3 Cables, Conduits and Conductors

570



Electrical installations in hazardous areas



20.3.2 Inductance and capacitance



The cable or conductor arrangement will have been selected to ensure that

its energy storage capability does not adversely affect intrinsic safety and

this normally involves inductancelresistance ratio and capacitance. (Inductance itself is normally not used as a selection parameter except where the

inductance/resistance ratio acceptable in the circuit is so low as not to be

usable with standard conductor configurations such as can be the case in

high current circuits.) The capacitance between the circuit conductors may

be based upon failures between the causing of interconnection of conductors as this increases capacitance and hence stored energy, rather than

reducing stored energy as is normally the case with inductance because of

current division. (This is normally only a problem in circuits using screens,

using more than two conductors or led through multicore cables which are

subject to fault - see Chapter 13). Experiments have shown that the multiconductor configuration does not lead to a directly additive situation and

the capacitance of typical multi-conductor situations approximates to the

figures given in Table 20.2.

Table 20.2 Effect on capacitance of interconnection of conductors in multicore cables

Number of

conductors

interconnected

None

None

2

2

3



3



Screen

fitted/not fitted



hc

Amount by w i h

the capaatance

between any two cores

should be multiplied

1.o

2.0 (note)

1.5

2.5 (note)

2.5



Not fitted



Fitted

Not fitted

Fitted

Not fitted

Fitted



3.0 (note)

~



~



Note: 1 The screen is assumed to be deliberately connected to one side of the circuit w i h is

hc

the normal situation.



Where conduit systems are used it is not possible to use separate

insulated conductors as is normally the case unless the inductance,

inductance/resistance ratio, and capacitance of the circuit can be defined in

the worst case because of the possible movement of conductors within the

conduit. It is normally necessary to ensure that the conductors are secured

together to guarantee their respective positions and to fit a screen which can

be connected to the potential equalization system to negate the influence

of the conduit if it is metallic. Any screen used for this purpose must, be

insulated to withstand a test voltage of 500 V rms.

Intrinsically safe circuits and non-intrinsically safe circuits must not be

contained within the same conduit.



Installation of intrinsically safe apparatus 571



20.3.3 Cable installation



Cables containing intrinsically safe circuits should be installed to minimize

the risk of damage. Although failure of a cable which contains only one

intrinsically safe circuit does not directly cause ignition-capable sparking

its interconnection with local structural steel, for example, can result in

invasion. Thus the precautions normally taken with industrial installations

in areas of similar mechanical and environmental risk are appropriate. If

the risk of mechanical intervention is high, consideration should be given

to using armoured or metal sheathed cable to minimize the risk of damage.

In particular multicore cables of Type ‘B’ (see Chapter 13) require special

precautions to be taken to prevent physical damage as they will contain

more than one intrinsically safe circuit, with separation only by conductor

insulation.

In order to avoid invasion, cables containing intrinsically safe circuits

preferably should not be mounted on the same cable tray or in the same

cable duct as those for non-intrinsically safe circuits, unless at least one of

the types of cable is steel wire or braid armoured or is metal sheathed.

If this is not the case then where they are led in the same ducting or on

the same tray, the ducting or tray should be fitted with a divider which

effectively divides the two types of cable (see Fig. 20.2). This divider should

be rigid, robust and, if conducting, be fitted in such a way that it makes

good contact with the local structural metalwork and hence the bonding

(potential equalization) system.

When cables containing intrinsically safe circuits are fitted in the proximity of other conductors and cables which contain non-intrinsically safe



Rigid

divider

(earthed metal

or insulating)

Securing

straps or loom



Securing



\



Cable duct



\



Intrinsically

safe circuit

cables

(labelled or

light-blue sheathed)



Non-intrinsically

safe circuit

cables

(must not be

light-blue sheathed)



Intrinsihally

safe circuit cables

(need not be

restrained

O marked)

r



\



Non-intrinsically

safe circuit

cables

(need not be

restrained but

must not be

blue sheathed)



Fig. 20.2 Cable tray and ducting containing mixed cables. Note: As the intrinsically

safe circuit cables are not necessarily marked or light-blue sheathed then

such marking on colouring is necessary on the part of the duct containing

intrinsically safe circuit cables



572



Electrical installations in hazardous areas



circuits, the possibility of induction of energy from those circuits into the

intrinsically safe circuits needs to be considered as in extreme cases it could

adversely affect intrinsic safety. This is only likely to be a problem when

intrinsically safe circuit cables are led parallel to and, in close proximity

with, single conductor cables carrying high currents parallel to high voltage

overhead power lines.



20.3.4 Conduit installation



All conduits are subject to the normal installation requirements for conduits

(see Chapter 18). The possibility of induction, while being less with

conduits, must still be considered in the cases identified in Section 20.3.3.



20.3.5 Marking of cables, cable bundles, cable trays, or ducts and conduits



There are currently no specific requirements in the UK for marking of cables,

cable bundles or conduits containing intrinsically safe circuits although, in

BS/EN 50020, marking of connection facilities in certified/approved apparatus is required. It is, however, wise to mark cables, cable bundles and

conduits in some way to ensure that the security achieved in initial installation is not later compromised by, for example, mixing of unarmoured

intrinsically safe and non-intrinsically safe cables on the same tray or in

the same duct, or pulling non-intrinsically safe conductors into conduits

containing intrinsically safe circuit conductors. BS/EN 60079-142 for the

first time sets out minimum marking requirements. These requirements are

minimum and it is recommended that the following procedure, which is

slightly more onerous, be followed.

1. Individual cables should be marked unless their installation makes it

obvious that they contain intrinsically safe circuits, for example, where

they are in a cable tray or duct or a clearly defined part thereof, which

is clearly marked (see 2).

2. Cable trays or ducts or parts thereof, which are reserved for intrinsically



safe circuit cables only, should be clearly marked if the cables within

them are not marked.

3. Points on conduit systems at which conductors may be drawn into

the conduit, such as junction boxes, should be marked to prevent nonintrinsically safe circuit conductors from being drawn into the conduit.

Marking may be by label or colour coding or by equally effective

means. It should be noted, however, that connection facilities within

certified/approved intrinsically safe and associated apparatus will be



Installation of intrinsically safe apparatus 573



marked by the colour light blue if a colour code is used, and thus where

colour coding of cables, cable trays or conduits or parts thereof and conduits

is used for this purpose then the same colour (light blue) should always be

used. It is necessary to ensure that no other cables have sheath colouring

of light blue (e.g., thermocouple cable) where sheath colour coding is used

to idenhfy intrinsically safe circuit cables. Where the colour coding is used

for tray/ducting or conduits, other cables can be light blue although this is

not ideal as it could cause confusion.



2 . . Additional requirements for Zone 0 (and Zone 20 where

036

appropriate)

Notwithstanding Chapter 13 intrinsically safe circuits entering Zone 0 (or

where appropriate Zone 20) should only be fed down Type 9 multicore

cables containing more than one intrinsically safe circuit where all intrinsically safe circuits within the cable are category ‘ia’ (see Chapter 13). (Category ’ib’ circuits are only intrinsically safe with one fault and thus, with

more faults must be considered as non-intrinsically safe, which means that

with two faults the circuits could operate in a way which could damage

the cable).

Intrinsically safe circuits may be fed down such multicore cables where

the circuits within the cable and its screens are connected to the potential

equalization system at the same point, or each screen within the cable is

insulated from all other screens with insulation capable of withstanding

a test voltage of 500Vrms (this means that the total insulation between

screens should be capable of withstanding a test voltage of 1OOOVrms). (If

the screens can become interconnected and are earthed at different points

it is possible to invade the enclosed circuits with any current which flows

when the two points on the potential equalization system are at different

potentials as may be the case in fault conditions).



20.4 Conductor terminations

In intrinsically safe installations, particularly where multicore cables are

used, it is common to find uncertified junction boxes in installations rather,

than the case in other protection concepts where any such box would

need certification/approvalbecause its content would be ignition capable if

sparking occurred. Junction boxes in intrinsically safe installations are used

to allow individual distribution of circuits where several are included in

the same cable and in individual circuits where advantage can be identified

(e.g., in cases where certified/approved apparatus has a flying lead rather

than terminals and thus no termination provision, where the terminals in

the certified/approved apparatus are in the same enclosure as the electrical

components and it is considered this constitutes an unacceptable risk of



574 Electrical installations in hazardous areas



damage in the particular installation, and where it is considered as advantageous to have a local isolation facility outside the apparatus or its terminal

box). In these cases it is necessary to ensure that the terminal box satisfies

those construction requirements appropriate which would be applied if it

were certified/approved and the duty here falls upon the user.

20.4.7 Terminal construction



The terminals used in such boxes need to be constructed so that the

clearance, creepage and distance through encapsulant or solid insulation

satisfy the requirements for intrinsic safety (see Tables 13.8, 13.9 and 13.10

in Chapter 13). In addition, to minimize the possibility of inadvertent earths

due to stray strands of terminated conductors touching other circuits or

earth, the minimum distances between the point of termination and earth,

and between the point of termination and other circuits need to satisfy the

requirements shown in Fig. 20.3.



sulation

d>6



d



Fig. 2 . Terminal construction

03



Installation of intrinsically safe apparatus 575



It should be noted that in BS 5345, Part 4 the separation from earthed

metal was 4mm rising to 6 m m at circuit voltage above 90V peak and

up to 374V peak. This was to take account of the minimum figures in

Tables 13.8, 13.9 and 13.10 which require clearances of 4 m m at 90V peak

and 6 m m at 375V peak. Such a precaution is not necessary as the clearance figure is determined by the tables where they exceed the clearances

given in Fig. 20.3, as the Fig. 20.3 clearances are only to give minimum

dimensions for physical reasons rather than electrical ones as is the case

with the tables.

Terminals in junction boxes also need to give confidence in the quality of

connection. This means that the method of clamping the conductors needs

to ensure adequate clamping of all conductor strands and of not damaging

them in doing so. While a normal screw terminal will satisfy this requirement where the conductor ends are fitted with a crimped ferrule to prevent

direct pressure on the conductor (see Fig. 20.4), the action of the screw when

it directly impinges on a conductor, particularly a stranded conductor, is

such that effective clamping cannot be guaranteed and possible damage to

the conductor or its strands cannot be ignored. The normal terminal used

for a single conductor is shown in Fig. 20.5 and for multiple termination

in Fig. 20.6.

The use of terminals can be summed up as follows. First, terminals

where the screw impinges directly on the conductor are only permissible

for conductors fitted with a ferrule, whether they are single conductors or

multistranded conductors (see Fig. 20.4). In addition, only one such ferruled

conductor may normally be terminated in such a terminal.

Second, terminals of the type shown in Fig. 20.5 may be used for a single

conductor with or without ferrule, whether it is a single conductor or a

multi-strand conductor. Only one such conductor should be terminated in

a terminal of this type.

Third, terminals of the type shown in Fig. 20.6 are specifically designed

to allow termination of more than one conductor, whether it is a single

conductor or multi-strand conductor. Conductors with or without ferrule

Solid or stranded

conductor



-



-



Condudtor

insulation



(Note 2)



(Note 1)



-



ferrule



Fig. 20.4 Conductor crimping. Notes: (1) Conductor must exit the ferrule but must

be cut off as close to the ferrule as possible. (2) Two Independent crimping

points are necessary to ensure reliable crimping and electrical connection



576 Electrical installations in hazardous areas



I



I



Terminal loose I Terminal clamping

conductor

to accept

I

conductor I

I



Spring anti-vibration

leaves relaxed

Terminal screw

Pressure plat



I

I

I

I

I

I

I

I

I



Spring anti-vibration



d

(Ferrule not



necessary - see Note)



Fig. 20.5 Typical terminal for termination of a single conductor. Note: If the pressure

plate were omitted a ferrule would be necessaly on the conductor



Fig. 20.6 Typical terminal for multi conductor termination. Notes: (1) Vibration security is achieved by slight spreading of wings, until retained by top clamp, on

tightening. (2) Conductors should be fitted with ferrules even if a pressure

plate is present



may be terminated provided the maximum number specified by the

terminal manufacturer is not exceeded.

Notwithstanding the above it is always best to use a ferrule with multistranded conductors if possible as it gives the opportunity to identify the

crimping of all strands in the multi-stranded conductor before termination

takes place.



Installation of intrinsically safe apparatus 577



It is also necessary to ensure that terminals are secure against loosening, particularly in the case of multi-circuit junction boxes as loose wires

could cause interconnection between circuits or inadvertent earths. Most

normal terminals achieve the necessary integrity in this regard but the

practice is to use terminals which satisfy the requirements of increased

safety 'e' (see Chapter 12) to ensure this. These terminals are not normally

certified/approved as such, but use the same forms of construction and are

readily available. Most ranges of these also have the possibility of interconnection of terminals by shorting bars which allow termination of more than

one conductor at a single electrical point without the necessity of using

terminals as described in Fig. 20.6.

20.4.2 Assemblies of terminals (junction and barrier boxes)

In the hazardous area terminal boxes can only contain intrinsically safe



circuits when mounted in a hazardous area unless their configuration is

certified/approved to another protection concept suitable for the hazardous

area of installation (e.g., flameproof enclosure 'd' or increased safety 'e').

In such cases not only must the box comply with the protection concept

in question, but the segregation of intrinsically safe circuit wiring and

non-intrinsically safe circuit wiring must comply with the separation

requirements which are necessary when they are mounted in a nonhazardous area.

Where only intrinsically safe circuit conductors are present the outer

sheath of the conductor is usually removed as soon as the cable enters

the box, and conductors inside the box are then usually loomed together

or fitted inside trunking to give security as they will not move much even

if they become disconnected. Figure 20.7 shows a typical arrangement for

such boxes. The screens are usually led through the box without being

connected to the potential equalization system in such boxes, either by being

gathered on a conducting bar or by use of additional terminals (although

this latter is difficult where multicore cables with an overall screen only

are concerned). It is necessary to exercise extreme care when this is done

as the screens are not connected to the potential equalization system and

should be capable of withstanding a 500 V rms test. As testing in hazardous

areas is often difficult, the physical arrangements for the screen are important and may require its sleeving to prevent inadvertent connections. If a

connection to the potential equalization system is intended and possible

then all of the screens intended to be connected should be gathered on a

conducting bar, and the bar should be C O M & ~ to the potential equalization system with an insulated conductor which has a level of insulation

capable of withstanding a 500 V rms test which gives a connection resistance

not exceeding 1G?.

The above constructionalspecificationsalso apply to junction boxes which

contain both intrinsically safe and non-intrinsically safe terminations. While

this should be avoided as far as possible where it becomes necessary the best

way is to divide the box into two compartments (as shown in Fig. 20.8). The



578 Electrical installations in hazardous areas

Terminal box

Terminals.

Individual wires

to terminals

/

(Note 1)



Individual wires

to terminals (Note 1)



-Harness or

trunking



Insulated

,conductor

bundles

Individual

screens’

(Note 3)



bar

where the conductors

enter the harnesdtrunking



Incoming cables

(one may be a multicore)



Fig. 20.7 Typical junction box containing only intrinsically safe circuits. Notes (1) The

wires are insulated up to the terminal. (2) Where screens are connected

to the potential equalization system (bonded) at the same point they may

be collected on a conducting bar which is insulated from the enclosure.

(3) Where individual screens are connected to the potential equalization

system (bonded) at different points one to another, they may not be

connected to a common bar but shall be sleeved with insulation and fed

through terminals as other conductors



partition in this case needs to be the same as would be required in associated

apparatus (see Chapter 13) in that it should extend to within 1.5mm of the

enclosure walls and is sufficiently robust. As before, this normally means

a metal partition of at least 0.45mm minimum thickness firmly connected

to the potential equalization system or an insulated barrier 0.9mm thick.

If the material used can be easily distorted by manual pressure at these

thicknesses they should be increased until it gives confidence that normal

termination activities will not cause significant distortion. As there are nonintrinsically safe circuits in this situation it is important that the connection

to the potential equalization (bonding) system is not combined with that

for the screen of intrinsically safe cables (see Section 20.5 of this chapter).



Installation of intrinsically safe apparatus 579



Terminal

(Note 4)



Rigid conducting or

insulating divider rising to

within 1.5 mm of box lid



Non-intrinsically

safe circuit

terminals

(Note 3)



Intrinsically safe circ

screen collection



Non-intrinsically safe

screen connection bar

(need not necessarily be

isolated from

enclosure)



Non-intrinsically

safe cables

(Note 2)

Intrinsically safe

circuit cables

(Note 1)



Fig. 20.8 Mixed circuit box with divider. Notes: (1) These cables should be fed

to dedicated intrinsically safe cable traydducts. (2) Non-intrinsically safe

circuit cables should be kept separate from intrinsically safe circuit cables.

(3) Installation of non-intrinsically safe circuit cable must comply with the

protection concept appropriate or industrial practice. (4) Box construction

(as far as is appropriate to non-intrinsically safe circuit cables) must comply

with industrial requirements. Where it is hazardous area mounted it must

satisfy a relevant protection concept (e.g., Exd or Exe) as far as nonintrinsically safe terminations are concerned



If such partitions are not present the configuration should be as shown in

Fig. 20.9. The termination points of the conductors and screens of the intrinsically safe and non-intrinsically safe circuits are separated by a clearance of

at least 50 mrn measured through air between them around any rigid insulating or metal partitions present. In this case, insulating rather than metal

partitions are to be preferred unless the metal partition is firmly connected

to the potential equalization system.

As previously stated, cable harness or trunking for the intrinsically safe

circuit conductors and screens is always necessary to prevent excessive

movement, but in the case where there is no partition such harness or

trunking becomes necessary for the non-intrinsically safe circuit conductors

and screens to prevent movement lessening separation from intrinsically

safe counterparts.

A particular application of the mixed junction box is the one where it is

used for the fitting of barrier devices and thus becomes the intrinsically safe

interface of several intrinsically safe systems. This is shown in Fig. 20.10.



580 Electrical installations in hazardous areas



r5°'mmj



Terminal



minimum



,

(N:g4)



Non-intrinsicallysafe

circuit cables



safe circuit

cables

(Note 1)



Fig. 20.9 Mixed circuit box without a divider. Notes: (1) These cables should be

fed to dedicated intrinsically safe cable trays/ducts. (2) Non-intrinsically

safe circuit cables should be kept separate from intrinsically safe circuit

cables. (3)If the non-intrinsically safe circuit screen collection bar is not

deliberately connected to the potential equalization system (possibly via

the enclosure) it must be 50 mm from the intrinsically safe circuit screen

collection box and the intrinsically safe circuit terminals. (4) Box and nonintrinsically safe parts must comply with industrial requirements if mounted

in a non-hazardous area, but if hazardous area mounted must satisfy the

requirements of an appropriate protection concept (e.g., Exd or Exe).

(5) Non-intrinsically safe conductors must be in a restraining harness

or trunk, and if a harness, then separation from a harness containing

intrinsically safe conductors should be at least in accordance with the

requirements of BS/EN 50020 (3mm minimum is suggested)



Terminal and barrier boxes for general use should be IP54,particularly if

used outside, and should be robust to allow for rough handling. This is, of

course, not necessary when the terminals or barriers are mounted within a

control room or similar, and even a box is not strictly necessary here if the

location is protected from the environment and interference by the configuration of the building. It is recommended, however, that a terminal or

barrier box should be used in all but the most exceptional of circumstances

as otherwise the degree of personnel control necessary to avoid danger will

be very high and is unlikely to be maintained over a long period.

It is slightly less usual, but sometimes necessary, to create a junction

box where the intrinsically safe system is composite whereby the voltage

limitation is in one item of apparatus and the current limiting circuits in

another, which is the case with some alarm and public address systems.

This involves three types of wiring in the box as follows: