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Cables for I.S.

installations

RTKtec 110

Intrinsic Safety Technical Paper

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Cables for I .S. insta l la t ions

In t roduct ion

Unlike most other methods of protection the requirements for cables for intrinsically safe circuits do not generally make thecables greatly different to the cable which would have been chosen for a similar but non-IS installation in a non-hazardousarea.

Nevertheless there are a considerable number of details which need to be checked, and the reader is recommended to studythe publications EN 60079-14 Electrical installations in hazardous areas (other than mines) ..and prEN 50039:2001Electrical Apparatus for potentially explosive atmospheres Specification for intrinsically safe electrical systems i whichgive full information on the subject.

There are two separate areas for consideration when selecting a suitable cable, namely the electrical characteristics and themechanical construction.

Elec t r ica l Charac te r is t ics

Any electrical cable will have the following characteristics:

Cc The total capacitance of the cable, ie. the capacitance per unit length multiplied by the length.

A worst-case capacitance per unit length figure can usually be obtained from the cable manufacturer. In the absence of suchinformation, tests may be conducted on a sample including core to core(s), cores to screen etc. to obtain a figure.Alternatively known figures from another cable of similar construction can be assumed although a safety factor should beadded to cover margins of uncertainty.

Lc The loop inductance of a cable pair, i.e. the inductance per unit length multiplied by the length.

Lc/Rc The cable characteristic of inductance per unit length divided by the resistance per unit length, which

gives a distributed inductance/resistance ratio which is independent of length.

Either (or both) of these parameters are again usually available from the cable manufacturer, or may be evaluated by tests.The spatial separation of the two cores comprising a loop has an influence on the loop inductance, therefore cables

comprising of individual pairs for each circuit will give lower figures than in a multicore cable in which one core of a pair mightnot be positioned adjacent to its corresponding return conductor.

Typical values might be as follows:-

For conductors of 0.5mm2 (e.g. 16/0.2mm):- For conductors of 1.5mm2 (e.g. 30/0.25mm):-

C = 12 to 20 nF/km C = 20 to 30 nF/km

L = 0.6 to 0.9 mH/km L = 0.7 to 1.0 mH/km

R = 40 to 45 !/km R = 13 to 15 !/km

L/R = 14 to 24 H/ohm L/R = 40 to 70 H/ohm

From the above it will be found that for gas group IIC applications a maximum conductor size of 0.5 or 0.75mm 2 will beneeded. In gas group IIB, where generally the IIC permitted reactance figures may be multiplied by three, a conductor sizeof 1.5mm2 is often adopted as standard.

Capacitance, unless cables are very long, is seldom a problem. The inductance Lc can sometimes exceed practicalinstallation values but in these cases the use of the alternative L/R ratio will meet the requirements if a cable of relatively smallcross section is used.

All cables must be able to withstand a dilectric strength test of 500 Vrms, both between conductors and between conductorsand screen if fitted. The test for some multicore cables is at a higher voltage (see below).

All conductors must have an appropriate radial thickness of insulation. Typically this will be 0.2mm minimum for the smallest

conductors, increasing for larger conductor sizes.The minimum diameter of conductors, or strands within a multi-strand conductor, is 0.1mm. Multistranded conductors musthave their terminations protected against separation of individual strands, for example by core-end sleeves.

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Mechan ica l Const ruc t ion

Individual cables

Where individual cables are used for each intrinsically safe circuit, such cables will incorporate two or three (occasionally four)conductor cores. Such cables may or may not be screened or enclosed within a metallic conducting sheath, and may be fixedin position or trailed to moving apparatus as required.

These individual cables must be able to withstand an electrical test of 500Vrms between conductors, conductors to earth, orconductors to screen/sheath if fitted. Flexible cables must be of double-insulated construction in which the outer sheath is intough rubber or polychloroprene, or consists of a plastic insulation of robust construction at least equal to that of heavy toughrubber sheathing.

The durability of these cables must be maintained over the whole ambient temperature range which may be encountered inservice.

Multicore or Multipair cables

Multicore cables may be used for multiple intrinsically safe circuits but may not contain both IS and non-IS circuits within thesame cable.

If the multicore cable incorporates conducting screens over the conductors of each circuit, e.g. screened pairs or screened

triples, then such cables may be used whether they are fixed in position throughout their length or are movable in servicealong part or the whole of their length.

If the multicore cable does not incorporate screens over individual circuits then it is only suitable for fixed installations, unlessthe parameters of the contained circuits are such that they will remain safe in the presence of mutual faults (full details ofwhich are given in the above referenced Standards).

Multicore cables not equipped with individual circuit screens must also withstand the elevated voltage test of 1000Vrmsbetween one bundle of half the conductors and another bundle of the remaining conductors.

Fau l t cons idera t ions fo r mu l t i -core cab les

Faults which, might have to be taken into consideration when using multi-core cables in intrinsically safe circuits, depend uponthe type of cables used.

Type A cable

Complies with the requirements for multi-cores as discussed before and in addition has conducting screens providingindividual protection for intrinsically safe circuits in order to prevent such circuits becoming connected to one another the coverage of those screens must be at least 60% of the surface area.

No faults between circuits are taken into consideration.

Type B cable

Is fixed and effectively protected against damage, complies with the requirements for multi-cores as discussed beforeand in addition has no circuit contained within the cable having a maximum voltage Uo exceeding 60V.

No faults between circuits are taken into consideration.

Other cable types

Complying with the requirements for multi-cores but not with types A or B, it is necessary to consider several faultssimultaneously. Therefore, these types of cable are not recommended.

Iden t i f ica t ion by co lour

If it is a desired practice to identify cables by colour, then intrinsically safe circuits should have their outer sheaths colouredlight blue. Similarly, cable trunking or raceways within cabinets and cubicles, or for cable runs within the plant, may be

coloured light blue to identify their duty to carry cables of intrinsically safe circuits. In some situations, extra steps may needto be taken to avoid possible confusion with blue power cables (e.g. single-phase neutral or three-phase line).

Cables for I .S. insta l la t ions

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Screens and Sheaths

On zener barrier installations, field cables incorporating screens with overall insulation will have their screens connected toIS earth at the barrier enclosure. RTK S90 series barriers incorporate a terminal for this purpose. Screens should be free ofconnection at the field end.

Single core screened cables (e.g. `concentric` or `coaxial` types) are only suitable for field apparatus connected to zenerbarriers if their combined screen and return conductor connects to earth at the barrier end and is insulated elsewhere fromexternal contact.

With isolation interfaces the treatment of screen connections is less important and they may be earthed as convenient foroperational needs.

Cables including a metallic sheath (e.g. mineral insulated) or a wire armour will have their sheath or armour bonded to localstructural earth, and not, in the case of zener barriers, to the IS earth system.

Separa t ion f rom o ther cab les

The cables of intrinsically safe circuits shall be kept segregated from other non-IS cables, either by spatial separation, or byphysical barriers such as armouring, conduits, metallic partitions or separate cable trays. For full details of requirements therelevant Standards or Codes of Practice should be consulted.

RTK Engineering Ltd St . Peters Square Ha rrogate N orth Yorkshire HG 2 0N P England

Telephone +44 (0)142 3 5 08 25 3 Fax + 44 (0)142 3 5 31 13 0 Emai l solutions@ rtkeng.co.uk