IEC 60079-27:2008

IEC 60079-27
Edition 2.0 2008-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 27: Fieldbus intrinsically safe concept (FISCO)

Atmosphères explosives –
Partie 27: Concept de réseau de terrain de sécurité intrinsèque (FISCO)

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IEC 60079-27
Edition 2.0 2008-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 27: Fieldbus intrinsically safe concept (FISCO)

Atmosphères explosives –
Partie 27: Concept de réseau de terrain de sécurité intrinsèque (FISCO)

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
M
CODE PRIX
ICS 29.260.20 ISBN 2-8318-9539-1

– 2 – 60079-27 © IEC:2008
CONTENTS
FOREWORD.3

1 Scope.5
2 Normative references .5
3 Terms, definitions and abbreviations .6
3.1 Terms and definitions .6
3.2 Abbreviations .6
4 Apparatus requirements .6
4.1 General .6
4.2 FISCO power supplies.6
4.2.1 General .6
4.2.2 Additional requirements of “ia” and “ib” FISCO power supplies .6
4.2.3 Additional requirements of “ic” FISCO power supplies .7
4.3 FISCO field devices.7
4.3.1 General .7
4.3.2 Additional requirements of “ia” and “ib” FISCO field devices .8
4.3.3 Additional requirement of “ic” FISCO field devices .8
4.4 Terminator.8
4.5 Simple apparatus .8
4.6 Marking .9
4.7 Examples of marking .9
5 System requirements.10
5.1 General .10
5.2 Additional requirements of “ic” FISCO systems.11

Annex A (informative) Typical system .12

Bibliography.13

Figure A.1 – Typical system.12

Table 1 – Assessment of maximum output current for use with “ia” and “ib” FISCO
rectangular supplies.7
Table 2 – Assessment of maximum output current for use with “ic” FISCO rectangular
supplies .7

60079-27 © IEC:2008 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 27: Fieldbus intrinsically safe concept (FISCO)

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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6) All users should ensure that they have the latest edition of this publication.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60079-27 has been prepared by subcommittee 31G: Intrinsically-
safe apparatus, of IEC technical committee 31: Equipment for explosive atmospheres.
This second edition cancels and replaces the first edition issued in 2005. It constitutes a
technical revision.
The significant change with respect to the first edition is that this standard replaces the
FNICO requirements with the requirements of an “ic” FISCO system.
The text of this standard is based on the following documents:
CDV Report on voting
31G/169/CDV 31G/176A/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.

– 4 – 60079-27 © IEC:2008
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60079 series, under the general title: Explosives atmospheres can
be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under “http://webstore.iec.ch” in the
data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
60079-27 © IEC:2008 – 5 –
EXPLOSIVE ATMOSPHERES –
Part 27: Fieldbus intrinsically safe concept (FISCO)

1 Scope
This part of IEC 60079 contains the details of apparatus, systems and installation practice for
use with the Fieldbus Intrinsically Safe Concept (FISCO). It is based on the concepts of
Manchester encoded, bus powered systems designed in accordance with IEC 61158-2 which
is the physical layer standard for Fieldbus installations.
The constructional and installation requirements of FISCO apparatus and systems are
determined by IEC 60079-11, IEC 60079-14, and IEC 60079-25, except as modified by this
standard. Part of a Fieldbus device may be protected by any of the methods of explosion
protection listed in IEC 60079-0, appropriate to the zone of intended use. In these
circumstances, the requirements of this standard apply only to that part of the apparatus
directly connected to the intrinsically safe trunk or spurs.
NOTE 1 Certification to the FISCO requirements does not prevent apparatus also being certified and marked to
IEC 60079-11 in the conventional manner so that they may be used in other systems. Some apparatus certified
before this standard was published but not necessarily complying with the electrical parameters of this standard
may be marked “Suitable for FISCO systems”. This apparatus may be accepted in a FISCO system, if the
comparison of the electrical parameters U , I , P , with U , I , P , demonstrate compatibility with the remainder of
o o o i i i
the system, and all the other requirements of this standard are met.
NOTE 2 A typical system is illustrated in Annex A.
NOTE 3 Generally, “ic” FISCO systems are intended for use in zone 2 locations. FISCO systems are
predominantly intended for use in zone 1 and 2 locations, but may enter zone 0 locations if specifically permitted to
do so by the documentation.
NOTE 4 Edition 1 of this standard introduced the FNICO concept to cover the use of Fieldbus concepts in zone 2
utilizing the energy-limited [nL] concept. This standard substitutes the “ic” concept for the energy-limited concept,
but permits the continued use of FNICO and nL apparatus.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60079-0, Explosive atmospheres – Part 0: Equipment – General requirements
IEC 60079-11, Explosive atmospheres – Part 11: Equipment protection by intrinsic safety “i”
IEC 60079-14, Electrical apparatus for explosive gas atmospheres – Part 14: Electrical
installations in hazardous areas (other than mines)
IEC 60079-15, Electrical apparatus for explosive gas atmospheres – Part 15: Construction,
test and marking of type of protection ”n” electrical apparatus
IEC 60079-25, Electrical apparatus for explosive gas atmospheres – Part 25: Intrinsically safe
systems
IEC 61158-2, Digital data communications for measurement and control − Fieldbus for use in
industrial control systems – Part 2: Physical layer specification and service definition

– 6 – 60079-27 © IEC:2008
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-0,
IEC 60079-11 and IEC 60079-15 as well as the following apply.
NOTE Additional definitions applicable to explosive atmospheres can be found in IEV 60050-426.
3.2 Abbreviations
FISCO Fieldbus Intrinsically Safe Concept
FNICO Fieldbus Non-Incendive Concept
4 Apparatus requirements
4.1 General
Apparatus shall be constructed in accordance with IEC 60079-11 except as modified by this
standard.
The apparatus documentation shall confirm that each apparatus is suitable for use in a FISCO
system in accordance with this standard.
4.2 FISCO power supplies
4.2.1 General
The power supply shall either be resistive limited or have a trapezoidal or rectangular output
characteristic. The maximum output voltage, U , shall be not greater than 17,5 V nor less than
o
14 V under the conditions specified in IEC 60079-11 for the respective level of protection.
The maximum unprotected internal capacitance C and inductance L shall be not greater than
i i
5 nF and 10 µH, respectively.
The output from the power supply may be connected to earth.
No specification of the internal parameters L and C or the maximum external parameters L
i i o
and C is required on the certificate or label.
o
The determination of power supply output parameters shall take into account the possible
opening, shorting and earthing of field wiring connected to the field terminals of the apparatus.
4.2.2 Additional requirements of “ia” and “ib” FISCO power supplies
The maximum output current I for any “ia” or “ib” FISCO power supply shall be determined in
o
accordance with IEC 60079-11 but shall not exceed 380 mA. For rectangular supplies,
Table 1 may be used for assessment.

60079-27 © IEC:2008 – 7 –
Table 1 – Assessment of maximum output current for use
with “ia” and “ib” FISCO rectangular supplies
Permissible current, for IIC Permissible current, for IIB
U
o
(includes 1,5 safety factor) (includes 1,5 safety factor)
V
mA mA
14 183 380
15 133 354
16 103 288
17 81 240
17,5 75 213
NOTE The two largest current values for IIB are derived from 5,32 W.
The maximum output power P shall not exceed 5,32 W.
o
4.2.3 Additional requirements of “ic” FISCO power supplies
The maximum output current I for an “ic” FISCO power supply shall be determined in
o
accordance with IEC 60079-11. For “ic” FISCO rectangular supplies, Table 2 may be used for
assessment.
Table 2 – Assessment of maximum output current for use
with “ic” FISCO rectangular supplies
U Permissible current, for IIC Permissible current, for IIB
o
V mA mA
14 274 570
15 199 531
16 154 432
17 121 360
17,5 112 319
NOTE The maximum output power P from “ic” FISCO power supplies is not restricted to 5,32 W.
o
4.3 FISCO field devices
4.3.1 General
These requirements apply to apparatus other than the power supply terminators and simple
apparatus connected to the intrinsically safe bus whether installed inside or outside the
hazardous area.
In addition to the relevant sections of IEC 60079-11, hand held terminals shall comply with the
requirements for field terminals.
The requirements are as follows:
a) field devices shall have a minimum input voltage parameter of U = 17,5 V;
i
b) the maximum unprotected internal capacitance C of each field device shall be not greater
i
than 5 nF;
c) the bus terminals shall be isolated from earth in accordance with IEC 60079-11;
d) the bus terminals of separately powered field devices shall be galvanically isolated from
other sources of power in accordance with IEC 60079-11, so as to ensure that these
terminals remain passive and multiple earthing of the bus is avoided;

– 8 – 60079-27 © IEC:2008
e) under normal or fault conditions as specified in IEC 60079-11 the bus terminals shall
remain passive, that is the terminals shall not be a source of energy to the system except
for a leakage current not greater than 50 µA;
f) field devices shall be allocated a level of protection and be suitable for apparatus group
IIC in accordance with IEC 60079-11;
g) field devices intended to be installed within the hazardous area shall be temperature
classified in accordance with IEC 60079-11.
4.3.2 Additional requirements of “ia” and “ib” FISCO field devices
The additional requirements of “ia” and “ib” FISCO field devices are as follows:
a) field devices shall have minimum input parameters of I = 380 mA and P = 5,32 W;

i i
b) field devices shall have an internal inductance L not greater than 10 µH.
i
4.3.3 Additional requirement of “ic” FISCO field devices
The additional requirement of “ic” FISCO field devices is that they shall have an internal
inductance L not greater than 20 µH.
i
4.4 Terminator
The line terminators required by the system shall comprise a resistor-capacitor combination,
which presents at its terminals a circuit equivalent to a resistor of minimum value 90 Ω in
series with a capacitor of maximum value 2,2 µF (including tolerances).
NOTE 1 IEC 61158-2 specifies the component values necessary for operational reasons.
The terminator shall
a) be allocated a level of protection and be suitable for apparatus group IIC;
b) have an input voltage parameter U not less than 17,5 V;
i
NOTE 2 If the capacitive component(s) are considered to be able to fail to create a short circuit then the required
power rating of the resistor(s) is 5,1 W and the temperature class should be determined with a power dissipation of
3,4 W.
c) be isolated from earth in accordance with IEC 60079-11;
d) have a maximum unprotected internal inductance L not greater than 10 µH;
i
e) terminators intended to be installed within the hazardous area shall be temperature
classified in accordance with IEC 60079-11.
The terminators may be incorporated within field devices or power supplies.
NOTE 3 For safety assessment purposes, the effective capacitance (C ) of the terminator is considered not to
i
affect the intrinsic safety of the system.
4.5 Simple apparatus
The requirement of simple apparatus used in an intrinsically safe system is that it shall
comply with IEC 60079-11. The capacitance and inductance of the simple apparatus shall be
known so that the system requirement in 5.1 can be assessed. The total inductance and
capacitance of each simple apparatus connected to a FISCO system shall be not greater than
10 µH and 5 nF, respectively.
NOTE Care should be taken in temperature classifying simple apparatus within an “ia” or “ib” system since the
maximum power available may be as high as 5,32 W. Temperature classification of an “ic” system is done in
normal operation.
60079-27 © IEC:2008 – 9 –
4.6 Marking
Each piece of apparatus, with the exception of simple apparatus, shall be marked with the
word “FISCO” followed by an indication of its function, i.e. power supply, field device or
terminator. In addition, each piece of apparatus shall be marked in accordance with
IEC 60079-11, except where modified by this standard. For example, the manufacturer’s
name and address shall still be marked.
Where apparatus is dual marked so that it can be used in both a FISCO system and a
conventional intrinsically safe system, care shall be taken to differentiate between the FISCO
marking and the marking for the conventional intrinsically safe system.
For FISCO power supplies, output parameters U , I , C , L , P and L /R need not be
o o o o o o o
marked. For FISCO field devices or terminators, input and internal parameters U , I , C , L , P
i i i i i
and L /R need not be marked.
i i
4.7 Examples of marking
a) Power supply
FISCO power supply
U = 250 V
m
[Ex ia] IIC
John Jones Ltd
SW99 2AJ UK
Type: DRG OOI
–20 °C ≤ Ta ≤ +50 °C
PTB Nr 01A 2341
Serial No: 014321
b) Field device
FISCO field device
Ex ia IIC T4
Paul McGregor plc
GL99 1JA UK
Type: RWS 001
–20 °C ≤ Ta ≤ +60 °C
c) Terminator
FISCO terminator
Ex ia IIC T4
James Bond plc
MK45 6BY UK
Type MI5 007
BAS 01 A 4321
Serial No: 012345
d) Dual marked field device
A McTavish plc
GL 98 1BA UK
Type RWS 002
– 10 – 60079-27 © IEC:2008
–20 °C ≤ Ta ≤ +60 °C
INERIS 02 A 2345
Serial No: 060128
- - - - - - - - - - - - - - -
FISCO Field device
Ex ia IIC T6
- - - - - - - - - - - - - - -
U = 28 V
i
C = 3 nF
i
I = 200 mA
i
L = 10 µH
i
P = 1,2 W
i
5 System requirements
5.1 General
A system is usually of the form illustrated in Annex A.
The cable used in the system shall comply with the following parameters:
a) loop resistance R 15 Ω/km to 150 Ω/km;
c
b) loop inductance L 0,4 mH/km to 1 mH/ km;
c
c) capacitance C 45 nF/km to 200 nF/km;
c
d) maximum length of each spur cable 60 m in IIC and IIB;
e) maximum length of each trunk cable, including the length of all spurs, 1 km in IIC and
5 km in IIB.
When cable, which complies with this standard, is used, no further consideration of cable
parameters is necessary.
NOTE 1 The installation and constructional requirements of individual cables and multicores carrying more than
one IS circuit are contained in IEC 60079-14. Annex C of that standard also gives guidance on the determination of
cable parameters.
Where a system comprises
a) one source of power,
b) any number of field devices up to 32, and
c) up to two terminators,
all complying with the requirements of this standard combined with a cable to the above
specification, then that system shall be considered to be adequately safe.
The system shall be allocated a level of protection (“ia”, “ib” or “ic”) determined by the least
onerous level of protection of the apparatus used in the system. The safety documentation
should record the allocated level of protection.
Sub-systems of the system may have different levels of protection where this is justified by
the assessment and recorded in the documentation. For example, an “ia” spur may be created
from an “ib” trunk by the insertion of a suitably certified interface.

60079-27 © IEC:2008 – 11 –
The terminator(s) shall be situated at the end(s) of the trunk. The power supply shall be
located not more than 60 m from one end of the trunk. Where the power supply is connected
via a spur, then that spur is restricted to a length of 60 m.
NOTE 2 The number of field devices which can be connected to a spur is restricted by operational constraints and
the requirement of this standard, which restricts the number of field devices in a system to a maximum of 32.
Connection facilities and/or switches may be added to a system without modifying the safety
assessment. Other types of simple apparatus complying with IEC 60079-11 may be connected
to a FISCO system provided that the total inductance and capacitance of each simple
apparatus is not greater than 10 µH and 5 nF respectively, and the total number of pieces of
such simple apparatus plus field devices does not exceed 32.
The safety documentation may be simplified to a list of the equipment, together with relevant
apparatus documentation used. The documentation should clearly identify the level of
protection of each part of the system.
The apparatus group of the power supply determines the apparatus group of the system.
The temperature classification of each piece of apparatus shall be determined and recorded in
its documentation. It is also necessary to confirm that the permitted maximum ambient
temperature of each piece of apparatus is suitable for its intended location.
5.2 Additional requirements of “ic” FISCO systems
Apparatus designed and approved to the FNICO requirements of the first edition of this
standard may be used in an “ic” FISCO system.
Field devices, terminators and other ancillaries complying with the requirements of intrinsic
safety but not as FISCO apparatus in accordance with this standard may be used with a
FISCO power supply in an “ic” FISCO system provided that they have input parameters of U
i
not less than 17,5 V and internal parameters of L and C not greater than 20 µH and 5 nF
i i
respectively.
Similarly, apparatus not approved as FISCO apparatus, but constructed in accordance with
the requirements of IEC 60079-15 energy-limited “nL” apparatus and having input parameters
of U not less than 17,5 V and internal parameters of
...