IEC 60079-25:2010

IEC 60079-25 ®
Edition 2.0 2010-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 25: Intrinsically safe electrical systems

Atmosphères explosives –
Partie 25: Systèmes électriques de sécurité intrinsèque

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IEC 60079-25 ®
Edition 2.0 2010-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 25: Intrinsically safe electrical systems

Atmosphères explosives –
Partie 25: Systèmes électriques de sécurité intrinsèque

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XC
CODE PRIX
ICS 29.260.20 ISBN 978-2-88910-048-4
– 2 – 60079-25 © IEC:2010
CONTENTS
FOREWORD.4
1 Scope.6
2 Normative references .6
3 Terms, definitions and abbreviations .7
3.1 Terms and definitions .7
3.2 Abbreviations .8
4 Descriptive system document .8
5 Grouping and classification.9
6 Levels of protection .9
6.1 General .9
6.2 Level of protection “ia”.9
6.3 Level of protection “ib”.9
6.4 Level of protection “ic”.9
7 Ambient temperature rating .10
8 Interconnecting wiring / cables used in an intrinsically safe electrical system .10
9 Requirements of cables and multi-core cables.10
9.1 General .10
9.2 Multi-core cables .10
9.3 Electrical parameters of cables.11
9.4 Conducting screens.11
9.5 Types of multi-core cables.11
9.5.1 General .11
9.5.2 Type A cable .11
9.5.3 Type B cable .11
9.5.4 Type C cable .11
10 Termination of intrinsically safe circuits .11
11 Earthing and bonding of intrinsically safe systems.12
12 Protection against lightning and other electrical surges .12
13 Assessment of an intrinsically safe system .13
13.1 General .13
13.2 Simple apparatus .14
13.3 Analysis of inductive circuits.15
13.4 Faults in multi-core cables.15
13.4.1 Type of multi-core cables.15
13.4.2 Type A cable .15
13.4.3 Type B cable .15
13.4.4 Type C cable .16
13.5 Type verifications and type tests .16
14 Marking .16
15 Predefined systems .16
Annex A (informative) Assessment of a simple intrinsically safe system.17
Annex B (normative) Assessment of circuits with more than one source of power .20
Annex C (informative) Interconnection of non-linear and linear intrinsically safe circuits .23
Annex D (normative) Verification of inductive parameters .59

60079-25 © IEC:2010 – 3 –
Annex E (informative) A possible format for descriptive systems drawings and
installation drawings .61
Annex F (informative) Surge protection of an intrinsically safe circuit.64
Annex G (normative) Testing of cable electrical parameters.67
Annex H (informative) Use of simple apparatus in systems .69
Annex I (normative) FISCO systems .71
Bibliography.74

Figure 1 – Systems analysis .14
Figure 2 – Typical system using simple apparatus .15
Figure B.1 – Sources of power connected in series.21
Figure B.2 – Sources of power connected in parallel.22
Figure B.3 – Sources of power not deliberately connected .22
Figure C.1 – Equivalent circuit and output characteristic of resistive circuits .24
Figure C.2 – Current and/or voltage addition for interconnections .26
Figure C.3 – Output characteristic and equivalent circuit of a source with trapezoidal
characteristic .29
Figure C.4 – Example of an interconnection.33
Figure C.5 – Sum characteristics for the circuit as given in Figure C.4.35
Figure C.6 – Current and/or voltage addition for the example given in Figure C.4 .36
Figure C.7 – Limit curve diagram for universal source characteristic − Group IIC .47
Figure C.8 – Limit curve diagram for universal source characteristic – Group IIB .57
Figure C.9 – Copy pattern for universal source diagrams.58
Figure D.1 – Typical inductive circuit .60
Figure E.1 – Typical block diagram for IS system descriptive system document .62
Figure E.2 – Typical installation drawing for IS system .63
Figure F.1 – Surge protection requirements of an instrument loop .66
Figure I.1 – Typical system .73

Table A.1 – Simple system analysis.19
Table C.1 – Parameters necessary to describe the output characteristic.28
Table C.2 – Assignment of diagrams to equipment groups and inductances.31

– 4 – 60079-25 © IEC:2010
INTERNATIONAL ELECTROTECHNICAL COMMISSION
______________
EXPLOSIVE ATMOSPHERES –
Part 25: Intrinsically safe electrical systems

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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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 60079-25 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 published in 2003 and constitutes a
thorough technical revision.
The significant changes with respect to the previous edition are listed below:
• extension of the scope from Group II to Groups I, II and III;
• introduction of level of protection “ic”;
• addition of requirements for cables and multi-core cables;
• reference to IEC 60079-11 regarding the termination of intrinsically safe circuits
• requirements for the assessment of an expanded and clarified intrinsically safe system
regarding level of protection “ic”, simple apparatus and faults in multi-core cables;

60079-25 © IEC:2010 – 5 –
• introduction of predefined systems and merging of the system requirements for FISCO
from IEC 60079-27;
• addition of requirements for simple intrinsically safe systems containing both lumped
inductance and lumped capacitance;
• addition of a method for testing the electrical parameters of cables;
• additional information for the use of simple apparatus in systems.
The text of this standard is based on the following documents:
FDIS Report on voting
31G/202/FDIS 31G/203/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of IEC 60079 series, under the general title Explosive atmospheres, can be
found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability 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.
– 6 – 60079-25 © IEC:2010
EXPLOSIVE ATMOSPHERES –
Part 25: Intrinsically safe electrical systems

1 Scope
This part of IEC 60079 contains the specific requirements for construction and assessment of
intrinsically safe electrical systems, type of protection “i”, intended for use, as a whole or in
part, in locations in which the use of Group I, II or III apparatus is required.
NOTE 1 This standard is intended for use by the designer of the system who may be a manufacturer, a specialist
consultant or a member of the end-user’s staff.
This standard supplements and modifies the general requirements of IEC 60079-0 and the
intrinsic safety standard IEC 60079-11. Where a requirement of this standard conflicts with a
requirement of IEC 60079-0 or IEC 60079-11, the requirement of this standard takes
precedence.
This standard supplements IEC 60079-11, the requirements of which apply to electrical
apparatus used in intrinsically safe electrical systems.
The installation requirements of Group II or Group III systems designed in accordance with
this standard are specified in IEC 60079-14.
NOTE 2 Group I installation requirements are presently not provided in IEC 60079-14.
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 60060-1, High-voltage test techniques – Part 1: General definitions and test
requirements
IEC 60079-0, Explosive atmospheres – Part 0: Equipment – General requirements
IEC 60079-11:2006, Explosive atmospheres – Part 11: Equipment protection by intrinsic
safety “i”
IEC 60079-14:2007, Explosive atmospheres – Part 14: Electrical installations design,
selection and erection
IEC 60079-15, Electrical apparatus for explosive gas atmospheres – Part 15: Construction,
test and marking of type of protection "n" electrical apparatus
IEC 60079-27:2008, Explosive atmospheres – Part 27: Fieldbus intrinsically safe concept
(FISCO)
IEC 61158-2, Industrial communication networks − Fieldbus specifications – Part 2: Physical
layer specification and service definition

60079-25 © IEC:2010 – 7 –
IEC 61241-0, Electrical apparatus for use in the presence of combustible dust – Part 0:
General requirements
IEC 61241-11, Electrical apparatus for use in the presence of combustible dust – Part 11:
Protection by intrinsic safety 'iD'
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions, specific to intrinsically
safe electrical systems, apply. They supplement the terms and definitions which are given in
IEC 60079-0 and IEC 60079-11.
3.1.1
intrinsically safe electrical system
assembly of interconnected items of electrical apparatus, described in a descriptive system
document, in which the circuits or parts of circuits, intended to be used in an explosive
atmosphere, are intrinsically safe circuits
3.1.2
certified intrinsically safe electrical system
intrinsically safe electrical system conforming to 3.1.1 for which a certificate has been issued
confirming that the electrical system complies with IEC 60079-25
3.1.3
uncertified intrinsically safe electrical system
intrinsically safe electrical system conforming to 3.1.1 for which the knowledge of the
electrical parameters of the items of certified intrinsically safe electrical apparatus, certified
associated apparatus, simple apparatus and the knowledge of the electrical and physical
parameters of the interconnecting wiring permit the unambiguous deduction that intrinsic
safety is preserved
3.1.4
descriptive system document
document in which the items of electrical apparatus, their electrical parameters and those of
the interconnecting wiring are specified
3.1.5
system designer
person who is responsible for the descriptive system document, has the necessary
competence to fulfil the task and who is empowered to enter into the commitments on behalf
of his employer
3.1.6
maximum cable capacitance
C
c
maximum capacitance of the interconnecting cable that can be connected into an intrinsically
safe circuit without invalidating intrinsic safety
3.1.7
maximum cable inductance
L
c
maximum inductance of the interconnecting cable that can be connected into an intrinsically
safe circuit without invalidating intrinsic safety

– 8 – 60079-25 © IEC:2010
3.1.8
maximum cable inductance to resistance ratio
L /R
c c
maximum value of the ratio inductance (L ) to resistance (R ) of the interconnecting cable that
c c
can be connected into an intrinsically safe circuit without invalidating intrinsic safety
3.1.9
linear power supply
power source from which the available output current is determined by a resistor; the output
voltage decreases linearly as the output current increases.
3.1.10
non-linear power supply
power supply where the output voltage and output current have a non-linear relationship
NOTE For example, a supply with a constant voltage output that can reach a constant current limit controlled by
semiconductors.
3.2 Abbreviations
FISCO Fieldbus Intrinsically Safe Concept
FNICO Fieldbus Non-Incendive Concept
4 Descriptive system document
A descriptive system document shall be created for all systems. The descriptive system
document shall provide an adequate analysis of the safety achieved by the system.
NOTE Annex E comprises examples of typical diagrams, which illustrate the requirements of the descriptive
system document.
The minimum requirements are as follows:
a) block diagram of the system listing all the items of apparatus within the system including
simple apparatus and the interconnecting wiring. An example of such a diagram is shown
in Figure E.1;
b) a statement of the group subdivision (for Groups II and III), the level of protection for each
part of the system, the temperature classification, and the ambient temperature rating in
accordance with Clauses 5, 6 an d 7;
c) the requirements and permitted parameters of the interconnecting wiring in accordance
with Clause 8;
d) details of the earthing and bonding points of the systems in accordance with Clause 11 .
When surge protection devices are used, an analysis in accordance with Clause 12 s h a l l
also be included;
e) where applicable, the justification of the assessment of apparatus as simple apparatus in
accordance with IEC 60079-11 shall be included;
f) where the intrinsically safe circuit contains several pieces of intrinsically safe apparatus
the analysis of the summation of their parameters shall be available. This shall include all
simple apparatus and certified intrinsically safe apparatus;
g) a unique identification of the descriptive system document shall be created;
h) the system designer shall sign and date the document.
NOTE The descriptive system’s drawing is not the same as the Control Drawing referred to in IEC 60079-11.

60079-25 © IEC:2010 – 9 –
5 Grouping and classification
Intrinsically safe electrical systems shall be placed in a Group I, Group II or Group III as
defined in IEC 60079-0. Groups II and III intrinsically safe electrical systems as a whole or
parts thereof shall be given a further subdivision of the Group as appropriate.
Apparatus within Groups II and III intrinsically safe electrical system, intended for use in
explosive gas or dust atmospheres, shall be given a temperature class or maximum surface
temperature in accordance with IEC 60079-0, IEC 60079-11, IEC 61241-0 and IEC 61241-11
as applicable.
NOTE 1 In Group II and Group III intrinsically safe electrical systems, or parts thereof, the subdivisions A, B, C
may be different from those of the particular intrinsically safe electrical apparatus and associated electrical
apparatus included in the system.
NOTE 2 Different parts of the same intrinsically safe electrical system may have different subdivisions (A, B, C).
The apparatus used may have different temperature classes and different ambient temperature ratings.
6 Levels of protection
6.1 General
Each part of an intrinsically safe electrical system intended for use in an explosive
atmosphere will have a level of protection of “ia”, “ib” or “ic” in accordance with IEC 60079-11.
The complete system need not necessarily have a single level of protection.
NOTE 1 For example, where an instrument is primarily an “ib” instrument but which is designed for the connection
of an “ia” sensor, such as a pH measuring instrument with its connected probe, the part of the system up to the
instrument is “ib” and the sensor and its connections “ia”.
NOTE 2 An “ia” field instrument powered via an “ib” associated apparatus would be considered as an “ib” system.
NOTE 3 A system may be “ib” in normal operation with external power, but when power is removed under defined
safety circumstances (ventilation failure) then the system could become “ia” under back up battery power. The level
of protection will be clearly defined for foreseeable circumstances.
Clause 13 contains details of the required assessment.
6.2 Level of protection “ia”
Where the requirements applicable to electrical apparatus of level of protection “ia” (see
IEC 60079-11) are satisfied by an intrinsically safe system or part of a system considered as
an entity, then that system or part of a system shall be placed in level of protection “ia”.
6.3 Level of protection “ib”
Where the requirements applicable to electrical apparatus of level of protection “ib” (see
IEC 60079-11) are satisfied by an intrinsically safe system or part of a system considered as
an entity, then that system or part of a system shall be placed in level of protection “ib”.
6.4 Level of protection “ic”
Where the requirements applicable to electrical apparatus level of protection “ic” (see
IEC 60079-11) are satisfied by an intrinsically safe system or part of a system considered as
an entity, then the system or part of a system shall be placed in level of protection “ic”.

– 10 – 60079-25 © IEC:2010
7 Ambient temperature rating
Where part or all the intrinsically safe system is specified as being suitable for operation
outside the normal operating temperature range of –20 °C and +40 °C, this shall be specified
in the descriptive system document.
8 Interconnecting wiring / cables used in an intrinsically safe electrical system
The electrical parameters of the interconnecting wiring upon which intrinsic safety depends
and the derivation of these parameters shall be specified in the descriptive system document.
Alternatively, a specific type of cable shall be specified and the justification for its use
included in the documentation. Cables for the interconnecting wiring shall comply with the
relevant requirements of Clause 9.
Where relevant, the descriptive system document shall also specify the permissible types of
multi-core cables as specified in Clause 9, which each particular circuit may utilize. In the
particular case where faults between separate circuits have not been taken into account, then
a note shall be included on the block diagram of the descriptive system document stating the
following: “where the interconnecting cable utilizes part of a multi-core cable containing other
intrinsically safe circuits, then the multi-core cable shall be in accordance with the
requirements of a multi-core cable type A or B, as specified in Clause 9 of IEC 60079-25”.
A multi-core cable containing circuits classified as level of protection “ia”, “ib” or “ic” shall not
contain non-intrinsically safe circuits.
“ic” multi-core cables may contain more than one intrinsically safe “ia”, “ib” or “ic” circuit
subject to the applicable faults specified in Clause 13 .
NOTE Multi-core cables not complying with type A or B are permitted if the specific combination of circuits is
examined against the requirements of IEC 60079-11.
Intrinsically safe “ic” circuits shall only be run together with intrinsically safe “ia” and “ib”
circuits provided they are run in a multi-core cable of type A or type B specified in 9.5 .
9 Requirements of cables and multi-core cables
9.1 General
The diameter of individual conductors or strands of multi-stranded conductors within the
hazardous area shall not be less than 0,1 mm.
Only insulated cables with insulation capable of withstanding a dielectric test of at least 500 V
a.c. or 750 V d.c. shall be used in intrinsically safe circuits.
NOTE This clause is not intended to prevent the use of bare conductors in a signalling system and these should
be considered as simple apparatus and not interconnecting wiring.
9.2 Multi-core cables
The radial thickness of the insulation of each core shall be appropriate to the conductor
diameter and the nature of the insulation with a minimum of 0,2 mm.
Multi-core cables shall be capable of withstanding a dielectric test of at least:
a) 500 V r.m.s. a.c. or 750 V d.c. applied between any armouring and/or screen(s) joined
together and all the cores joined together.
b) 1 000 V r.m.s. a.c. or 1 500 V d.c. applied between a bundle comprising one half of the
cable cores joined together and a bundle comprising the other half of the cores joined

60079-25 © IEC:2010 – 11 –
together. This test is not applicable to multi-core cables with conducting screens for
individual circuits.
The dielectric strength test shall be carried out in accordance with an appropriate cable
standard or dielectric strength tests of IEC 60079-11.
9.3 Electrical parameters of cables
The electrical parameters (C and L or C and L /R ) for all cables used within an intrinsically
c c c c c
safe system shall be determined according to a), b) or c):
a) the most onerous electrical parameters provided by the cable manufacturer;
b) electrical parameters determined by measurement of a sample, with the method of testing
electrical parameters of cables given in An ne x G;
c) where the interconnection comprises two or three cores of a conventionally constructed
cable (with or without screen): 200 pF/m and either 1 μH/m or an inductance to resistance
ratio (L /R ) calculated by dividing 1 μH by the manufacturers specified loop resistance per
c c
meter. Alternatively, for currents up to I = 3 A an L/R ratio of 30 μH/Ω may be used.
o
Where a FISCO or FNICO system is used, the requirements for the cable parameters shall
comply with Annex I.
9.4 Conducting screens
Where conducting screens provide protection for separate intrinsically safe circuits in order to
prevent such circuits becoming connected to one another, the coverage of those screens shall
be at least 60 % of the surface area.
9.5 Types of multi-core cables
9.5.1 General
Multi-core cables shall be classified as either type A, type B or type C for the purposes of
applying faults and assessing the safety of the cabling within an intrinsically safe system. The
cable types are specified in 9. 5. 2, 9 . 5. 3, an d 9. 5. 4 .
The use of multi-core cables that do not comply with the requirements for types A, B, or C is
not permitted.
9.5.2 Type A cable
A cable whose construction complies with 9. 1, 9.2 , 9.3 and has conducting screens providing
individual protection for each intrinsically safe circuit according to 9. 4.
9.5.3 Type B cable
A cable whose construction complies with 9. 1, 9.2 an d 9.3, is fixed and effectively protected
against damage and does not contain any circuit with a maximum voltage U exceeding 60 V.
o
9.5.4 Type C cable
A cable whose construction complies with 9. 1 , 9. 2 and 9. 3.
10 Termination of intrinsically safe circuits
Intrinsically safe systems that contain junction boxes or marshalling cubicles where
intrinsically safe circuits are terminated shall comply with the terminal requirements in the
facilities for the connection of external circuits of IEC 60079-11.

– 12 – 60079-25 © IEC:2010
11 Earthing and bonding of intrinsically safe systems
In general, an intrinsically safe circuit shall either be fully floating or bonded to the reference
potential associated with a hazardous area at one point only. The level of isolation required
(except at that one point) is to be designed to withstand a 500 V insulation test in accordance
with the dielectric strength requirement of IEC 60079-11. Where this requirement is not met,
the circuit shall be considered to be earthed at that point. More than one earth connection is
permitted on a circuit, provided that the circuit is galvanically separated into sub-circuits, each
of which has only one earth point.
Screens shall be connected to earth or the structure in accordance with IEC 60079-14. Where
a system is intended for use in an installation where significant potential differences (greater
than 10 V) between the structure and the circuit can occur, the preferred technique is to use a
circuit galvanically isolated from external influences such as changes in ground potential at
some distance from the structure. Particular care is required where part of the system is
intended to be used in Zone 0 or Zone 20 locations or when the system has a very high level
of protection so as to conform to EPL Ma requirements.
The descriptive system document should clearly indicate which point or points of the system
are intended to be connected to the plant reference potential and any special requirements of
such a bond. This may be achieved by adding a reference to IEC 60079-14 in the descriptive
system document.
NOTE IEC 60079-14 does not apply to electrical installations in mines susceptible to firedamp.
12 Protection against lightning and other electrical surges
Where a risk analysis shows that an installation is particularly susceptible to lightning or other
surges, precautions shall be taken to avoid the possible hazards.
If part of an intrinsically safe circuit is installed in Zone 0 in such a way that there is a risk of
developing hazardous or damaging potential differences within Zone 0, a surge protection
device shall be installed. Surge protection is required between each conductor of the cable
including the screen and the structure where the conductor is not already bonded to the
structure. The surge protection device shall be installed outside but as near to the boundary
of Zone 0 as is practicable, preferably within 1 m.
Surge protection for apparatus in Zones 1 and 2 shall be included in the system design for
highly susceptible locations.
The surge protection device shall be capable of diverting a minimum peak discharge current
of 10 kA (8/20 μs impulse according to IEC 60060-1 for 10 operations). The connection
between the protection device and the local structure shall have a minimum cross-sectional
copper. The cable between the intrinsically safe apparatus in Zone 0
area equivalent to 4 mm
and the surge protection device shall be installed in such a way that it is protected from
lightning. Any surge protection device introduced into an intrinsically safe circuit shall be
suitably explosion protected for its intended location.
The use of surge protection devices which interconnect the circuit and the structure via non-
linear devices such as gas discharge tubes and semiconductors is not considered to
adversely affect the intrinsic safety of a circuit, provided that in normal operation the current
through the device is less than 10 μA.
NOTE If insulation testing at 500 V is carried out under well-controlled conditions, then it may be necessary to
disconnect the surge suppression devices to prevent them invalidating the measurement.
Intrinsically safe systems utilizing surge suppression techniques shall be supported by an
adequately documented analysis of the effect of indirect multiple earthing, taking into account

60079-25 © IEC:2010 – 13 –
the criteria set out above. The capacitance and inductance of the surge suppression devices
shall be considered in the assessment of the intrinsically safe system.
Annex F illustrates some aspects of the design of surge protection of an intrinsically safe
system.
13 Assessment of an intrinsically safe system
13.1 General
Where a system contains apparatus which does not separately conform to IEC 60079-11, then
that system shall be analysed as a whole, as if it were an apparatus. A level of protection “ia”
system shall be analysed in accordance with the level of protection “ia” criteria of
IEC 60079-11. A level of protection “ib” system shall be analysed in accordance with the level
of protection “ib” criteria of IEC 60079-11. A level of protection “ic” system shall be analysed
in accordance with the level of protection “ic” criteria of IEC 60079-11. In addition to the faults
within the apparatus, the failures within the field wiring listed in 13.4 shall also be taken into
account.
NOTE It is recognized that applying faults to the system as a whole is less stringent than applying faults to each
piece of apparatus; nevertheless, this is considered to achieve an acceptable level of safety.
Where all the necessary information is available, it is permissible to apply the fault count to
the system as a whole even when apparatus conforming to IEC 60079-11 is being used. This
is an alternative solution to the more usual straightforward comparison of input and output
characteristics of the separately analysed or tested apparatus. Where a system contains only
separately analysed or tested apparatus conforming to IEC 60079-11, the compatibility of all
the apparatus included in the system shall be demonstrated. Faults within the apparatus have
already been considered and no further consideration of these faults is necessary. Where a
system contains a single source of power, the output parameters of the power source take
into account opening, shorting and earthing of the external interconnecting cable, and
consequently these failures do not need to be further considered. Annex A contains further
details of the analysis of these simple circuits.
When a system contains more than one linear source of power, then the effect of the
combined sources of power shall be analysed. Annex B illustrates the analysis to be used in
the most frequently occurring combinations.
If an intrinsically safe system contains more than one source of power, and one or more of
these sources are non-linear, the assessment method described in Annex B cannot be used.
For this kind of intrinsically safe system, Annex C explains how the system analysis can be
conducted if the combination contains a single non-linear power supply.
Figure 1 illustrates the principles of the system’s analysis.

– 14 – 60079-25 © IEC:2010
Analyse an
intrinsically
safe system
No
Is apparatus in
Use principles of
accordance
IEC 60079-11
with
IEC 60079-11
Yes
Is only one Follow Annex A
Yes
power supply
used?
No
Are the power Follow Annex B
Yes
supplies
linear?
No
Use guidance of Create a
Annex C and/or descriptive
take expert system
advice document
IEC  244/10
Figure 1 – Systems analysis
13.2 Simple apparatus
Switches, terminals, terminal boxes, plugs and sockets complying with the simple apparatus
requirements of IEC 60079-11, may be added to a system without modifying the safety
assessment. The possible heating effects on simple apparatus shall be considered. When
other types of simple apparatus consisting of energy storing components for example
capacitors or inductors complying with IEC 60079-11 are added to a system, the safety

60079-25 © IEC:2010 – 15 –
assessment shall take into account their electrical parameters. A typical system using simple
apparatus is shown in Figure 2.
Where simple apparatus are intended to contain several separate intrinsically safe circuits,
e.g. connectors, plugs and sockets or a resistance thermometer with two separate resistance
windings, the separation requirements of IEC 60079-11 apply. If they do not conform, then the
interconnected circuits shall be assessed as a single intrinsically safe circuit.

1 2
U U
o m
Hazardous area Non-hazardous area
IEC  245/10
Key
1 certified intrinsically safe apparatus
2 certified associated intrinsically safe apparatus
3 cable
4 simple apparatus
Figure 2 – Typical system using simple apparatus
13.3 Analysis of inductive circuits
Where an apparatus has a well-defined inductance and resistance either by virtue of its
documentation or construction, then
...