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

LADDER FOR

MOON TWO

CABLE SAMPLES

TiEiGHT OF ENCLOSURE = 3,,

ACCESS DOOR ,VITH

OBSERVATION WINDOW

FIG. 6.104 3-metre smoke cube

source and photoelectric cell assembly to be mounted externally, so that the attenuation of light with increasing smoke density can be measured. The photocell is designed to have a response close to that of the human eye. A fan is provided to mix the smoke and give reproducible results. Since a large number of manufacturers and test authorities have constructed this test equipment, there is clearly a need to ensure they all have similar characteristics. This can be achieved by burning known ratios of toluene/alcohol in the cube and checking the light attenuation against reference values. The 3-metre cube is now a well established test equipment that is relatively simple and very practical.

Having established suitable test equipment to measure smoke from a representative section of cable installation, we must now consider the fire model. The fire model used by LTE consists of burning a number of cables by placing them in horizontal formation over a fire source consisting of a tray containing I litre of alcohol. The number of cable samples is selected such that the test configuration contains 2-3 kg/m of combustible material which is considered typical of an underground railway installation. However, within a power station, higher densities of cables are involved and a number of routes are vertical; the LTE fire model is therefore not suitable for power stations. Considering power station applications, quite clearly for smoke tests it is desirable to use a vertical array of cable samples known to be the more onerous case. This follows the same principle as for reduced propagation tests. Taking this one step further to be consistent, the same type of fire model should be used for both the reduced propagation and smoke tests (i.e., a vertical ladder with a propane-gas burner fire source). In practice it is not possible to use the identical arrangement for smoke

tests because of limitations of the 3-metre cube te st equipment, e.g., the cable quantity subjected to the test has to be limited to avoid smoke saturation i n the cube. Although it is not possible to carry out smoke tests in the 3-metre cube on the largest array of cabli ng used in a power station, it is possible to use a representative sample which is adequate to correlate smoke emission with the effects of propagation.

The fire model selected for type tests is shown in Fig 6.105 and consists of cable samples 2 metres long, wired to a vertical ladder. The number of cable sampl es is selected to give a loading of 5 kg/m of non-met allic material and the cables are mounted in the same formation as detailed for reduced propagation in Section 8.2 of this chapter, i.e., power cables spaced and control cables bunched. It is important to note that cable formation is a dominant factor in smoke production and that variations in cable spacing can significantly affect results. The fire source itself consists of the same propane-gas ribbon burner and flow rates as used for the propagation test. The test regime consists of apply. ing the fire source for a period of 25 minutes, after which it is extinguished and the samples are left burn or smoulder until maximum smoke productioi: has been achieved. By this method both flaming and non-flaming modes can be assessed.

CABLE SAMPLES 2m LONG

LAODER

GAS BURNER

II

FIG. 6.105 Cable sample arrangement for smoke test

During the test, the intensity of the light received by the photocell is monitored on a chart recorder as

a measure of transmittance. The recorded light trans- mittance can be converted to absorbance using the

formula:

A, = login 1 0/ 1- t

initial luminous intensity

kl here IC)

-- intensity of light beam through smoke

t

where D = visibility distance

K = parameter dependent on light level and type of exit sign

From Equations (6.8) and (6.9):

ormal to present the results as standard absor- ;,,i ,1, n A„ by the formula

A,) -= A, x

-.s. volume of test chamber, m 3 t' = optical path length, m

,is hen defined as the absorbance produced across he faces of 'a 1-metre cube when the test installation

!, burnt under the given conditions.

The calculation for A o ignores the effect of any ‘riloke deposited on the windows of the optical system.

flis is because experience has shown that the absorb-

.111,:e associated with this deposition on the windows ,, ;mall compared to the absorbance associated with he smoke in the cube, and negligible error is caused i2noring this factor. Where it is wished to take :MN factor into account, the absorbance associated with Jcposition on the windows can be measured after clearmoke from the cube and the appropriate correc-

!i ons made.

Clearly A 0 can be calculated at any time during ;he test but the most significant measurements are „on,idered to be at the end of the test flame appli- -aion period, A, (ON), and the maximum after the :lame is extinguished, A 0 (OFF). Quite clearly the inft,t critical factor is the total smoke generation drid since this is achieved during the A o (OFF) period,

,hould be used for the acceptance criteria. A typical !e,i requirement would be that A, (OFF) should not ,A,eed 10.

84.2 Use of test information

I la ,,ing defined a test method we can now try to re-

The 'expected absorbance' in a particular location

A0 --: absorbance across the faces of a 1 m cube found from test

nfactor depending on quantity of cable involved

Now the factor 'n' must relate the quantity of cable material tested to that contained in the power station installation. It is suggested that the smoke test be carried out on an array containing 5 kg/m of non-metallic combustible material. Therefore, assuming a linear relationship:

Typical values for the parameter K are given in Table 6.23. These parameters are based on experiments carried out in a 3-metre cube, using a random sample of observers to assess the visability of typical signs over a range of distances through a known density of smoke.

TABLE 6.23

Typical values for the parameter K

As an example, consider a cable flat 2 m wide by 2 m high and 50 m long. Using a test acceptance criterion of A 0 (OFF) = 10 and assuming one cable tray carrying a non-metallic combustible cable mass of 10 kg/m, for self-illuminated signs the visibility would be:

The results obtained by this process should only be used for guidance since the process assumes an even dispersion of smoke particles and ignores any irritant affect that smoke may have on the eyes.

8.5 Corrosive gas emissions

At present, bench-top tests on samples of materials taken from cables are considered the most practical method of assessing corrosive gas emissions.

The most commonly used of these bench-top tests is that specified in IEC 754. This test method requires

that approximately one gram of material is pyrolysed in a combustion tube using an electric furnace. Ramp heating is used with a maximum temperature of 800 ° C and the resultant gases are analysed by titration methods. Hydrogen chloride and hydrogen bromide are assessed as hydrogen chloride and the limit of sensitivity of this method is 0.5%. The difficulty with this test method is that it only assesses two of the halogen gases and, whilst hydrogen chloride may be potentially the most aggressive, other corrosive gases cannot be ignored. Another drawback with this method is that the limit of detection is 0.5% and may not be adequate for recent developments in low fire risk cables.

The CEGB has therefore developed a test method which monitors pH and conductivity which can detect all acid species. A diagrammatic arrangement of the test apparatus is given in Fig 6.106. The combustion arrangement is similar to EEC 754 in that the sample is pyrolysed in a combustion tube using an electric furnace and ramp heating up to 800 ° C. A sample size of one gram is used and the combustion products are drawn through distilled water of which the pH and conductivity is measured. Since hydrogen chloride is

of particular concern, a sensitive chloride ion electrode is included which can monitor to a level of 0.05%.

Experimental work has been carried out for the CEGR to assess the corrosive effects of the more commo n acid gas species on items such as printed circuit boards and relay contacts. From this work it is possible to judg e the maximum concentration of gas in parts per milli on (PPM) that is tolerable from a corrosion aspect. Using this information, together with a knowledge of th e quantity of cable material likely to be consumed in a fire and the station volume in which it may be released, it is possible to predict pH acceptance levels for the test method discussed.

8.6 Toxic gas emissions

At present, bench-top tests on samples of material taken from cables are considered to be the most practical way of assessing toxic gas emissions. There are several such tests available but the most established in this country is that defined in the Ministry of Defence specification NES 713. This test procedure has therefore been used as a basis for CEGB assessment of toxic gas emissions.

test method consists of . burning a four

,1::: e, aCaEl pGmetre. The chamber is constructed from a

1(typically polypropylene or polycar- iiilai [‘ei rni,a a tranyarent access door in wohnieside.irin a sealed chamber having a volume of

obtained from cable insulating materials. ()\icity index can be calculated from the measured

ncentrations using IDLH values. The immediate

j.til2er to life and health (IDLH) value, as defined r, the NIOSH/OSHA 'Guide to chemical hazards',

a maximum level from which one could ,.„re %satin 30 minutes without any escape-impairing

:liriorns or any irreversible effects on health.

Cable accessories

iNSULATED SECT:ON

9 Cable accessories

9.1Cable glands

g1.1 Background to gland design

elands are used to terminate the outer finishes ables (i.e., inner sheath, armour and outer sheath) to take the insulated conductors through into the

•.j uiptucnt. The gland therefore locates the cable at the

.,;inpment and is required to form a moisture seal to

..,,t11 the cable and the equipment. Glands also provide iieans of making connections to the armour for rig when required. Where connections to earth are

.,itlircd, it is important that the gland is fitted with an

!cgal earth lug so that a bond can be installed to

•.kc the fault current direct to the earth bar. Without provision the fault current would have to flow to

1,ia the less secure route of gland threads, gland H.:!c securing bolts and equipment casing.

Hie requirements for cable glands are specified (ii)CD Standard 190: 'Insulated mechanical cable

...111J5 Part I — metallic glands'. Insulated glands the cable armour to be isolated from the gland ;'.ate and hence earth. Typical arrangements of insu-

•kd glands are shown in Fig 6.107. Insulated glands with GDCD Standard 190 must incorporate following features and offer certain advantages

cr metallic glands to BS6121, Part 1:

•

Insulated glands allow single-point bonding of sin- gle-eore cables to prevent armour circulating currents

and hence permit current rating to be maximised.

FIG. 6.107 Typical arrangements of insulated cable glands

•Insulated glands allow single-point bonding of control cables to prevent circulating or fault currents flowing in the armour and hence reduce the risk of interference in control and instrumentation circuits.

•Insulated glands allow cable armour to be isolated from earth to enable cable outer sheath integrity to be tested.

•An integral earth lug is specified to provide an adequately tested means of connecting the cable armour to earth.

•GDCD Standard 190 requires that all glands for use on power cables are subjected to short-circuit tests.

•A screen terminator can be provided, as shown in Fig 6.108, to allow the copper tape screen of 11 kV cables to be electrically connected to the cable armour.

•GDCD Standard 190 requires glands to accept a wider range of cable sizes.

9.1.2 Gland construction

GDCD Standard 190 covers a range of glands which are identified by 'Class Number according to armour type and gland configuration, i.e., whether bonding connectors or screen terminators are required. The armour

types covered are aluminium wire, aluminium strip, steel wire and double steel tape. To avoid corrosion problems, for aluminium armour the gland body is constructed from aluminium and for steel armour the gland body is made from brass. The glands include an outer seal, to seal between the gland and the cable outer sheath. A gland plate seal is also provided to seal between the gland and the gland plate. Inner seals, to seal between the cable inner sheath and the gland to prevent moisture entering the cable armour area are not considered necessary, as cable boxes are normally kept in a dry condition. In addition it is essential that compression-type inner seals are not used on cables insulated with materials such as low-density polythene, since this is likely to flow from under the seal during load cycling and reduce the dielectric strength of the cable.

Multipair cables, as described in Section 3.6 of this chapter, have a drain wire provided to enable connections to be made to the screen. Whilst insulated terminators can be provided on the gland for this drain wire to be terminated, it is normally more convenient to provide an insulated block of terminals in the equipment to terminate the drain wires for connection to each other, or to earth, as appropriate.

The nisulated portion of the gland is designed and tested to withstand 2 kV for I minute. The figure of 2 kV matches the design criteria for rise of potential on single-point bonded cable armours under fault conditions, this figure also being the minimum voltage withstand for cable sheaths. Quite clearly, if gland bodies can rise to voltages of up to 2 kV at the 'floating end' then insulated shrouds must be fitted for personnel safety. Where control cable glands are not bonded to earth these must also be shrouded to protect personnel

from transferred potentials. There is no requirement to shroud glands that are bonded to earth and

this would he physically difficult because of tj-k: ing connections.

With respect to short-circuit tests, it is jud_ . all power cables that could be fitted into gla of 40 mm and smaller would be fuse prote , therefore these sizes of glands are tested at 13.1 kA for 0.1 s. Gland sizes of 50 mm an may accept cables that are circuit-breaker and these are tested at a short-circuit level

for up to 1 s depending on the capability of armour.

Insulated glands for use with wire braid, -dell as on flexible cables, are not required since such cables are normally short and there is no risk of circulating currents. Standard CX glands to B56121 are therefore used for braided cables.

9.1.3 Gland sizing

When sizing cable glands the following factors must be taken into account:

•The dimension over the cable inner sheath (under armour) must be smaller than the gland ! - ore.

• The dimension over the cable outer she smaller than the fully-opened outer seal

than the test mandrel size to ensure an ad _late seal.

•The gland must be selected for the armour Mgt material and size.

There is not a large overlap in cable accommodation between gland sizes and therefore gland sizing should be carried out on the cable manufacturers' dimensional

•Electrical load cycling test — 2000 cycles with resistances and temperatures measured every 100 cycles.

•Tensile test.

The test results are analysed to check whether the compression fittings have remained stable throughout the test.

Engineering Recommendation C79 was published before the large scale introduction of elastomeric cables; hence the load cycle test temperature of 80°C above ambient and the short-circuit test temperature of 160 ° C are appropriate to the cables commonly in use at that time, i.e., paper or PVC insulation. When dealing with fittings for elastomeric cables (e.g., XLPE or EPR), it is considered appropriate to increase the load cycle test temperature to 90 ° C above ambient and the short-circuit test temperature to 250 ° C.

As already stated, the short-circuit tests are optional and the choice depends on whether the fittings are going to be used on circuits protected by HBC fuses. As discussed in Section 4.2.1 of this chapter, multicore power cables are always fuse protected and there is no requirement to carry out short-circuit tests on fittings for shaped aluminium conductors. Single-core cables with stranded aluminium conductors are normally associated with circuit-breaker controlled circuits. Fittings for these must therefore be short-circuit tested. Fittings for circular solid aluminium conductors are associated with earth cables. Earth cables of 150 mm 2 and larger are used to bond plant that is associated with circuitbreakers, so the conductor fittings must be short-circuit tested. Earth cables smaller than 150 mm 2 are used to bond plant protected by HBC fuses and there is no requirement to have short-circuit tests carried out on these sizes of conductor fitting.

At the time of writing there is no published British or IEC Standard to control dimensions of conductor fittings. It is therefore left to the user to specify dimensional constraints, which is essential to ensure that clearances and creepage distances are maintained within standard terminal arrangements. British Standard BS5372: 1976 gives requirements for cable terminations for electrical equipment.

9.2.2 Fittings for copper conductors

The construction method for compression fittings for copper conductors is similar to that for aluminium which was discussed in the previous section. Although not mandatory it is normal practice to tin copper fittings. The requirements for tools and dies, and for marking (given in Section 9.2.1 for aluminium fittings) are considered to be equally relevant to copper fittings.

Test requirements for copper fittings are given in BS4579: Part 1: 1980 — 'Performance specification for compression joints in electric cable and wire connectors'. This specification requires six specimens to be prepared and subjected to:

•Initial resistance measurement.

•500 load cycles with resistance measurements every

50 cycles.

•Final resistance measurement.

•Tensile test.

It should be noted that, unlike aluminium, the test regime for copper fittings does not include an optio n of short-circuit tests. Experience from short-circuit tests on equipment in which conductor fittings have necessarily been included have demonstrated a need to design fittings to meet these conditions adequately. For fittings to be used on circuit-breaker controlled circuits, it is therefore recommended that short-circuit tests are carried out in a similar manner to that already prescribed for aluminium conductor fittings. A further point to bear in mind is that BS4579: Part 1 limits the fittings to use on conductors having a maximum conductor temperature of 85 ° C. Therefore, where fittings are required for use on elastomeric cables (e.g., XLPE or EPR) which operate at a continuous conductor temperature of 90 ° C, a more onerous test regime is required. In these cases it is recommended that the load cycling be carried out at 90 ° C above ambient as previously proposed for aluminium fittings in Section 9.2;1 of this chapter.

In practice, the vast majority of copper power cables used in power stations are small multicore types insulated with PVC and protected by fuses for which the test requirements given in BS4579: Part 1 for conductor fittings are more than adequate.

The comments given in the previous Section 9.2.1 regarding conductor lug dimensions and termination accommodation are equally applicable to copper fittings.

9.2.3 Formed terminations

Formed terminations are a means of terminating solid aluminium conductors by squashing the conductor flat and punching a hole through it for the terminal equipment fixing bolt. The operation is carried out using a hydraulic tool and special dies. An arrangement of formed terminations is shown in Fig 6.109.

This idea was proposed in the early 1960s (Burki and Sabine, 1963 [221), but it did not gain any immediate popularity possibly because of concerns about the mechanical and electrical integrity of the flattened palm. Formed terminations were reassessed by the Central Electricity Research Laboratories during the early 19705 and considerable effort was devoted to obtaining optimum palm dimensions for mechanical and electrical performance. The advantages of this system are:

•The sensitive interface between conductor and compression fitting is eliminated.

•The termination length is considerably reduced, relieving space problems in terminal boxes.

Plain copper or brass should also be cleaned with a stainless steel wire brush and immediately coated with petroleum jelly. Tinned surfaces need only be cleaned with a degreasing agent before a light coat of petroleum jelly is applied. Copper compression fittings should be supplied and packaged in a clean condition and only need a light coat of petroleum jelly on the mating palm surface.

It is important that the wire brush is made from stainless steel and that a separate brush is kept for each type of metal. If a joint is slackened for any reason it is essential that it is completely undone and remade using the appropriate surface preparations.

If aluminium is not involved, after suitable surface preparation the joint can be completed using normal diameter washers (to BS4320) between the joint materials and the fastener. With copper joints, the thin copper oxide film on the joint surfaces ruptures relatively easily to give metal-to-metal contact, therefore, the clamping pressure and hence the fastener torque is not critical.

Where aluminium is involved, because of the tenacious nature of the surface oxide, it is essential that the clamping pressure is sufficient to rupture the oxide fil m at contact spots so that metal-to-metal contact is formed. These contact points are actually crests or peaks that are formed on the contact surface by the preparation using a stainless steel wire brush. This contact pressure is clearly dependent on the surface area under the clamping washer and on the torque applied to the fastener. Because aluminium creeps under applied pressure and the dimensional change causes a relaxation in clamping load, it is important that the load is applied over as large an area as practicable. For this purpose, large diameter washers having an area complying with BS4320, Table 2, form C are used. These large-diameter washers are of sufficient area to restrict the aluminium creep whilst being small enough to generate sufficient clamping pressure to rupture su 1. face oxides using acceptable fastener torques. Washers complying with BS4320 have insufficient thickness to avoid deformation and this results in a reduction 01