Electrical Installations in Hazardous Areas

580 Pages·1998·27.161 MB·English

by  Alan McMillan

20
Installation of intrinsically safe
apparatus/associated apparatus
and intrinsically safe systems T
BS 5345, Part 4(1977)
BS/EN 60079-14 (1997)
Intrinsic safety is very different to the other protection concepts in that it
does not operate by prevention of the release of ignition-capable energy,
the prevention of access of the explosive atmosphere to such releases, or
the control of the ignitions which result when such releases occur, but by
the prevention of ignition-capable energy from entering the hazardous area
and control of energy stored in the hazardous area by electrical components
which can store energy, such as inductors and capacitors. Thus release of
energy in the form of arcs or sparks in an explosive atmosphere is permitted
and this feature dictates the installation practice together with the fact that
some intrinsically safe installations are permitted in Zone 0 (and probably
in Zone 20 - see Chapter 15).
Intrinsically safe installations are also normally based upon instrumenta-
tion rather than electrical bases. A result of this is that in many, if not all,
use is made of multicore cables which carry more than one intrinsically safe
circuit, the installation splitting into individual cables to supply individual
circuits in a particular location at each end of the multicore. Figure 20.1
shows the typical way in which instrumentation systems are installed and
this also applies to intrinsically safe circuits.
These attributes require the following features of any installation to be
carefully considered.
1. Any invasion of the intrinsically safe circuit by energy from other intrin-
sically safe circuits or non-intrinsically safe circuits may add energy to
a particular circuit and hence may make any release of energy ignition
capable. Thus separation of intrinsically safe circuits from other circuits is
an important consideration whether those circuits are intrinsically safe or
non-intrinsically safe.
2. When fault currents are flowing in earth and bonding conductors are
in conditions of electrical fault, different parts of a potential equalization
systems may be at different voltages. Although these differences are not
large and have little effect in the case of installations with other protection
Fig. 20.1 Basic multicore cable intrinsically safe installation
concepts, they may be significant if added to the potential of an intrinsically
safe circuit and thus installations must take this situation into account.
3. The energy storage capability of certified/approved apparatus is defined
and its safety can be determined. Any energy stored in interconnecting
cables can add to the total and thus cable construction is more important
in this regard than in respect of non-intrinsically safe installations.
4. Intrinsically safe installations in Zone 0 (and where possible in Zone 20)
achieve a much higher level of integrity than those in Zones of lower risk.
The effects of this must be taken into account for installations in Zone 0.
20.1 Standards and Codes
Due to its unique approach, the division between construction and
installation in intrinsic safety is not as clear as is the case for other
protection concepts in that installation matters, such as choice of cable
and termination, also impinge upon apparatus and system design. Some
information, particularly that in the intrinsically safe system Standard, is
also installation information (e.g., the choice of type of cable - A, B, C
or D - is a matter for intrinsically safe system design where a system is
certified/approved and also is a matter for installation where a system is
produced on the basis of the parameters of certified/approved apparatus
and is not itself certified). While the basic installation is currently in the
Code of Practice, BS 5345, Part 41 (now being replaced by BS/EN 600792),
relevant information also appears in the apparatus construction Standard
BS/EN 500203, and its predecessor, BS 5501, Part 7 Part 94.
20.2 Basic installation requirements
As with other protection concepts, apparatus should be installed where it is
accessible and its labels are clearly visible to facilitate inspection. Also, not
withstanding that physical damage will not result in the release of ignition-
capable energy, it should be remembered that the construction Standards do
not include any enclosure strength requirements and thus, to ensure that
reliable operation is achieved, locations should be such that mechanical
damage is unlikely unless apparatus with a suitably rugged enclosure has
been chosen (see Chapter 17). Also the possibility of hostile environments
for which the apparatus is not designed should be taken into account. It
is true that most manufacturers will enclose intrinsically safe apparatus for
normal outdoor location but it must be remembered that the tests associated
with certification/approval do not usually confirm this. In addition, much
associated apparatus is designed for indoor use in a control room or similar.
Mains powered associated apparatus should also be fitted with appro-
priate short circuit overcurrent and mains/earth fault protection to ensure
that if a fault occurs in the part of the apparatus where the mains is present,
continuous arcing or heating which could adversely affect the intrinsically
safe circuit does not occur.
The use of barrier devices either of the shunt zener diode type or of more
complex types, some of which offer galvanic isolation between their inputs
or outputs, is now very common and the majority of installations use such
devices. Barrier devices which do not provide galvanic isolation (e.g., shunt
zener diode barriers) are currently in the majority and, unless precautions
are taken, the use of such devices can cause danger by providing unplanned
connections to the potential equalization system and thus the possibility of
invasion of the intrinsically safe circuits. Where such devices are used, and
where any other associated apparatus not providing galvanic isolation from
the mains supplies is used, the apparatus to which their non-hazardous area
terminals are connected must provide such isolation. It is sufficient that any
mains fed apparatus connected to their non-hazardous area terminals be fed
from the mains via a double wound transformer (not an auto transformer),
and the primary of that transformer be appropriately fused (fused as would
be required in normal industrial use).
20.3 Cables, conduits and conductors
Cables and interconnecting conductors used within conduits should be
chosen in accordance with the requirements set out in Chapters 13 and
17, with particular consideration given to the reactance parameters of the
cable or the conductors and their possible heating due to current in the
circuit. For new plants these factors will probably have been covered in the
design phase where the cables and conductors were initially selected, but
in the case of modifications this may not be so as existing multicore routes,
for example, may be used for economy. An installation check is necessary
to ensure errors have not occurred in either case.
Intrinsically safe and non-intrinsically safe circuits must not be connected
to conductors in the same cable except in exceptional circumstances. (It is
noted that it is, for example, possible to use an intrinsically-safe circuit to
monitor a plug and socket so that if they are separated the non-intrinsically
safe circuits within them are isolated before the pins part. This is dealt with
later in this chapter but the intrinsically safe circuit in this case must not be
mixed with other intrinsically safe circuits.).
20.3.7 Conductor temperature
The conductors of cables and those led in conduit are no different to those
inside apparatus and the conductor size (including the screen size if one
is fitted) should satisfy the requirements for conductors within apparatus
(see Table 20.1)
Table 20.1 Field wiring surface temperature classification
Conductor Conductor Maximum permissible
nominal nominal current in Amps for
diameter cross-sectional temperature
area classification of:
(mm) (mm2)
T1-T4 T5 T6
0.035 0.000962 N/A N/A N/A
0.05 0.00196 N/A N/A N/A
0.1 0.00785 2.1 1.9 1.7
0.2 0.0314 3.7 3.3 3.0
0.35 0.0962 6.4 5.6 5.0
0.5 0.196 7.7 6.9 6.7
Notes:
I N/A because conductors of less than 0.1 mm diameter should not be used in intrinsically safe
circuits because they are not considered sufficiently robust to have the necessary integrity.
2 Conductors made up of several strands are only acceptable if no strand is less than 0.1 mm
diameter for the same reason, even though their total equivalent cross-sectional area may
exceed 0.00785 mm2.
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 inductance/resistance ratio and capacitance. (Induc-
tance 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 conduc-
tors 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 multi-
conductor 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 multi-
core cables
Number of Screen Amount by which
conductors fitted/not fitted the capacitance
interconnected between any two cores
should be multiplied
None Not fitted 1.0
None Fitted 2.0 (note)
2 Not fitted 1.5
2 Fitted 2.5 (note)
3 Not fitted 2.5
3 Fitted 3.0 (note)
Note: 1 The screen is assumed to be deliberately connected to one side of the circuit which is
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 SOOVrms.
Intrinsically safe circuits and non-intrinsically safe circuits must not be
contained within the same conduit.
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 prox-
imity of other conductors and cables which contain non-intrinsically safe
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
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 appa-
ratus is required. It is, however, wise to mark cables, cable bundles and
conduits in some way to ensure that the security achieved in initial instal-
lation 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 non-
intrinsically 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
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 identify 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.
20.3.6 Additional requirements for Zone 0 (and Zone 20 where
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 intrinsi-
cally safe circuits within the cable are category 'ia' (see Chapter 13). (Cate-
gory '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 SOOVrms (this means that the total insulation between
screens should be capable of withstanding a test voltage of 1000 Vrms). (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/approval because 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
damage in the particular installation, and where it is considered as advan-
tageous 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.
20.4.1 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.
Fig. 20.3 Terminal construction
It should be noted that in BS 5345, Part 4 the separation from earthed
metal was 4mm rising to 6mm 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 4mm at 90V peak
and 6mm at 375V peak. Such a precaution is not necessary as the clear-
ance 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 require-
ment 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
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