spacing.

':ome references will be found quoting the effective J,:nth to be in the range 0.6 to 0.75 times the electrode Spaing. If the soil is reasonably homogeneous, it will he found that the resistivity becomes substantially xnstant with depth as shown in Fig 6.131. The interpretation of effective depth is not critical under these

..onditions and to assume the effective depth is equal to electrode spacing should give acceptable results in practice. The formula used is derived on the assumption that the soil through which the current circulates is homogeneous, and this would be borne out if the soil- :-c.istivity remained constant as the electrode spacing

was varied. In practice it will be found that the soil is non-homogeneous. To consider this in a simplified way, the value of resistivity for small values of electrode spacing can be said to represent the soil resistivity near the surface. Also, where the soil resistivity becomes sensibly constant with electrode spacing, this can be said to represent the underlying strata. The formulae

used in Section 11.4 of this chapter assume homogeneous soil conditions and therefore, where they are applied to non-homogeneous soil conditions, it is the 'effective' value of soil resistivity that should be used. The deduction of this effective value is complex, but it must lie between the values deduced for the surface and underlying strata soils. As a practical way of dealing with these difficulties, it is suggested that acceptable results will be obtained by taking the mean value of the measured values down to the maximum depth the proposed electrode will be driven.

For example, if it is required to drive an electrode to a depth of 10 metres, using the results given in Table 6.27 the effective resistivity is taken as:

Where the apparent resistivity starts increasing as the electrode spacing 'a' is increased, the underlying soil strata will be of higher resistivity and there will be less benefit in driving an electrode to this depth.

Soil resistivities will vary over a site and it is important to make measurements at each proposed electrode position. Where possible, these measurements should be made before site work commences since any buried earth conductors, cables or pipes can change the pattern of current flowing between the test electrodes, thus reducing the accuracy of the measurements.

11.6.2 Earth electrode resistance measurement

The resistance to earth of an electrode is measured using the same principle as for earth resistivity measurements, i.e., by passing current through the ground and measuring the potential difference across it. In this case the current is made to flow between the electrode under test and an auxiliary earth electrode driven into the ground. This auxiliary electrode may be specially driven for the test or could be any conveniently placed existing electrode. This auxiliary electrode should be placed at a sufficient distance away, such that it does not interfere with the natural distribution of current from the electrode under test. This distance will depend on the size and efficiency of the electrode under test and this will also affect the test method selected. For relatively small electrodes, such as individual pile electrodes recommended for power stations, a proprietary earth resistance measurement instrument can be used as described in (a) below. However, for large low-resistance electrodes such as the combined system on a power station or EHV system, the required distance is so large that a power frequency current injection system as outlined in (b) is necessary:

(a)Measurement of resistance by composite instrument

Measurements are made using the same instrumen t as that discussed in the previous Section 11.6.1 which is fitted with terminals for current injection and potential measurements, and provides a direct reading of resistance in ohms. A diagram_ matic arrangement of the test is shown in Fig 6.132 where X is the electrode under test, Y is the current electrode and Z the potential probe. To use the instrument in this mode, one of the potential terminals is connected to a current terminal and the electrode under test, the remaining terminals being connected to the potential and auxiliary current electrodes as appropriate.

The electrodes themselves are driven in a straight line. With the current electrode Y fixed, resistance measurements should be taken with the potential electrode Z at various distances from X. These measurements should be most frequent at the location where XZ = 0.6 XY since this is where the neutral zone or true resistance should be found. The results should be plotted as shown in Fig 6.133 and the flat portion which forms the neutral zone gives the resistance of the electrode under test. If a neutral zone is not found, the auxiliary electrode should be moved further away at, say, twice the previous distance from the electrode under test, and the test procedure repeated. If the neutral zone cannot be found with a practical distance between the electrode under test and the auxiliary current electrode, then the 61.8% distance rule as described by Tagg [29] should be invoked. This work by Tagg indicates that the true resistance should be indicated at a potential electrode position of 61.8% from the electrode under test to the auxiliary electrode position. Under these circumstances, it is suggested that a family of four graphs is constructed for various current electrode positions and the electrode resistance taken as the mean of the four resistance measurements at the 61.8wo potential electrode distance position. A typical set of results is shown in Fig 6.134.

(b)Measurement of impedance by power frequency current injection

For a large earth system it may be difficult to estimate its centre to apply the 61.8% rule dis-

CURRENT

SOURCE N..,

FIG. 6.132 Measurement of earth electrode resistance