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

ARRANGEMENT 1;

SINGLE CORE POWER CABLES

CANTILEVER ARM

TSINGLE CLAW CLEATS

WITH SPACING STRAP

LARGE MULTICORE

POWER OR LARGE

C AND I CABLES

NOT MIXED ON

THE SAME CANTILEVER

ARMS)

ARRANGEMENT 2:

muLTIconE POWER

AND CONTROL CABLES

CANT , LEVER ARM

WHERE A VERTICAL

CABLE RUN EXCEEDS

. CI M A PRESSED STEEL CLEAT IS USED ON EVERY THIRD CANTILEVER ARM

SMALL MULTICORE

POWER OR

C AND I CABLES

(NOT MIXED ON

THE SAME CANTILEVER

ARMS

000mm

installed in preformed sectionalised concrete troughs. These troughs also provide protection for cables installed in areas liable to settlement where directly buried cables might be damaged.

The trough types used by the CEGB have a U- section and they are provided with close fitting concrete lids of the reinforced pathway type. Three basic sizes are used:

•685 mm wide x 345 mm deep.

•360 mm wide x 205 mm deep.

•175 mm wide x 215 mm deep.

Cable cleating within these troughs depends upon the cable type. For both straight runs and bends, control

cables are laid direct into the base of the trough, being neither cleated to the trough nor to one another. For power cables in the two large sizes of trough, lengths of cable support channel are installed in the base of the trough at 1 metre intervals although they are not anchored down to allow for some settlement. Single core power cables are cleated to these channels using the zig-zag cleats at 2 metre intervals. Where multicore power cables are installed in troughs, they are laid on the channels spaced using thermal spacers at 1 metre pitch for cables of 35 mm 2 size and above.

Where the number of power cables is sufficient to warrant their use, J-brackets are mounted on the walls of the trough down each side. The cables are installed on the brackets in the same manner as for normal

Furthermore, the sheath of a cable could sustain damage if it were to be dragged across any sharp edges or rough surfaces during installation. In these cases, rollers and skid plates such as those shown in Fig 6.97 may be used to lift the cable away from the hazard and hence prevent scuffing. As an additional precaution, the CEGB has devised mechanical tests (see Section 3.11 of this chapter) to demonstrate that the cables which are used in modern power stations have some degree of resistance to this type of damage.

HOOP ROLLER

SKID PLATE ROLLER

FIG. 6.97 Typical rollers and skid plates

Pulling cables into ducted runs brings with it its own unique problems, mainly due to the restricted space within the duct and the inaccessibility of the cable once the pull has commenced. These problems become worse as the length of duct run increases.

In addition to the number of, and radius of, the bends in a ducted run, the force required to pull a cable through a duct will depend upon the coefficient

520

of friction between the cable sheath and the inner surface of the duct. This coefficient of friction can be reduced by the use of proprietary cable pulling lubricants. Care must be taken when selecting such pulling lubricants to make sure that they have no long term detrimental effect on the cable sheathing materials.

Another problem which needs Lo be considered i s that of jamming. Jamming is the wedging of cables within the duct where three or more cables are laying side by side in a flat plane. This situation can easily lead to excessive tension being applied to the cable, hence damaging it. This condition is most likely t o occur when pulling cables around bends and can be avoided by the careful selection of cable fill for a particular size of conduit.

There is another potential source of installation damage to cables which is specific to ducted runs, if more than one cable is to be routed in a given duct, then they should all be installed at the same time. Additional cables are never installed over existing ones because the winch wire used to pull the cable can easily cause cut-through damage to the sheaths of th e original cables.

It has been pointed out in the previous section on cable support steelwork, that the CEGB favours the use of cantilever type cable support arrangements. When cables are being installed, it allows them to be pulled from the drum, laid alongside their support steelwork and then lifted into place. This will in turn reduce the loads to which the cables are subjected during installation. Where the use of trapeze type cable support structures is unavoidable (usually only for certain seismic applications where additional rigidity is required), installation is greatly complicated by the need to thread cables through the supports.

When the cable is being pulled from the drum, it is important to ensure that the drum itself is conveniently placed, so that the cable does not have to negotiate any unnecessary bends. The drum itself should be supported on a freely turning axle with the cable being removed over the top of the drum. The cables must never be removed from the drum by rolling the drum along on its flanges, since the arc length at the drum flange will be greater than that of the cable on the drum. Attempting to do this will lead to the cable being used to skid the drum round on its flanges.

The CEGB normally uses two basic pulling techniques, hand pulling and winch (or nose) pulling.

The former is self-explanatory. Its advantage is that cables are unlikely to be subjected to excessive pulling tensions. The drawback is of course, that the technique is highly labour intensive and hence expensive. Good access is also essential for the whole of the route- length, requiring complicated and expensive scaffold ing. Obviously, the larger the cable, the more difficult it will be to hand pull it.

In the alternative method, the pulling force is pro - vided by a motorised winch. Here, a winch rope (usually of the plastic coated steel type) is connected to the

NR. 6.98 Method of attaching pulling bond to multipair cables

It is also important to ensure that a non-stretch

,inch rope is used, as this will remove the tendency 'or a succession of impulse loads to be applied to the

,able during pulling.

Since a motor is providing the pulling power, there al,o more potential for applying excessive load to

'he cable using this pulling technique. It is essential :icrel'ore that the winch operator is in close corn- FrItinication with other members of the pulling gang ,',Itioned along the run, who can warn him if the cable , ecoilies snagged or caught. When using this installa- 'ion method, it is also necessary to monitor the load

placed on the cable during the pull for comparison the maximum allowable pull tension for the cable.

I is is usually achieved by means of a dynamometer to the winch rope.

\nother method of ensuring that excessive loads %ire not applied to the cable, is by placing a shear device tu e Pulling wire which will fail at a predetermined !enNion. The CEGB does not favour the use of such

.1,2\ ices as they can cause a safety hazard, due to whip-

if they fail suddenly.

Finally, as an additional precaution against damaged a 1.5 metre length is always

trom the nose of the cable following the pull. More sophisticated cable pulling methods also exist,

‘tich as motorised rollers and running bond techniques. (hese are discussed briefly as follows.

•It offers the potential for pulling cables through very complicated routes without the risk of applying too much load to the cable. The tensile load applied to the cable only amounts to that required to pull the cable from the previous motorised roller.

•There is, in theory, no limit to the length of an individual cable pull.

The disadvantages with this technique are:

•The setting up of the rollers, including a complex alignment process, is time consuming and expensive.

•Generally individual cables (or at best, two or three cables of the same diameter), only can be pulled through motorised rollers in one pass. After each pull, individual cables would therefore have to be removed from the rollers before another cable can be pulled through the same roller set-up.

•The space available between individual cable carriers is limited and may physically prevent the use of these motorised rollers.

•The pulling equipment is much more sophisticated than that required for nose pulling and is therefore more expensive.

Running bond techniques

The principle of the running bond cable pulling technique is shown in Fig 6.99. The technique is widely used and has been developed by the distribution side of the electricity supply industry for the installation of large, delicate, high voltage cables in trenches.

A steel bondwire, of at least twice the length of the cable section length over which pulling is to be carried out, is run-out through the whole section length over cable rollers positioned along the line which the cable is to follow. As with nose pulling techniques, those rollers should be installed at sufficient frequency to prevent the cable from dragging along the support steelwork.

The cable is then tied to the bond wire at 2 metre intervals along its entire length, increasing the frequency of ties if the cable is to be installed on a steep incline or through vertical run sections. At each change of direction during the pull, the bond ties are released and the cable is taken round the bend using separate skid plates and rollers with the bond wire passing through a separate snatch block. After each bend, the bond ties are replaced. Whilst traversing each bend, the nose of the cable is guided over the corner rollers to ensure that a positive tension is maintained to prevent build up of slack at the bend. The advantages of this pulling technique are similar to those of the motorised roller technique, namely:

•The reduction of tensile load applied to the cable.

•The potential to pull through complex routes.

•The potential to pull through very long routes.

The disadvantages again revolve around the time and effort required to set up the rollers and snatch blocks prior to pulling. The tying and untying of the bond wires at each bend is also very time consuming and labour intensive.

In general, the cable support steelwork design and layout philosophy used by the CEGB, in particular the use of cantilever supports, means that the motorised roller and running bond cable pulling techniques are not necessary. They do, however, have a potential for application to the problem of pulling cables through successive trapeze type supports.

8 Cable performance under fire conditions

This section deals with cable performance under fire conditions with respect to flame spread and fume emissions, and with the tests used to evaluate these factors. Whilst the burning characteristics with respect (0 flame spread and fume emissions are controlled, such cables may not necessarily be designed to continue

functioning under fire conditions. Where circuit integrity is required, then short-time fireproof (STEP)

cables as described in Section 3.7 of this chapter must be used.

•Category 1 - single core cables tested in spaced formation.

•Category 2 .- multicore power cables 16 mm 2 arid above tested in spaced formation.

•Category 3 - multicore control cables and small power cables up to and including 16 mm 2 tested in touching formation.

•Category 4 - multipair control and instrumentation cables tested in touching formation.

All categories were tested with a non-metallic mass of 10 kg/ni.

The test rig specified in the first issue of GDCD Standard 21 was based on the Italian CESI laboratory test rig in Milan, the general arrangement being shown in Fig 6.101. The test method consists of mounting cables in a vertical arrangement within a chimney to produce an onerous condition. The cables are heated at the base, using electrical hot plates, with a minimum temperature of 600 ° C and pilot flames are provided to ignite flammable gases that are driven off. Once the cables have ignited they are left to burn and, after one hour, if they have not self-extinguished they are manually extinguished. The test is considered satisfactory if the traces of charred damage on the cables do not extend more than 1.5 rn above the hotplates. All

test rigs used must be proved by testing untreated PVC cables to ensure that propagation occurs.

In 1982, EEC Publication 332: Part 3 'Tests on bunched wires or cables' was issued. This specifies a test similar to the first issue of GDCD Standard 21, the major differences being that the IEC specifies a gas burner instead of electric hot plates and in addition the acceptance criteria is increased from 1.5 m to 2.5 m. The general arrangement of this test rig is shown in Eig 6.102. The IEC burner is of the ribbon propane-gas type with a fuel input rate of 73.7

proximately 2.5 kg/rn, 5 kg/rn and 10 kg/rn using a typical density for the cable combustible materials. A test rig complying with IEC 332: Part 3 had been built in the UK at Queen Mary College Industrial Research Limited and the CEGB evaluated this against the existing GDCD Standard 21 protocol during 1982. Tests were carried out at 10 kg/rn on at least one cable in each of the four GDCD Standard 21 Categories. In addition, an untreated PVC cable was tested to ensure that the rig was capable of producing propagating fires. These tests demonstrated that the EEC test rig could be used as an acceptable substitute for tests to GDCP Standard 21; the test rig was therefore adopted for cable constructions containing PVC materials. Because 01 the size and complexity of power station installatioflS

_

TEST CABLES

6.101 Reduced propagation test rig (electric hotplates)

:,;,iitional requirements beyond the IEC test - method

.pecified to reduce the risk of fires propagating. Plc maximum density of cables that the IEC specifies ID kg/m which is the same as GDCD Standard 21, 1 -, Lle 1. This generally results in an economic arrange-

.i cuories 3 and 4, the cables are installed touching

.a1,1 a restriction to 10 kg/m of non-metallic material in the cable tray being under-loaded as far as ra,c and weight considerations are concerned. For

' 4 output. The reason for this is that it is possible

APERTURE

tat GENERAL ARRANGEMENT

(b) TYPICAL TEST RESULT

FIG. 6.102 Reduced propagation test rig (gas burner)

to design a cable with suitable heat barrier tapes to pass the test with a 70 000 Btu/h burner. However,

when this cable is subjected to a greater heat source, as may be the case in a real fire situation, then the barrier tapes may break down allowing the more flammable insulation within to be exposed and propagation can occur. By trying to obtain penetration to the conductor during the test this risk is reduced.

During the 1970s special PVC compounds were formulated to enable cables to be constructed to pass these propagation tests. This was achieved by the use of various additives which did not generally affect the mechanical performance of the PVC. The most commonly used additive today is antimony trioxide which acts together with the chlorine in PVC to suppress the flames.

The use of PVC produces cables that have high smoke and acid emissions due to the halogens in the material. A requirement for low smoke and acid emission requires the use of non-halogenated materials, but means that the halogen gases are not available to interrupt the combustion process. One of the polymers that is becoming popular for limited fire hazard (LFH) cables is ethylene/vinyl acetate (EVA) and, to give this reduced propagation performance, it can be heavily filled with aluminium hydroxide (alumina trihydrate). This filler contains about 35% water which is released at temperatures above 200 ° C with the absorption of heat. The steam produced also dilutes the flammable gases given-off from the polymer.

Whilst Category 1 and 2 LFH cables were found to behave in a similar manner to PVC cables, when testing non-PVC Category 3 and 4 cable designs (which are required to be mounted in a close touching formation) it was found that they behaved i,n a manner different to PVC cables. With PVC cables there is a greater tendency for propagation with increasing mass. This was not apparent with new limited fire hazard designs. It was noted that cables nearest the burner became involved in the fire but any cables in the second or subsequent further-removed rows were substantially unaffected. With PVC cable designs it had been normal for all cables at burner level to become involved in the fire. In service installations however, cables are not normally tightly held in regimented rows as specified for this fire test. In a cable riser, the cables are cleated in small bundles having a maximum diameter of 75 mm, whilst on horizontal runs the mix of different sizes of cables gives a natural break-up from the close array that may be attained with cables of one size. Tests were therefore conducted with cables fixed to the ladder in bundles approximately 60 mm wide with 20 mm between bundles. When tested in this manner, it was found that cables tended to propagate fire more readily and this tendency increased with the mass being tested. Indeed, tests were carried out with cables mounted in blocks over the range from 2.5 kg/m up to 30 kg/m of non-metallic material and consistent results were obtained throughout.

In summary, For Categories 3 and 4, a close regimented touching array as specified by IEC 332, Part 3,

is considered acceptable for PVC, since normally a ll non-metallic material is involved during test at bur ner level. However, this method of mounting is not typi. cal of an actual service condition and for new cable designs it may give inconsistent results. The CEGB Standard for li mited fire hazard cables therefore r e. quires Category 3 and 4 cables to be tested

bundles.

In practice, the number of cables that can be placed on a cable tray is limited by the space available on th e cable supporting steelwork or the maximum weight it can carry. For Categories 1 and 2, where the cables are installed in spaced formation to achieve adequate current rating, space is the limiting factor and the maximum non - metallic mass that can he usefully used is 10 kg/m. For Categories 3 and 4, where the cables are bunched, the maximum non - metallic mass is con- trolled by the maximum cable dead-load that the steel. work can support. As discussed in Section 7 of this chapter, GDCD Standard 197 600 mm wide ladder racks are designed to carry a dead-load of 50 kg/rn. This means that a test with a non-metallic mass of 20 kg/m is adequate for Categories 3 and 4 to match the steelwork design.

8.3 Oxygen index tests

The oxygen index (01) of a material is defined as the minimum concentration of oxygen, expressed as a percentage by volume, in a mixture of oxygen and nitrogen that will just support combustion of material under defined conditions. A suitable test method is given in Appendix A of BS4066: Part 3: 1986. In this test a sample of material is mounted vertically in a glass chimney through which a known mixture of oxygen and nitrogen is passed. The top of the sample is ignited and its behaviour noted to see if it burns beyond a set distance or if it self-extinguishes within a prescribed ti me. The oxygen/nitrogen ratio is adjusted until the material under test just supports combustion, the concentration of oxygen as a percentage by volume is then recorded as the OI of the material.

The 01 of a typical untreated PVC will be of the order of 25%, whilst a treated PVC used in reduced propagation cables may have an oxygen index in the range 30 to 40%. However, it must not be assumed that a compound with a higher 01 will automatically produce a cable with greater resistance to fire propaga. tion. Two materials having the same OI may behave completely differently depending on the polymer type and the additives used to give flame retardence. It must also be borne in mind that the test is carried out at ambient temperature and also that the flame is re- quired to burn downwards like a candle; the test conditions are therefore not representative of a typical real

fire situation. For these reasons an oxygen index test must not be considered as a substitute for the proPa'

gation tests described in the previous Section 8.2.