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Fic. 6.111 Control cable crimped terminations

type terminations are also subjected to tensile tests to ensure that they can be retained in a spring-loaded insertion terminal block with the terminal clamping screw fully released. The mechanical reliability of snapon terminations is assessed in accordance with Clause 10 of BS5057: 1973.

The ring type terminals may also be used to terminate s mall power cables but consideration must be given to the maximum operating temperature, particularly with the pre-insulated type.

9.3.2 Wire wrapped terminations

A wire wrap termination is formed by wrapping the stripped conductor tightly around a sharp cornered terminal to form a sound electrical and mechanical joint without soldering. Such an arrangement is shown in Fig 6.112. General requirements for wire wrap terminations are given in EEC Publication 352. The method is suitable for use with solid single-strand conductors of the type used in multipair cables described in Section 3.6 of this chapter. The conductor size of 1/0.8 mm is relatively large by conventional wire-wrapping standards but is well within the capability of the method. The wire wrap method offers the advantages of a compact, consistent, reliable and fast termination.

The type of termination used should be the modified wire wrap connection which wraps a minimum of one half turn of insulation around the post as well as the turns of uninsulated wire that form the electrical connection. This half turn of insulation gives improved vibration characteristics. Wire wrap joints can be produced by stripping the insulated wire for the set distance to give the required number of turns and then using a pre-strip bit. An alternative method is to use

Fic. 6.112 Wire wrap termination

a 'cut, strip and wrap' bit which severs the conductor, strips the insulation and wraps the termination in one operation. The 'cut, strip and wrap' technique is faster and hence cheaper than using separate operations to strip and wrap. A typical cut, strip and wrap operation is shown in Fig 6.113. The bit is used in conjunction with a high torque hand-held electrically-operated gun of the type shown.

The principle of the method is that the wire is wrapped under a high level of tensile stress. Notches are formed in the wire by the edges of the terminal

INSULATION REMOVED WIRE WRAPPING BIT WIRE WRAPPING TOO'.

r.... 1110611W

CABLE----CC71E-

TERMINAL BOARD

FIG. 6.113 Wire wrapping tool operation

of high stress can cause partial discharges within the insulation, which result in erosion and eventually failure of the cable at that point. It can be seen from Fig 6.114 that there is also a very high stress in the air where the screen terminates, and this may be sufficient to cause discharges in the air at working voltage. It is therefore necessary to incorporate a stress control system into the termination of II kV cables.

Stress control can be provided by a number of methods such as the use of high permittivity materials, non-linear materials or resistive coatings applied over the insulation from the screen end for a defined distance. These materials may be obtained in heat shrink or tape forms. Heat shrink components are manufactured from cross-linked materials that have been expanded at high temperature and then cooled in this expanded condition. Since the material is cross-linked, it retains a 'memory' of its original shape and size. When it is subsequently heated during installation (using a blow-lamp or hot-air gun), it tries to recover its original size and hence shrinks down tight onto the cable.

A further method of stress control is the use of a stress cone. This works by controlling the capacitance in the area of the screen termination to reduce the stress, as shown in Fig 6.115. Stress cones were originally formed by applying layers of insulating tape to build up the cone shape, then applying layers of semi-conducting tape. This process was very skilled and time consuming, and has now been replaced by the use of moulded rubber terminations. Moulded rubber stress cones are manufactured for use on cables having extruded screens, so an adapter has to be included when they are used on cables having varnish and tape screens. This adapter fits over the varnish

:NSULATCN

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REINFORCING

INSULATING

PORTION

FLUX LINES

Fic. 6.115 11 kV cable screen termination with stress control

and tape screen to present an interface to the stress cone similar to that of an extruded screen. A co rn _ plete termination is shown in Fig 6.116. From t his figure, the cable copper tape screen can be seen t er _ minated on the gland and, above this, the screen adapt er and stress cone. Stress cones of this type are used withi n cable boxes having air clearances which are located indoors in controlled environmental conditions, such as switchrooms, where condensation will not occur.

In locations where terminal boxes are liable to con_ densation (such as on motors and transformers) o r where air clearances cannot be achieved, it is necessary to use a fully insulated termination. A fully insulated termination is shown in Fig 6.117 and these are kno wn as elbow terminations because of their shape.

Elbow terminations are fully screened by an oute r layer of semi-conducting material which is earthed. An inner semi-conducting screen is provided which is held at phase potential by contact with the conductor fitting. This arrangement prevents high stresses and hence discharges in the air surrounding the conductor fitting. A stress relief adapter is provided to control the stress, in a similar manner to the stress cone, where the cable insulation screen is terminated. Quite clearly these elbow terminations are more expensive to purchase and install than stress cones and should only be used where essential.

Moulded rubber terminations have been used extensively in power stations since the introduction of elastomeric cables. Their selection was based on the results of a series of tests carried out by the Central Electricity Research Laboratories. These tests showed that moulded rubber terminations were relatively easy to install and provided excellent stress control as indicated by a lack of partial discharge activity at up to three times working voltage. Moulded rubber terminations also provide a ready means of disconnecting cables for testing.

Suitable test criteria for terminations are given in Engineering Recommendation C89, May 1986 — 'Performance specification for terminations on polymeric insulated cables'. It is important to ensure that the partial discharge performance of terminations is matched by that of the cable to enable the completed installation to undergo a partial discharge test. It is recommended that all 11 kV polymeric cables (complete with terminations) be subjected to a partial discharge test as part of the commissioning procedure.

10 Fire barriers

10.1Introduction

A fire barrier is, as the name suggests, a physical barrier which inhibits the progress of a fire and hence

li mits its damaging effects. In this context, the tern) 'damaging effects' includes not only the flames and their associated heat of combustion, but also anY

s moke and corrosive and toxic gases generated by that combustion.

Fire barriers form an integral part of the CEGB's design philosophy for cable installations since they are used to provide 'segregation' between groups of cables and/or electrical plant. The concept of segregation is described in detail in Section 2.1 of this chapter.

A fire barrier may be part of the power station civil structure itself, i.e., concrete or blockwork walls, ceilings or floors, or it may be a purpose designed prefabricated partition. Requirements for segregation are therefore taken into account when the civil structure of the power station is being designed. However, there are many cases where it is impracticable or prohibitively expensive to provide civil structure fire barriers for segregation and, in situations like these, pre-fabricated

fire barriers are used; it is these pre-formed partitions, colloquially known as fire barriers, with which this

section is chiefly concerned.

Pre-fabricated fire barriers (Fig 6.118) are formed from non-combustible fibreboard panels which are mounted onto a framework, usually of C-section steel channels. The panels are generally mounted on either side of the steel framework leaving a gap of typically 100 mm. This 'double-skin' format is required to provide the necessary level of fire withstand performance. In some cases, the air gap between the two panels is filled with mineral fibre to further increase the degree of thermal insulation provided by the barrier which, in turn, reduces the temperatures on the unexposed face during a fire.

The panels themselves are proprietary items and may typically be composed of an asbestos-free fibre/ cement reinforced by galvanised steel sheet, mechani- cally-bonded to the fibre during manufacture by pressing tangs into the fibre surface. This is the type of panel illustrated in Fig 6.118. An alternative panel type is formed from glass fibre reinforced cement. The steel sheet and the glass fibre reinforcement are provided to improve the mechanical properties of the panel.

The steel framework onto which the panels are fixed will itself be rigidly fixed to the civil structure at the barrier location.

10.2 Performance requirements

The level of fire performance required from fire barriers varies dependent upon the type of power station in which they are installed. In fossil-fired and hydro power stations, segregation is provided primarily to limit economic loss resulting from a 'small' fire. (A small fire is defined as one which can be extinguished within one hour.) Consequently, fire barriers used for this type of segregation must be capable of containing the effects of that fire for a minimum period of one hour.

In the case of nuclear power stations, irrespective of the type of reactor employed, additional segregation is required to ensure that the reactor can be safely shutdown in the event of a fire. As explained in Section 2.1.2 of this chapter, the segregation philosophy used for nuclear power stations dictates that two different

of segregation, or defined. ['I ce are, therefore, two different levels of fire barrier ;, ,Jformance needed to provide these two classes of

Fire barriers for segregation Class I need to be able ,ontain the effects of a 'major' fire, defined as one can be extinguished within four hours. They also be required to withstand the effects of other

. .:/ards which may occur locally to that barrier, such ini Aks from a turbine disintegration or a hot gas (the latter being specific to gas-cooled reactor Because of these more onerous requirements, •::-e barriers for segregation Class I are generally part

he civil structure.

egregation Class II fire barriers must he capable containing the effects of a 'small' fire (as defined The same designs of barrier are therefore

o provide Class II segregation as are used for

0,

11-tired and hydro stations.

\I the time of writing, pre-formed fire barriers have : I fo rmally been demonstrated as being capable of

.niding Class I segregation. However, development

work aimed at the formal approval of partition fire barriers (and their associated fixings and penetration seals) is in progress, which will provide this performance. In addition to this, the UK design for PWR power stations also calls for a 3-hour rated fire barrier. Development work in this area is also in progress.

It is important to note that the basic performance requirements and hence test methods for such barriers, will be of the same form as those set out for Class II fire barriers except that they must be maintained for the appropriate longer period.

Class II fire barriers are further divided into two categories dependent upon the function which they fulfil. Category 1 fire barriers are used to provide the required segregation defined above. Category 2 fire barriers are employed within a given segregation 'zone', acting as smoke and fire stops, thus restricting the extent of fire damage and also assisting fire detection and fire fighting. This categorisation is merely one of functional definition as the performance requirements for both Category I and Category 2 fire barriers are the same.

Cabling

This distinction can be seen more clearly by examining typical cable installation fire barrier configurations, examples of which are shown in Fig 6.119.

Configuration 1 shows a simple plain wall, partition type fire barrier. This type of arrangement is most Category 1

Chapt er 6

fire barrier. The second configuration shows an ex_ ample of a Category 2 fire harrier used to provid e zoning within a particular segregation area. Here, the most important features are the so-called 'penetration s,. Since Category 2 barriers provide fire protection withi n a segregation area they will, by definition, have cable

BARRIER , x NG

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GLAZED PIPE DOOR iNCLUDING OPERATING

.41EChIANISMS AND SEALS

CARIES SUPPORTED

ON STEELWORK

FIRE BARRIER

BARRIER

FIXING

FRAMEWORK

•

CABLE PENETRATIONS

i SINGLE AND MULTI OPENING' MAY OR MAY NOT INCLUDE CABLE suppoRT STEEL

CANTILEVER ARMS

CABLE RISER WALLS

FIRE BARRIER

INCORPORATING

SUPPORT ARRANGEMENT

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:

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• • •

SECTION X X

CONFIGURATION I TYPICAL CATEGORY 1 PLAIN WALL TYPE FIRE BARRIER

CONFIGURATION TYPICAL CATEGORY 2

CROSS-BARRIER

INCORPORATING CABLE AND PIPE PENETRATIONS AND FIRE DOOR

CONFIGURATION 3 TYPICAL CATEGORY 2 HORIZONTAL CROSS BARRIER IN A CABLE RISER

•The ability of these materials to propagate a fire, both vertically and horizontally.

•\\. hen PVC and to some extent the other common-

used materials burn, they generate considerable quantities of dense smoke and toxic, acidic fumes and gases which may rapidly reduce visibility and render the atmosphere hostile to humans in confined spaces. This is aggravated by the presence of the Category 2 fire barriers. The main constituent of these combustion products is hydrogen chloride ‘‘hich readily combines with available moisture to Form hydrochloric acid. (Water is produced during combustion as well as being present in the atmosphere.) Even after a fire has been extinguished, the corrosive deposits which result may cause long term damage to sensitive equipment such as relay contacts.

•The gases driven off by the combustion of PVC are also highly flammable. Indeed, it is believed that It is the burning of these gases, with recorded flame

10.2.2 Proximity of the fire to the barrier

Having identified the nature of the fuel source and its combustion characteristics, it is necessary to consider how near a potential hazard may be to the protective barriers.

An ideal cable installation layout would ensure that the fire barriers were installed some distance away from the major source of combustible material, hence reducing the level of performance required from the barriers and consequently their cost. This invariably proves impractical, for two reasons.

Firstly, the space available for cable routes is limited by civil structure costs and, secondly, as has already been mentioned, that cables need to pass through Category 2 barriers (i.e, those used to provide zoning within a given segregation area). This type of fire barrier will, therefore, have the principal source of combustible material in intimate contact with the barrier.

These two factors dictate that all Class II fire barriers must be designed to provide the required level

of performance assuming the fire to be immediately adjacent to the barrier. In this way, the most onerous practical conditions are catered for at the design stage.

It is clear from the above, that the nature of the fire hazard present in power station cable installations is unique. The CEGB has therefore produced its own internal technical specification which sets out performance requirements, This specification is becoming increasingly well known throughout the fire barrier manufacturing industry. These technical requirements are discussed in more detail in the following section.

10.3 Fire test requirements

It is considered that the only representative method of assessing the performance of a fire barrier is to conduct a full scale fire test, i.e., to place a sample of the fire barrier in front of a 'test fire' and examine how it performs. To make this test fully representative of 'as-installed' conditions, the specimen barrier must include all fixings, accessories, penetrations, doors, etc., which would be included in use.

This is the philosophy set out in British Standard Specification BS476: Part 8: [24], the document which forms the basis of the CEGB's fire test requirements. This specification primarily addresses test methods and performance criteria for materials used in building construction. Whilst this standard was conceived for more conventional applications, such as factories and other public buildings, its philosophies are obviously equally applicable to the power station cable installation.

In order to make a fire test 'repeatable and hence provide a means of comparing the performance of one barrier construction with another, it is necessary to define a standard set of fire conditions. The severity of this test fire is specified in terms of a time/temperature curve. BS476: Part 8: 1972 specifies such a time/ temperature relationship, to which the test furnace should be closely controlled within laid down limits for the duration of the test. This is the so-called `BS476 fire test curve' and it has been derived from the burning characteristics of typical materials found in normal building environments (such as wood, fabrics, etc.). General construction fire barriers such as blockwork walls have been designed and tested to these requirements. This fire test curve is shown in Fig 6.120.

Other time/temperature test curves exist for special applications however. A typical example is the hydrocarbon fire curve developed by the Mobil Oil Company which is used extensively throughout the petrochemical industry. This fire test curve recognises that when hydrocarbons burn, they do so initially at much higher temperatures than do the materials considered by the British Standard. The Mobil curve therefore features a higher initial rate of temperature rise than does the British Standard curve. In the longer term, if there is sufficient fuel available, a building material fire will

1000

900

800

7 00

I., 600

2500

400

300

200

1 00

FIG. 6.120 Fire test curves (BS476 and Appendix A)

attain higher temperatures than the hydrocarbon fire. Again, this is reflected in the profile of the two ti me/temperature curves.

Similarly, cable insulation, bedding and sheathing materials have different burning characteristics from conventional fire hazard materials as a result of the combustion of the gases driven off. The CEGB fire barrier specification therefore contains a fire test curve, known as the 'Appendix A' curve, which acknowledges this fact. This test profile has the high initial rate of temperature rise of the hydrocarbon curve, flattening out at a temperature of 1050 ° C and is also shown in Fig 6.120 for comparison with the British Standard curve.

Since Class II fire barriers must be capable of containing the effects of a cable installation fire for a minimum of one hour, a one hour long fire test is conducted on representative barrier specimens.

BS476: Part 8: 1972 identifies three major properties of the barrier which are indicative of its level of protection and these are:

•Stability The ability of the fire barrier to remain intact and in a stable condition without excessive deformation for the complete duration of the test.