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

• v.craa soil resistivity to a depth of 10 m at any rroposeci electrode location is 19.9 Sim.

I 1 implicity, all calculations are carried out assum-

.•.....Arode is approximately 1 m below the surface as ,: ; ,m I) in Figs 6.124 and 6.125. The errors from this :; TroNimation are considered negligible, bearing in ihe limitations in the accuracy of soil resistivity

114,1 Sheet steel piles

....!rode location to he 19.9 Sim. This figure can be to determine the smallest electrode area capable

•

SECTION B-B

FIG. 6.124 Arrangement of sheet steel pile electrode

where: Q = soil resisitivity, Sim

t = duration of fault, s

For the given conditions:

=57.66 x 10 6 A/m 2

19.9x 1

Minimum length of single pile required:

1

L

i x (2w 4s)

where 1 = earth fault current, A

= maximum permissible current density, A/m 2

w = width of base of pile, m s = length of side of pile, m

28 000

0.17 x 10 4 x ((2 x 0.248) + (4 x 0.145)) L = 15,3 m

ater depth than to add further electrodes.

ftc o‘erall station earth resistance and rise of earth are then determined in the same manner as

11.4.1of this chapter.

11.4.3Earth rods

, ection is based on the use of 16 mm diameter

.vensible steel-cored copper earth rods complying with Standard 43-94.

ihe general formula for permissible current loading al he transformed by using the surface area of a

16 min earth rod to yield the following:

This section is based on the use of 50 x 6 mm copper strip. For an earth fault duration of 1 second and a soil resistivity of 19.9 12m, the permissible current rating is once again:

Total length of strip required is:

fault current

L -

current density x circumference

28 000

0.17x 10 4 x [(2 x 0.05) + (2 x 0.006)]

=147 m

In order to obtain reasonable current dissipation and obtain the required order of resistance, this tape would need to be buried over an area in excess of 300 m 2 . As discussed for earth rods, this amount of free space is not normally available at the required location. An alternative would be to install the electrode in contact with the earth under the foundation slab concrete.

Earth networks are formed and bonding of plant is carried out using PVC insulated cables. Earth cables run on the power cable supporting steelwork are considered to give a more economic installation than copper or aluminium bar clipped to the building surface. The use of earth cables allows the same installation and termination techniques as for power cables, giving a unified approach to labour requirements and the use of standard tools and equipment.

All cables used for earthing and bonding comply with the insulation, construction and testing requirements given in 3S6004: 1975. The cables may have either green or green/yellow insulation and the following rationalised range of sizes is used:

2.5Stranded copper/PVC

All earth cables are terminated, bolted to equipment and where necessary protected from corrosion using the techniques given in Section 9.2 of this chapter.

11.5.1 Main earth network

The main earth network is formed by interconnecting the earth electrodes and forming rings around main plant areas as shown in Fig 6.122. These rings are extended into all levels of the building where major plant is located. Interconnections are made using 'teeoff' bars of the type shown in Fig 6.126. These `tee-off" bars are also inserted in the ring at strategic locations to enable connections to be made to electrical plant.

As discussed in Section 11.3 of this chapter, the largest current that the station earth network will normally have to carry is the 43.1 kA associated with the 415 V system. Major plant such as switchgear is

FIG. 6.126 Arrangement of tee-off connections

given a 3 -second rating and it is considered appr o , priate to give this rating to the main earth network. However, this fault duration is considerably in excess of that associated with back-up protection and such faults must be considered extremely rare. Under these circumstances it is considered acceptable to allow the earth cable to reach a final temperature of 325 ° C, as allowed by Engineering Recommendation S5/1 for aluminium bar. Whilst this ensures mechanical integrity of the earth system, it will be appreciated that the PVC cable sheathing will disintegrate and any affected cables would have to be replaced. However, for fault durations appropriate to back-up protection, the earth network should not be damaged and therefore the final conductor temperature of earth cables is limited to 160° C.

The required cable size is calculated using the formula given in Section 4.3.1 of this chapter:

1

148log s [(325 + 228)/(25 + 228)]

=570 mm 2

The use of 500 mm 2 cable is accepted as being adequate for forming the station earth network because of the extremely low probability of a 3-second fault and also, since the network is a ring, there will be a degree of current sharing between cables. For a cable temperature limit of 160 ° C the maximum allowable fault duration is 1.26 seconds which is adequate to cater for back-up protection.

11.5.2 Instrument earth network

To reduce the risk of interference on control and instrumentation equipment, a 'clean' earth termed the station instrument earth is provided. This instrumegt

S = 372 mm 2

considering the electrode itself, the total cross- -,: uional area for two steel piles is:

= 2 x 14 x [248 + (2 x 145)] = 15 064 mm 2

llov,e\.er, allowance has to be made for surface corro,ion of the electrode over the life of the station. 1:1:‘, will depend on a number of factors of which the ,,ignificant is the soil pH which should be es- :,,h1Hhed from soil surveys. For this example, it is as- •11111Cd that the corrosion reduces the electrode thickness h, 3 mm (1.5 mm per surface) over the station life.

I here rore after corrosion the cross-sectional area of ,N o piles is:

---- 2 x 11 x [248 + (2 x 145)] = 11 836 mm 2

19.9

R –

4V [(2 x 0.248) + (4 x 0.145)1 x 10 x 2

=1.90

To avoid an excessive rise of earth potential, an overall station resistance typically of less than 0.1 0 is required. Therefore, even with the eight proposed electrodes in parallel, an individual electrode resistance of 1.9 0 is not acceptable and the number of pile sections must be increased. Increasing the number of pile sections to 25 would result in an electrode 10 m long having a resistance of:

The resistance of the remaining electrodes is then calculated assuming the same number of pile sections but using the soil resistivity appropriate to the location. For the purpose of this example let us assume that the remaining primary electrodes have resistances of 0.41, 0.51 and 0.49 0, and the secondary electrodes have resistances of 0.50, 0.58, 0.45 and 0.62 0. The overall station resistance is given by:

It can be seen that this is considerably in excess of !Ile required 372 mm 2 .

With respect to the connections from the earth cable , Litmection bar down to the earth electrode, it is nor- ,mal practice to use four 100 x 12 mm steel bars (as , hom,ri in Fig 6.124) to provide adequate mechanical trength. Again assuming 1.5 mm of surface corrosion

L)N•er the station life, the final cross-sectional area will he:

This example is based on a typical cylindrical pile having an external diameter of 1.05 m and a radial thickness of 6 mm. A typical arrangement is shown in Fig 6.125.

Reference should be made to Section 11.4.1 of this chapter for explanation of formulae and definitions where they are not given in this section.

Maximum allowable current density:

= 0.17 x 104 A/m 2

Minimum length of pile required:

L

i xrxd

where d = external diameter of pile

Although the minimum length required for current dissipation is 5 m, it is proposed that a 10 m length be used to obtain a lower value of resistance.

Now considering the current carrying capability of the electrode material itself and the connections to

As discussed previously in Section 11.4.1, a value of resistance of 1.06 0 is considered too large for a mai n electrode. It will therefore be necessary to incredse the electrode length or put a number of electrodes in parallel.

The following method of dealing with electrodes in parallel is given by Tagg [291 The method uses the 'equivalent hemisphere' principle where the rod electrode is replaced by a hemisphere having the same resistance. The radius of this equivalent hemisphere is given by:

r = Li[log n (8L/d) — 11

where L = length of rod, m

d = diameter of rod, m

For two rods in parallel:

where a = r/S and S = spacing of electrodes, m

And for three rods in parallel, arranged in a triangle:

Maximum csa of pile = r(R 2 — r 2 ) r(525 2 — 519 2 )

= 19 679 mm 2

After corrosion csa of pile = r(523.5 2 — 520.5 2 ) = 9839 mm 2

This can be seen to be considerably in excess of the minimum requirement of 372 mm 2 that was derived

From the above formulae it can be seen that spacing S increases, a will tend to zero and

value of 0.5 for the ratio will be approached. However

be required to approach the ideal value. For practical purposes, setting the spacing equal to the rod length normally gives an acceptable design.

148log„ 0325 + 228025 + 228)]

=521 mm 2

For a short-circuit duration of 1-second:

S —

148V log o [(325 + 228)/(25 + 228)]

=301 mm 2

For the 3-second rating, the nearest rationalised cable size of 500 mm 2 is selected to bond plant. For switchgear the 3-second rating is met by using two 300 mm 2 cables, one connected to each end of the switchgear earth bar.

For motors, other plant and cable gland bonds where a I-second rating is required, a 300 mm 2 earth cable is used. Alternatively, for cable gland bonds, aluminium bar having the same cross- sectional area may be used.

—

148log o [(325 + 228)/(25 + 228)]

=578 mm 2

For a short - circuit duration of 1 -second:

43 700 \I

148log e [(325 + 228)/(25 + 228)]

=334 mm 2

For the 3-second rating, the nearest rationalised cable size of 500 mm 2 is selected to bond plant. For switchgear, the 3 - second rating is met by using two 300 mm 2 cables, one connected to each end of the switchgear earth bar.

For motors, other plant and cable gland bo nds where a 1-second rating is required, a 300 rn ni z cable is used. Alternatively, for cable gland bond s, aluminium bar having the same cross-sectional area may be used.

(c)415 V, 31 MVA system

For aluminium cables, constant K -= 148 and 3

=228

Therefore, for a short-circuit duration of 3 seconds and a final conductor temperature of 325 ° C:

43 100 ,\,/ 3

S

148log i, [(325 + 228)/(25 + 228)]

=570 mm 2

For a short-circuit duration of 1-second and a final conductor temperature of 160 ° C:

S —

148log, [(160 + 228)/(25 + 228)1

=445 mm 2

For a short-circuit duration of 1-second and a final conductor temperature of 325 ° C:

43 100 V

s=

148log o [(325 + 228)/(25 + 228)1

=329 mm 2

For the 3-second rating, the nearest rationalised cable size of 500 mm 2 is selected to bond plant. switchgear the 3-second rating is met by using two mm 2 earth cables, one connected to each end of switchgear earth bar.

For motors and other plant where a 1 -second rating is required, a 500 mm 2 earth cable is used. For cable gland bonds, aluminium bar should be used and since this can be operated at 325 ° C without harm, the crosssectional area required is 300 mm 2 .

11.5.4 Plant bonding arrangements

This section gives specific plant bonding arrangements which are based, where appropriate, on the fault levels and calculated cable sizes given in the previous Sub- section.

Atransformer neutral earthing The 3.3 kV

•:-.insformer neutral earth connections should be by

means of a 500 mm 2 earth cable connected from the [rar4ormer neutral earthing resistors, either direct to an adjacent earth electrode or to the station earth

uetwork, v,hichever is the nearer.

, ,:: -ect) the switchgear neutral and earth bars via a !i nk.

i'lint bonding arrangements for earth connections to plant are as follows:

Earthing systems

In addition, a 150 mm 2 bond is to be provided between the motor and adjacent plant and structural (or other) steelwork to which personnel may make simultaneous contact.

• II kV/415 V gas turbine unit transformers The earth terminals of these transformers are connected to the station earth network by means of a 500 mm 2 earth cable.

•3.3 kV switchgear For switchgear connected to a transformer and not fuse-protected, the switchgear earth bar is connected to the main station earth network by two 300 mm 2 earth cables, one connected to each end of the bar.

For switchgear fed by fuse protected cables, the power cable armours will be utilised to provide the earth return path. A cable gland bond, of crosssection not less than half that of the power cable core, being provided between the gland and the switchgear earth bar with the exception of single-core power cables which utilise a 300 mm 2 bond.

•3.3 kV motors For 3.3 kV motors fed by three single-core power cables, an earth return cable of 300

mm 2 cross-section is provided from the motor earth terminal to the supply switchgear earth bar. The earth cable should be laid in proximity to the power cables..

When the motor is fed by a multicore cable, the cable armour is utilised to provide the earth return path, connection being trade to the cable armour from the motor earth terminal by a bond of crosssection not less than half that of the power cable core.

In addition, local bonding is provided between the motor earth terminal and adjacent plant, and structural or other . steelwork to which personnel may make simultaneous contact. The size of bond shall be half that of the power cable core cross-sectional area.

diesel generators, their tanks are to be connected to the station earth network by means of a 500 mm earth cable.

•3.3 kV diesel generator neutral earthing resistors

The 3.3 kV diesel generator liquid earthing resistor tanks are to be bonded to the station earth network by means of a 500 mm2 earth cable.

•415 V switchgear For switchgear fed by single-core cables, the switchgear earth bar is connected to the main earth network by means of two 300 mm 2 cables, one connected to each end of the bar.

For switchgear fed by fuse-protected multicore cables, the power cable armouring is utilised to pro-

vide the earth return path. A cable gland bond of cross-section not less than half that of the power cable core is provided between the gland and switchgear earth bar.

• Transformers having 415 V low voltage windings

Transformers of 1 MVA rating and larger have a 500 mm 2 cable connection between the transformer earth terminal and the 415 V switchgear earth bar.

Transformers of rating in the range 0.5 to 0.8 :VI VA have a similar connection of 300 mm 2 crosssection.

Transformers of rating less than 0.5 MVA have a si milar connection of cross-section not less than one half that of the power cable core.

•415 V plant other than switchgear and transformers

For plant within this range fed by a multicore cable the cable armour shall be utilised as the earth return. The gland bond size between the plant and the gland shall be of cross-section not less than half that of the power cable core, with a minimum cross-section

of 4 mm 2 .

When the plant is fed by single-core cables, an earth return cable of 300 mm 2 cross-section shall be provided from the equipment earth terminal to the supply switchgear earth bar. The earth cable shall be laid in close proximity to the single core power cables.

Any plant which is fed by a conduit or trunking system shall also have an earth cable included in the conduit of the same cross-section as that given by the above criteria.

For motor circuits of 1.5 kW and above, additional local bonding is provided between the motor and adjacent plant and structural or other steelwork to which personnel may make simultaneous contact. The bond size should not be less than half that of

the power cable core, with a minimum size of 4 mm 2 .

•Control and instrumentation marshalling boxes and cubicles The steelwork of all panels, marshalling

)oxes and the like should be bonded to the station earth network using 4 mm 2 earth cable. Where equipment is mounted directly onto earthed metalwork, no bond is necessary.

Where a power cable is taken into a panel, the cable armour is bonded to the panel and a separate bond to the station earth network is not required.

Plant bonding arrangements for insulated cable glands of the type described in Section 9.1 of this chapter are normally used for both power and control cables. This means that where a connection is required to the cable armour, a bond has to be connected from the gland integral earth tag to the equipment earth bar/stud or other glands as appropriate. In order to ,pass this information to site, a series of bond codes is used. The gland bond codes are recorded on the block cable dia-

grams and also on cable schedules and work cards used on site. A list of typical bonding codes is as follows:

•11 kV, 3.3 kV, 415 V power cables and multicore control cables

Earthed: A The armour is connected to the station earth or, where applicable, the equipment earth stud.

Floating: B The armour is insulated from earth. If the cable has a screen then this also is insulated. An insulating shroud is fitted over the gland body.

•Multipair cables

Earthed: H The armour is connected to the station earth system and the cable screen is connected to the instrument earth system.

FIn situations such as outbuilding control panels where a separate instrument earth system is not available, both armour and screen are connected to station earth.

Floating: B The armour is insulated from earth. If the cable has a screen then this also is insulated. An insulated shroud is fitted over the gland body.

PThe armour of 'digital control cables' is connected to the armours of other cables of the same discipline in the same box. The cable screen is connected to the screens of other cables of the same discipline in the same box.

RThe armour of 'analogue control cables' is connected to the armours of other cables of the same discipline in the same box. The cable screen is connected to the screens of other cables of the same discipline in the same box.

SSpecial requirement. To obtain the correct bonding information refer to the block cable diagram.

Actuator cables:

/ The armours of the actuator cables (composite power/control design) in the load centre are connected together and to the armour of the incoming multipair cable. The actuator cable outer screen and the incoming multipair cable screen are connected to the insulated screen bar in the load centre. The actuator cable inner screen is connected to station earth.