Designing of an Industrial Earthing System

For the designing of an industrial earthing system, first of all, a layout regarding the location of all machinery and apparatus along with their loads is prepared, wherein the horsepower or kilowatt rating of machinery is clearly being mentioned. Then according to this load, sizes of earth continuity conductor, earthing lead, and earth electrodes are determined for proper cable wrings of machinery. An example has been given below, through the understanding of which, sizes of earth conductor, earth lead, and earth electrodes can be ascertained. This example pertains to a factory, the layout of which has been illustrated in Figure 8.6.

Example;

Determine the sizes of earth conductor, earth lead, and earth electrodes of an earthing system installed for protection of various installations in the factory. The complete detail of electrical machinery and electrical appliances installed in the factory, is as below;

Motors

(i). Two number of 50 horsepower, 440 volts three-phase induction motors, along which 19 / .064” size cable has been run.

(ii). One number of 20 horsepower induction motor, 440 volts three-phase, along which 7 / .064” size cable has been run.

(iii). Two induction motors of 15 H.P each, 440 volts three-phase, along which 7 / .052” size cable has been run.

(iv). Three ten H.P single phase 230 volts induction motors, along which 19 / .052” size cable has been run.

(v). Twelve two H.P 230 volts single-phase induction motors, along which 7 /. 036” size cable has been run.

Figure 8.6 – Factory layout as detailed in example

Transformer

A 250 KVA three-phase 440 / 11000 volts transformer has been installed in the factory, the neutral of which is desired to be earthed. On the LT side of the transformer, 37 / .083 size cables have been used.

Water Level

The water level is approx. 30 ft. deep.

Solution;

In figure 8.7, the entire scheme of the earthing system of this factory has been illustrated. Here, the major possible source of the fault current is a transformer, therefore an earthing plate must essentially be nearby it. The best place for it is pointed A shown in the figure. As the water level is about 30 ft. deep, therefore a den 30 ft. deep is excavated and both the plates can be installed in this one den. Therefore, there will be no need to make two holes, which will also involve saving money. However, for a better protection point of view, an extra plate should be installed 12 ft. deep on point B, which is located nearby point A. And, in order to preserve moisture in it, a watering facility should also be available in it whenever required.

A connecting point has been built at point C nearby the main switchboard, whereas the second has been made at point D, while the third one is at point E, which is exactly below the combination switch fuse board, but slightly above the ducting. Then, these connecting points are connected through wires run through the cable ducts. As this example completely pertains to the earthing of electrical apparatus, therefore two separate earth continuity conductors have been operated in the entire factory. (The earth continuity conductors of other metalwork e.g., distribution fuse boards, switch fuses, etc., have not been shown in the figure for avoiding complications as well as for easy understanding purposes).

After the selection of places for plates and connecting points, the sizes of earth continuity conductors as well as earthing leads are determined in the figure. In this table, a suitable size of earth conductor has been given in front of every cable size. For simplicity purposes, the earthing system of the factory has been distributed into three sections. The explanation of each of these sections is as follows;

Section A

Motors	Cable sizes for motors	Earth continuity conductor
50 Horsepower	19 / .064”	4 S.W. G
20 Horsepower	7 / .064”	10 S.W. G
15 Horsepower	7 / .052”	12 S.W. G
10 Horsepower	19 / .052”	6 S.W. G

The largest motor in this section is 50 horsepower. Therefore, the main pair of earth continuity conductors must have the capacity to pass any value of fault current produced by means of any one of these two motors, from within it. As every 50-horsepower motor is located on the far end of the distribution, therefore the size of each earth continuity conductor revolving around this section must be 4 S.W.G. The sizes of earth continuity conductors existing between other small motors and the main pair in section A have been given in Figure 8.7.

Section B

Motors	Cable sizes for motors	Earth continuity conductor
10 Horsepower	19 / .052”	6 S.W. G
2 Horsepower	7 / .036”	14 S.W. G

Under this section, the size of every conductor within a pair of earth continuity conductors tends to be 6 S.W.G from 10 horsepower motor through the connecting point E., however, for a 2-horsepower motor, its size tends to be 14 S.W.G. Now if a fault occurs on 10 horsepower motor, thus a safe passage becomes available.

Section C

Motors	Cable sizes for motors	Earth continuity conductor
15 Horsepower	7 / .052”	12 S.W. G
2 Horsepower	7 / .036”	14 S.W. G

In this section, the sizes of the earth continuity conductors from 15 horsepower motor to connecting point D, are 12 S.W.G, however, conductor sizes of every 2-horsepower motor tend to be 14 S.W.G.

Now sizes of the earthing leads can also be determined. As cable sizes being used on the L.T side of the transformer are 37 / .083”, therefore a 3/0 SWG copper wire has been used with the help of the illustrated table. If a fault occurs within the transformer, the fault current will flow towards anyone or both of the earth plates fitted on point A or B. However, it depends on the earth’s resistance. Thus, all these leads must have the capacity to pass fault current. For this purpose, all leads from the transformer to C, from C to A, from C to D, and from D to B will be of 3 / 0SWG. Wires running between D to E do not come in the way of the transformer’s fault current, therefore their size tends to be 4 SWG, which is the size of the largest earth continuity conductor fitted along with E.

As both plates are below the water level, therefore their sizes of 2 ft. x 2 ft. x 1/8 inch will be sufficient.

Table 8.1 – Earthing Conductor

Size of copper wire & strip for earth continuity conductors and earth leads

Cable Size (Number & Diameter of Wires)

Copper Wire S.W. G

Copper Strip Size

1/ .044

3/ .029

3/ .036

7 / .029

7 / .036

7 / .044