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

panels when vertically orientated, and a classification system based on the rate and extent of flame spread.

The test method has been designed to take account of the combined effect of factors such as ignition characteristics and the extent to which the surface of the test specimen spreads the flame. The influence on these factors of any underlying materials, in relation to their ability to influence the rate of fire growth, is also taken into account.

In this method, specimens of the fire barrier panels are subjected to a specified radiative heating regime v.ith a small gas ignition flame at one end. The horizontal spread of flame from this ignition source is measured at 1.5 minutes and 10 minutes into the test, and the progress of the flame-front determines the surface spread-of-flame classification given to the material.

The most onerous classification, which is that required by the CEGB for its fire barrier panels, is Class I. To meet Class I requirements, the flame front must not progress beyond 165 mm from the ignition source after the 10-minute test period.

10.4 Additional performance criteria

In addition to the basic fire test performance criteria, the CEGB also dictates that its fire barriers provide the following features:

•Design performance over a lifetime of 40 years in a power station environment without the need for maintenance.

•Suitability for use in damp and wet conditions.

•Freedom from asbestos in their construction.

•Freedom from materials which emit any corrosive or toxic fumes or smoke on the unexposed face of the barrier under fire conditions.

•Defined minimum mechanical strength.

•Capability of withstanding the pressure rise associated with fires in sealed cable tunnels.

•Capability of withstanding in-service levels of vibration without reduction in fire performance.

•Barriers must not fail in such a way as to damage other plant (including cables) during a seismic disturbance of a specified magnitude (requirement for nuclear power stations only).

A wide range of tests are carried out on barrier samples which are designed to demonstrate the above qualities. These tests include:

•A hose stream test to verify that the thermal and mechanical shock of a hose stream being suddenly played on a hot fire barrier does not cause catastrophic failure.

•The following mechanical strength tests:

548

(a)I mpact withstand.

(b)Modulus of elasticity and bending strength.

(c)Mechanical strength after wet cycling.

(d)Pressure rise due to combustion (simulated statically or by analysis).

•Water absorption.

•Seismic qualification, usually by analysis based upo n dynamic tests on representative fire barrier sampl es.

It is more difficult to demonstrate that the materials used will not age significantly over their 40-year de s ign life. The type of testing which is carried out very much depends upon the materials used, but a common feature

is the application of heat, either a continually -applied elevated temperature or some form of thermal cycling.

In some cases, the materials used to construct the fire barrier are standard building materials for which ageing data is already well established. In these cases further testing is not necessary.

10.5 Fire doors

It is not possible to specify requirements for fire barriers without giving careful consideration to the design of fire doors.

Fire doors may effectively be considered as personnel penetrations, and they bring with them their own problems in ensuring adequate fire performance. Obviously, the presence of the door must not detract from the fire performance of the barrier in which it is located. This means that special care must be taken at the jamb assembly to ensure that integrity and insulation requirements are met. It may be particularly difficult to achieve the required fire performance whilst still providing doors which provide satisfactory emergency egress for personnel.

Another problem which arises with doors is that for ventilation purposes they often need to be latched open during normal operation. This means that a fastacting fire detection system must be provided to trigger door release in the event of a fire. The closing mechanism must obviously ensure that the required seal is made on closing.

10.6 Penetrations

Penetration seals are required in order to seal around openings where cables (and other services such as pipework) pass through fire barriers. It is a fundamental requirement, therefore, that the presence of these penetration seals does not degrade the fire performance of the barrier in which they are located.

Configurations 2 and 3 in Fig 6.119 show typical examples of the application of these seals, specifically installed in pre-formed type fire barriers. They could, however, equally be installed

As with fire barriers, in addition to the basic fire test performance criteria, the CEGB also dictates that its penetration seals provide the following features:

•Design performance over a lifetime of 40 years in a power station environment,

•Suitability for use in damp and wet conditions.

•Freedom from asbestos in their construction.

•Capability of withstanding the pressure rise associated with combustion in sealed cable tunnels.

•Compatibility with cable sheathing materials.

•Capability of withstanding in-service vibration and cable movement due to short-circuit and thermal cycling without reduction in fire performance.

1013: 1965, Clause 215, it is a requirement that if two or more stations are adjacent on what may be considered to be one site, then the earthing systems are to be interconnected to form a single earthing system. This applies to a number of generating stations on one site, and also to power stations and transmis s i on substations on a common site.

When reading this section it should be borne i n mind that earthing is closely associated with light n i ng protection, power cable ratings and gland bonding, and therefore reference will also need to be made to th e sections covering these topics.

11.2 Differences in earth potential

•They must not damage cables during a seismic disturbance of specified magnitude (requirement for nuclear power stations only).

A further design criterion which must be considered when selecting penetration seal materials, is the effect which the seals may have on the current carrying capacity of power cables which pass through them. Here, two conflicting interests exist. In order to achieve good insulation properties to restrict unexposed face temperatures during a fire, it is desirable for the seal to be a good thermal insulant. This does, however, restrict the conduction away of heat generated in the power cables due to copper losses under normal operating conditions. Thus it may become necessary to derate the power cable to accommodate the seals. This in turn may lead to the use of larger cables sizes to supply particular loads. The rigid plaster type seals will be less of a problem in this respect than the other seal types, since they are more thermally conductive.

11 Earthing systems

11.1Introduction

The purpose of an earthing system is to provide an adequate path for earth fault currents to return to the system neutral. This has to be performed in a manner which ensures 'safety to personnel', i.e., without giving rise to dangerous touch, step or transferred potentials. In addition, to prevent damage to plant, the rise of earth potential under fault conditions must not result in breakdown of insulation. In all cases, the system design must be such that enough fault current will flow to operate the protection devices and disconnect the fault. To meet these needs, an earth system must include earth electrodes to allow current to flow into the ground to remote system neutrals, and also a network of conductors to allow fault current to flow between plant within the station site. In this context, in accordance with British Standard Code of Practice

11.2.1 Definitions

When earth fault currents flow to ground, a potential gradient is formed around the earth electrode due to the resistance of the ground. This potential gradient is greatest adjacent to the earth electrode and reduces to zero or true earth at some distance from the earth electrode. Three methods of contact with these pot. tials are considered and defined as step, touch and transferred potentials. These are shown diagramma.. tically in Fig 6.121 which is discussed as follows:

•Step potentials Person 'a' on Fig 6.121 illustrates

'step potential'. Here the potential difference V 1 seen by the body is limited to the value between two points on the ground separated by the distance of one pace. Since the potential gradient in the ground is greatest immediately adjacent to the electrode area, it follows that the maximum step potential under earth fault conditions will be experienced by a person who has one foot on the area of maximum rise and the other foot one step towards true earth.

•Touch potential Person 'b' on Fig 6.121 illustrates 'touch potential'. Here the potential difference 112 seen by the body is the result of hand-to-both-feet contact. Again the highest potential will occur if there were a metal structure on the edge of the high potential area, and the person stood one pace away and touched this metal. The risk from this type of contact is higher than for step potential because

the voltage is applied across the body and could affect the heart muscles.

•Transferred potential The distance between the high potential area at the electrode and that of true earth may be sufficient to form a physical separation rendering a person in the high potential area i mmune from the possibility of simultaneous contact with zero potential. However, a metal object having sufficient length, such as a fence, cable sheath or cable core may be located in a manner that would

bridge this physical separation. By such means, zero earth potential may be transferred into a high potentiaI area or vice-versa. Person 'c' in Fig 6.12 1

illuqrates the case of a high potential being trans- ferred into a zero potential area via the armour of a cable. Since the armour is bonded to the main earth

In the illustrated case, the person is making simultaneous contact hand-to-hand with the cable sheath

and true earth. However, if the person is standing an [rue earth then the voltage V3 seen by the body ,: ould be the result of a hand-to-both-feet contact. Person 'd' in Fig 6.121 represents the case of zero [' o[ential being transferred to a high potential area \ id a cable core which is earthed at the remote point. In this case, the voltage V4 is lower than V3 o hich represents the station rise of earth potential, because person 'd' is located some distance from the main earth electrode and therefore is subject to the ground potential gradient. Quite clearly if person 'cl'

ii ad been on or touching the main electrode he would hl asre experienced the full rise of earth potential V3. Franslerred potentials are therefore considered to

..airy the greatest risk since the shock voltage may he .equal to the full rise of earth potential and not

a iraction of it, as is the case with step or touch pa [ entials.

11 2.2 Acceptance criteria

cceptance criteria are related to the rise of earth ;otential and its duration in the following manner:

(a)For high reliability systems, i.e., systems having high speed protection, the maximum permissible rise of earth potential without special precautions is 650 V. This requirement, given in Engineering Recommendation S5/I, is based on limits set by the International Telegraph and Telephone Consultative Committee for installing telephone equipment without special protection for personnel or equipment. A duration is not given for the clearance time associated with high reliability systems, but this is generally accepted as 0.2 seconds. Although this value of 650 V originates from requirements for telephone equipment, it is now also used as a criterion for safety generally. Therefore, providing this limit is not exceeded, experience has shown that no special measures are necessary in respect of potential rise, step, touch or transferred potentials.

(b)For systems protected by overcurrent protection, the maximum permissible rise of earth potential without special precautions is 430 V.

This requirement, also taken from Engineering Recommendation S5/1, is again based on CCITT recommendations for telephone equipment. Once again, no fault duration is given for this condition

but using the criterion given in (a) and extrapolating on an 1 2 t basis, for the 430 V limit a maximum duration of 0.46 seconds is obtained. As for (a), providing the criterion is not exceeded, no special

measures are required in respect of step, touch or transferred potentials.

(c)British Standard code of practice CPI013: 1965 (Clause 213) requires that the potential difference between two normally earthed items to which personnel may have simultaneous contact should not

exceed 55 V. Bearing in mind the criterion given in (b), this limit of 55 V is applied to all rises of earth potential that exist for durations in excess of 0.46 seconds.

(c1) If the criterion given in (a), (b) and (c) cannot be complied with, then special precautions must be taken to protect personnel and plant. An example of precautions to protect against transferred potentials is the use of isolation transformers on incoming telephone lines. A further example is the provision of local bonding to give immunity by ensuring that all metalwork to which simultaneous contact can be made is at the same potential. Guard rings buried at increasing depths around an electrode can be used to modify the ground surface potential to protect against step potentials.

11.3 Earthing systems design

When considering the design of earthing systems it is useful to bear in mind that earth fault currents have to return to their own system neutrals. This means that whilst a common earth system for all equipment is employed, it can be analysed in two parts depending on whether the neutral of the system feeding the fault is within the power station or is remote. The following Section 11.3.1 deals with 400 kV, 275 kV and 132 kV systems where a significant proportion of the earth fault current will flow via the earth electrode to a remote supply source. Section 11.3.2 deals with system voltages up to and including generator voltage, where the system neutral is within the power station and therefore the vast majority of earth fault current will flow in the metallic bonding system.

11.3.1 Systems having remote neutrals

400 kV, 275 kV and 132 kV systems have neutrals that are outside the confines of the power station and therefore the earth fault current flows back to the source via the ground and any EHV cable sheaths or aerial earth conductors. The proportion of fault current returning down each path will be dependent on the path impedance. Under such fault conditions the whole power station earth network will be raised in potential with respect to remote earth. The rise of earth potential will be the product of the current returning through the ground and resistance of the station earth. Since these EHV systems are classed as high reliability, the maximum allowable rise of earth potential under fault conditions without additional precautions is 650 V as discussed in Section 11.2.2 (a) of this chapter.

Since the earth fault current that flows into the ground is associated with the EHV systems, the mai n earth electrodes should be adjacent to plant connected to the EHV system (e.g., generator and station transformers). This is to enable earth currents to flo w to ground close to the fault location, thus restricting th e fault current flowing through the station earth system interconnections and minimising the potential gradients across the station. In addition to these main ea rt h electrodes, it is normal practice to install a number of secondary electrodes to reduce the overall station resistance and limit potential gradients across the site A typical station earth system arrangement is show n in Fig 6.122.

The earth electrode system is designed by calculating in sequence the following parameters:

•Maximum earth fault current that has to return to remote sources.

•Minimum size of electrode required to transfer fault current into the ground.

•Resistance of these minimum size individual electrodes.

•Consequent overall station earth resistance.

•Proportion of current that returns to source via the ground.

•Resultant rise of earth potential.

•If the rise of earth potential is unacceptable, the size of individual earth electrodes is increased and the calculations repeated.

•If the rise of earth potential cannot economically be reduced to an acceptable level, other measures to protect personnel and plant must be considered.

This process is explained in greater detail in the remainder of this section and a worked example is given in Section 11.4 of this chapter.

For earth electrode sizing it is only that proportion of EHV system earth fault current that has to return to remote neutrals that is of interest. Therefore, due account should be taken of on site' contributions to fault currents. For example, where generator transformers are connected to the 400 kV system, any contribution to the 400 kV earth fault level from on site' generation can be ignored when sizing the earth electrodes since the current will flow through metallic connections and not into the ground.

Before electrode sizing can be started, the soil resistivity at each proposed location must be established using the Wenner four-electrode test described in Section 11.6.1 of this chapter. The first step in sizing an electrode is to calculate the minimum surface area that is required to dissipate the current into the ground without undue heating and drying out of the soil local to the electrode. For these calculations it is considered prudent to ignore all EHV cable sheaths and overhead

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line earth conductors, and to assume that all earth fault current returns to the remote neutrals via the electrode adjacent to the faulted plant. Whilst this may appear conservative, in reality the electrode size is normally dictated by its resistance (since this governs the rise of earth potential) and not by its current carrying capability. At this stage the cross-sectional area of the electrode itself should be checked to ensure that it is capable of carrying the anticipated fault current. Such calculations should take into account the estimated loss of metal due to corrosion during the station life.

Having established the minimum size of electrode required to carry the fault current, the next step is to calculate the resistance of the station earth system and assess whether the rise of earth potential is acceptable. The first step in this process is to calculate the resistance of each individual electrode as shown in Section 11.4 of this chapter. The overall station resistance is then calculated for all electrodes in parallel ignoring the impedance of interconnecting cables. This greatly si mplifies the calculations and experience has shown that this also compensates for ignoring fortuitous earth paths such as pipes and base slab concrete which cannot be readily identified or calculated.

This rise of earth potential will be the product of the station earth electrode resistance and the current flowing through it. As already stated, the earth fault current will return to remote neutrals via the ground and any overhead line aerial earths or cable sheaths that may exist. The current division will be dependent on the relative impedances of each route. Since the electrode design is dictated by the allowable rise of earth potential, it would be grossly uneconomic to base these calculations on the assumption that all current flows in the ground. Therefore, the division of current between the various paths must be assessed. The amplitude of the earth current returning to neutrals via overhead line earth wires and towers can be 30 - 70% depending on the number of lines and their length [27]. Therefore, quite clearly, the value of current flowing into the ground will be dependent on the physical location of EHV substations relative to the power station.

Where the power station and EHV substation share a common site, at least one metallic connection per generating unit will be provided between the two earthing systems. Earth fault currents can therefore flow via these connections to the earth wires and towers associated with the EHV substation. The magnitude of these currents will depend on the number of overhead lines or cables entering the substation and their lengths. Since power station earth electrodes normally consist of steel piles they have, of necessity, to be designed and installed at an early stage of power station construction. This frequently means that accurate predictions of current division cannot be made at the design stage. It is therefore normal practice to calculate the rise of earth potential assuming the worst case of 70%

of the fault current returning through the ground. The combined resistance of the power station and EHV substation earth systems is used for this calculation.

Another arrangement that may be encountered is a power station located a few miles from its associated EHV substation. En such cases it can be assumed that for an earth fault at the power station only 10Wo of th e current would flow in the ground. The rise of earth potential would therefore be the product of this pro.. portion of current and the power station earth syste m resistance.

If the calculated rise of earth potential exceeds th e 650 V limit, the size of the electrodes should be increased and their resistances and the potential rises recalculated.

In some cases it may not be economically or technically possible to provide an electrode system that will li mit earth potential to 650 V. En such cases the po- tential rise should be reduced to an economic minimum and additional precautions taken to safeguard against transferred, step and touch potentials. Such measures against transferred potentials would include isolation transformers on incoming telephone and pilot cables, and possibly isolated sections in pipelines or railway tracks entering the site. In addition, step and touch potentials would have to be assessed using the type of techniques given in IEEE Standard 80 [28]. If step or touch potentials are found to be excessive then additional provisions such as guard rings will be necessary. Guard rings consist of a number of ground conductors, connected to the main electrode, and buried at increasing depths around it so that the ground surface potential is modified. A typical arrangement is shown in Fig 6.123.

11.3.2 Faults on internal systems

For system voltages up to and including generator voltage, the system neutrals are earthed within the boundary of the power station and therefore the vast majority of the earth fault current will flow via metallic bonds and not into the ground itself. Equipment operating at generator voltage is considered a special case since it is all bonded to a special earth bar run with the main connections. Consequently, any earth fault current will preferentially flow in this special earth bar rather than in the station earth network. Details of generator main connections are given in Chapter 4.

Earth fault currents associated with 11 kV, 3.3 kV and 415 V systems will however return to their neutrals via the station earth network. The station earth net- work is a mesh of metallic bonds interconnecting the earth electrodes and forming a ring around plant areas.

We must now consider the magnitude of earth fault currents associated with these systems. If the zero phase sequence impedance of the system were the same as the positive phase sequence impedence, then the earth fault current would have the same magnitude as that for a three-phase symmetrical fault. In practice for

Thipsformers it is not possible to obtain identical -dues for positive and negative phase sequence im-

..1:11ices. Transformers are therefore specified to have ,mo phase sequence reactance as close as possible 10.0' of the positive phase sequence reactance, with

.Ellowable minimum of 90 07o. With this minimum he earth fault current could be 3.510 higher than :hrce-phase symmetrical fault current. Since every made to keep the negative phase sequence :;•:..Llarice as near as possible to the positive value,

.::.d since in practice earth fault currents rarely achieve

. or full prospective value, it is normal within power - .;:ion design to assume that the unrestricted earth Huh ..urrent is the same as the three-phase symmetrical

The fault levels associated with internal systems u .L follows:

I kV and 3.3 kV systems which limit the fault current

. d pproximately 1000 A per infeed. However, the

N±,stem is solidly bonded to earth to make avail- -lc he

full prospective fault current to allow proper

clearance of fuses. The maximum current returning to an internal system neutral under a single earth fault condition is therefore that associated with the 415 V system, and this fixes the size of the station earth network for internal faults. There is a very low probability of the NER flashing over, allowing unrestricted earth fault current to flow for 11 kV and 3.3 kV system faults. If this were to occur on the 3.3 kV system, the fault current flowing through the station earth network would be greater than the design value associated with the 415 V system. The increase in fault current would however be so marginal that the integrity of the station earth system would not be threatened. For outbuildings which do not directly form part of the main station earth network, the size of its earth ring will be dependent on the system voltage and fault levels for equipment within that building.

For faults on internal systems, the allowable rise of earth potential should be that appropriate to overcurrent protection as defined in Section 11.2.2 of this chapter. This means that the rise of earth potential should be limited to 430 V for faults cleared in less than 0.46 seconds and 55 V for longer clearance times. Where these limits cannot he economically achieved, special measures (such as local bonding to give immunity from transferred potentials) can be used as discussed later in this section. For these internal faults,

the rise of earth potential at the faulted equipment is taken to be the product of the impedance of the metallic bonding system path and the total earth fault current.

To keep the potential gradient across the station to a minimum, the impedance of the fault current return path must also be kept to a minimum. Furthermore, to reduce the risk of electrical interference in control and instrumentation circuits, it is desirable to keep the fault current supply and return paths as close as possible. For rnulticore power cables these aims are achieved by using the armour as the main earth return path. For single-core cables which have their armours single - point bonded, these requirements can best be achieved by running an earth cable in close proximity with the power cables.

In the case of large transformer feeds and interconnectors, the single-core cable circuits will be short or will follow a major cable route where an earth cable forming part of the station earth network will already exist in close proximity. However, for motors fed by single-core cables, there is not likely to be an existing earth cable in close proximity to the power cables for the entire length of the route and, in these cases, it is necessary to provide an earth cable between the supply switchgear and the motor terminal. This earth cable is run in close proximity to its associated motor single-core cables.

For 11 kV and 3.3 kV plant, the 430 V and 55 V rise of earth potential criteria may not both be met. The possibility of transferred zero earth potential, via structural steelwork or adjacent electrical plant with its own earth return connection, cannot be ignored. Therefore, where paths exist for such 'transferred potentials, immunity must be given. This immunity is provided in these specific cases by installing local bonding between all adjacent plant and between plant and steelwork to which personnel may make simultaneous contact. Alternatively if there is a station earth bar connection point locally, the plant may be bonded to this.

For 415 V fuse-protected circuits the rise of earth potential under earth fault conditions will be in excess of 55 V and therefore local bonding may be necessary dependent on the protection clearance time. As shown in Section 4 of this chapter for motor circuits, the fuse size is selected for starting conditions rather than full-load current and is therefore relatively large. This means that to keep the conductor/armour loop resistance to a sufficiently low value to clear an earth fault in less than 0.46 seconds, the cable route length would have to be severely restricted or the cable size would have to be substantially increased. In this situation the economic solution is to provide local bonding on 415 V motors in a manner similar to that for I I kV and 3.3 kV circuits, to give immunity from transferred potentials. However, for feeder circuits other than those for motors, the fuse size will be more closely matched to the cable and therefore it is considered more economic to ensure that the pro-

tection operates in less than 0.46 seconds than t o provide local bonding. In making this judgement, account has been taken of the cost and difficulty of providing local bonding on all small plant such a s switches and light fittings. To ensure that the protectio n operates within the required time limit, the

armour loop resistance is controlled as

Section 4 of this chapter.

For remote outbuildings which are well outside th e area covered by the main earth electrode system, such as might be the case for a CW pumphouse, there i s some advantage in installing local earth electrodes.

Normally a pair of electrodes should be provided t o enable one to be disconnected for testing with the station operational. These electrodes will assist in reducing the rise of earth potential and hence transferred, touch and step potentials.

11.3.3 Lightning protection

As discussed in Section 12 of this chapter, it is necessary to connect the lightning protection system to the station earth network. The details of the required connections are also given in Section 12.

11.3.4 Additional considerations

The requirements of British Standard code of practice CP1013: 1965 should be complied with by earthing each of the following:

• The neutral points of each separate voltage ,istem.

transformer tanks and the armours of pow. Ales.

•Extraneous metalwork not associated with power systems, e.g., boundary and transformer fences.

11.4Earth electrodes

The purpose of this section is to give a worked example for sizing an earth electrode system using the principles already given in Section 11.3.1 of this chapter. During this process, alternative types of electrodes are assessed for their suitability for power station applications. The types of electrode investigated are as follows:

•Sheet steel piles.

•Cylindrical steel piles.

•Vertical earth rods.

•Buried horizontal strip.

The calculations show that a considerable length of buried strip or a large number of earth rods would be required to dissipate the fault currents associated with