IEC 61241-1-1:1999

INTERNATIONAL IEC
STANDARD
61241-1-1
Second edition
1999-06
Electrical apparatus for use in the presence
of combustible dust –
Part 1-1:
Electrical apparatus protected by
enclosures and surface temperature limitation –
Specification for apparatus
Matériels électriques destinés à être utilisés en présence
de poussières combustibles –
Partie 1-1:
Matériels électriques protégés par enveloppes et limitation
de la température de surface –
Spécification pour les matériels

Reference number
Numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series.
Consolidated publications
Consolidated versions of some IEC publications including amendments are

available. For example, edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the
base publication, the base publication incorporating amendment 1 and the base

publication incorporating amendments 1 and 2.

Validity of this publication
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology.
Information relating to the date of the reconfirmation of the publication is available
in the IEC catalogue.
Information on the subjects under consideration and work in progress undertaken
by the technical committee which has prepared this publication, as well as the list
of publications issued, is to be found at the following IEC sources:
• IEC web site*
•
Catalogue of IEC publications
Published yearly with regular updates
(On-line catalogue)*
• IEC Bulletin
Available both at the IEC web site* and as a printed periodical
Terminology, graphical and letter symbols
For general terminology, readers are referred to IEC 60050: International
Electrotechnical Vocabulary (IEV).
For graphical symbols, and letter symbols and signs approved by the IEC for
general use, readers are referred to publications IEC 60027: Letter symbols to be
used in electrical technology, IEC 60417: Graphical symbols for use on equipment.
Index, survey and compilation of the single sheets and IEC 60617: Graphical symbols
for diagrams.
* See web site address on title page.

INTERNATIONAL IEC
STANDARD
61241-1-1
Second edition
1999-06
Electrical apparatus for use in the presence
of combustible dust –
Part 1-1:
Electrical apparatus protected by
enclosures and surface temperature limitation –
Specification for apparatus
Matériels électriques destinés à être utilisés en présence
de poussières combustibles –
Partie 1-1:
Matériels électriques protégés par enveloppes et limitation
de la température de surface –
Spécification pour les matériels

 IEC 1999  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http://www.iec.ch
Commission Electrotechnique Internationale
PRICE CODE
V
International Electrotechnical Commission
For price, see current catalogue

– 2 – 61241-1-1 © IEC:1999(E)
CONTENTS
Page
FOREWORD . 3

INTRODUCTION .5

Clause
1 Scope . 6

2 Normative references . 6

3 Definitions. 7
4 Construction. 9
5 Enclosure materials. 10
6 Fasteners. 11
7 Interlocking devices. 11
8 Bushings. 11
9 Materials used for cementing. 12
10 Connection facilities and terminal compartments . 12
11 Connection facilities for earthing or bonding conductors. 12
12 Cable and conduit entries . 13
13 Supplementary requirements for electrical apparatus for practice B for use in zone 20 or 21. 14
14 Rotating electrical machines. 17
15 Switchgear . 18
16 Fuses. 19
17 Plugs and sockets . 19
18 Luminaries . 19
19 Caplights, caplamps and handlamps. 20
20 Verification and tests. 20
20.1 General. 20
20.2 Verification of documents . 21
20.3 Compliance of prototype or sample with documents. 21
20.4 Type tests . 22
20.4.1 General. 22
20.4.2 Mechanical tests . 22
20.4.3 Tests for dust exclusion by enclosures . 24

20.4.4 Torque test for bushings in enclosures for use in zone 20 or 21 . 25
20.4.5 Thermal tests . 26
20.4.6 Thermal shock test. 27
20.4.7 Tests of non-metallic enclosures or of non-metallic parts of enclosures
for use in zone 20 or 21. 27
21 Routine verifications and tests. 29
22 Manufacturer’s responsibility . 29
23 Verifications and tests on modified or repaired electrical apparatus. 29
24 Clamping tests of non-armoured and braided cables. 29
25 Clamping tests of armoured cables. 31
26 Marking. 32
27 Examples of marking. 34

61241-1-1 © IEC:1999(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
ELECTRICAL APPARATUS FOR USE IN THE

PRESENCE OF COMBUSTIBLE DUST –

Part 1-1: Electrical apparatus protected by enclosures and

surface temperature limitation – Specification for apparatus

FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61241-1-1 has been prepared by subcommittee 31H: Apparatus for
use in the presence of combustible dust, of IEC technical committee 31: Electrical apparatus
for explosive atmospheres.
This second edition cancels and replaces the first edition, published in 1993, and constitutes a
technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
31H/90/FDIS 31H/96/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.
A bilingual version of this standard may be issued at a later date.

– 4 – 61241-1-1 © IEC:1999(E)
IEC 61241 consists of the following parts under the general title: Electrical apparatus for use in

the presence of combustible dust:

– Part 1: Electrical apparatus protected by enclosures and surface temperature limitation

– Part 2: Test methods
– Part 3: Classification of areas where combustible dusts are or may be present

1)
– Part 4: Type of protection pressurization "p"

1)
– Part 5: Intrinsically safe apparatus

________
1)
Under consideration.
61241-1-1 © IEC:1999(E) – 5 –
INTRODUCTION
Combustible dust can be ignited by electrical apparatus in several main ways:

• by surfaces of the apparatus that are above the minimum ignition temperature of the dust

concerned. The temperature at which a type of dust ignites is a function of the properties of

the dust, whether the dust is in a cloud or layer, the thickness of the layer and the geometry

of the heat source;
• by arcing or sparking of electrical parts such as switches, contacts, commutators, brushes,

or the like;
• by discharge of an accumulated electrostatic charge;
• by radiated energy (for example electromagnetic radiation);
• by mechanical sparking or frictional sparking or heating associated with the apparatus.
In order to avoid ignition hazards it is necessary that
• the temperature of surfaces, on which dust can be deposited, or which would be in contact
with a dust cloud, is kept below the temperature limitation specified in IEC 61241-1-2;
• any electrical sparking parts, or parts having a temperature above the minimum ignition
temperature of the dust
– are contained in an enclosure which adequately prevents the ingress of dust, or
– the energy of electrical circuits is limited so as to avoid arcs, sparks or temperatures
capable of igniting combustible dust;
• any other ignition sources are avoided.
The protection specified in this standard will not provide the required level of safety unless the
electrical apparatus is operated within its rating and is installed and maintained according to
the relevant codes of practice or requirements, for example in respect of protection against
over-currents, internal short circuits, and other electrical faults. In particular, it is essential that
the severity and duration of an internal or external fault be limited to values that can be
sustained by the electrical apparatus without damage.
Two different types of practice, A and B, are specified in this standard. Both are intended to
provide an equivalent level of protection.

– 6 – 61241-1-1 © IEC:1999(E)
ELECTRICAL APPARATUS FOR USE IN THE

PRESENCE OF COMBUSTIBLE DUST –

Part 1-1: Electrical apparatus protected by enclosures and

surface temperature limitation – Specification for apparatus

1 Scope
This part of IEC 61241 is applicable to electrical apparatus protected by enclosures and
surface temperature limitation for use in areas where combustible dust may be present in
quantities which could lead to a fire or explosion hazard. It specifies requirements for design,
construction and testing of electrical apparatus.
NOTE – IEC 61241-1-2 gives guidance on the selection, installation and maintenance of the apparatus. Apparatus
within the scope of this standard may also be subject to additional requirements in other standards – for example,
IEC 60079-0.
The ignition protection is based on the limitation of the maximum surface temperature of the
enclosure and other surfaces which could be in contact with dust and on the restriction of dust
ingress into the enclosure by the use of "dust-tight" or "dust-protected" enclosures.
The application of electrical apparatus in atmospheres which may contain explosive gas as well
as combustible dust, whether simultaneously or separately, requires additional protective
measures.
Where the apparatus has to meet other environmental conditions, for example, protection
against ingress of water and resistance to corrosion, additional methods of protection may be
necessary. The method used is not to adversely affect the integrity of the enclosure.
The principles of this standard may also be followed when combustible fibres or flyings cause a
hazard.
This standard does not apply to dusts of explosives which do not require atmospheric oxygen
for combustion, or to pyrophoric substances.
This standard is not applicable to electrical apparatus intended for use in underground parts of
mines as well as those parts of surface installations of such mines endangered by fire damp
and/or combustible dust. This standard does not take account of any risk due to an emission of
flammable or toxic gas from the dust.
This standard does not include other types of protection and is only applicable to protection by
enclosures and surface temperature limitation.

2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 61241. For dated references, subsequent amendments
to, or revisions of, any of these publications do not apply. However, parties to agreements
based on this part of IEC 61241 are encouraged to investigate the possibility of applying the
most recent editions of the normative documents indicated below. For undated references, the
latest edition of the normative document referred to applies. Members of IEC and ISO maintain
registers of currently valid International Standards.
IEC 60034-5:1991, Rotating electrical machines – Part 5: Classification of degrees of
protection provided by enclosures of rotating electrical machines (IP code)
IEC 60050(426):1990, International Electrotechnical Vocabulary (IEV) – Chapter 426: Electrical
apparatus for explosive atmospheres

61241-1-1 © IEC:1999(E) – 7 –
IEC 60079-0:1998, Electrical apparatus for explosive gas atmospheres – Part 0: General
requirements
IEC 60079-7:1990, Electrical apparatus for explosive gas atmospheres – Part 7: Increased

safety "e"
IEC 60079-11:1991, Electrical apparatus for explosive gas atmospheres – Part 11: Intrinsic

safety "i"
IEC 60093:1980, Methods of test for volume resistivity and surface resistivity of solid electrical

insulating materials
IEC 60192:1973, Low-pressure sodium vapour lamps
IEC 60243-1:1998, Electrical strength of insulating materials – Test methods – Part 1: Tests at
power frequencies
IEC 60216-1:1990, Guide for the determination of thermal endurance properties of electrical
insulating materials – Part 1: General guidelines for ageing procedures and evaluation of test
results
IEC 60216-2:1990, Guide for the determination of thermal endurance properties of electrical
insulating materials – Part 2: Choice of test criteria
IEC 60529:1989, Degrees of protection provided by enclosures (IP Code)
IEC 60662:1980, High-pressure sodium vapour lamps
IEC 60947-3:1990, Low-voltage switchgear and controlgear – Part 3: Switches, disconnectors,
switch-disconnectors and fuse-combination units
IEC 61241-1-2:199X, Electrical apparatus for use in the presence of combustible dust –
Part 1-2: Electrical apparatus protected by enclosures – Selection, installation and maintenance
IEC 61241-2-1:1994, Electrical apparatus for use in the presence of combustible dust – Part 2:
Test methods – Section 1: Methods for determining the minimum ignition temperatures of dust
IEC 61241-3:1997, Electrical apparatus for use in the presence of combustible dust – Part 3:
Classification of areas where combustible dusts are or may be present
ISO 178:1993, Plastics – Determination of flexural properties

ISO 527 (all parts), Plastics – Determination of tensile properties
ISO 4225:1994, Air quality – General aspects – Vocabulary
3 Definitions
For the purpose of this part of IEC 61241, the following definitions apply:
3.1
dust
small solid particles in the atmosphere which settle out under their own weight, but which may
remain suspended in air for some time (includes dust and grit as defined in ISO 4225)

– 8 – 61241-1-1 © IEC:1999(E)
3.2
combustible dust
dust, fibres or flyings that can burn or glow in air and could form explosive mixtures with air at

atmospheric pressure and normal temperatures

3.3
conductive dust
dust fibres or flyings with electrical resistivity equal to or less than 10 Ωm

3.4
explosive dust atmosphere
mixture with air, under atmospheric conditions, of flammable substances in the form of dust or
fibres in which, after ignition, combustion spreads throughout the unconsumed mixture
[IEV 426-02-04]
3.5
minimum ignition temperature of a dust layer
lowest temperature of a hot surface at which ignition occurs in a dust layer of specified
thickness on this hot surface
[see 3.3 of IEC 61241-2-1, modified]
3.6
minimum ignition temperature of a dust cloud
lowest temperature of the hot inner wall of a furnace at which ignition occurs in a dust cloud in
air contained therein
[see 3.5 of IEC 61241-2-1, modified]
3.7
dust ignition protection (DIP)
all relevant measures specified in this standard (for example, dust ingress protection and
surface temperature limitation) applied to electrical apparatus to avoid ignition of a dust layer
or cloud
3.8
dust-tight enclosure
enclosure capable of preventing the ingress of all observable dust particles

3.9
dust-protected enclosure
enclosure in which the ingress of dust is not totally prevented but does not enter in sufficient
quantity to interfere with the safe operation of the equipment. Dust should not accumulate in a
position within the enclosure where it is liable to cause an ignition hazard
3.10
maximum surface temperature
highest temperature which is attained by any part of the surface of electrical apparatus when
tested under the defined dust-free or blanket conditions
NOTE – This temperature is attained under the test condition. Increasing the layer thickness can increase this
temperature due to the thermal insulation properties of dust.

61241-1-1 © IEC:1999(E) – 9 –
3.11
maximum permissible surface temperature
highest temperature a surface of electrical apparatus is allowed to reach in practical service to

avoid ignition. The maximum permissible surface temperature will depend upon the type of

dust, whether as a cloud or layer, if a layer, its thickness, and the application of a safety factor

[see IEC 61241-1-2, clause 6]
3.12
zones
see 2.10 of IEC 61241-3
3.13
zone 20
see 2.11 of IEC 61241-3
3.14
zone 21
see 2.12 of IEC 61241-3
3.15
zone 22
see 2.13 of IEC 61241-3
4 Construction
4.1 Electrical apparatus for use in potentially explosive dust atmospheres shall comply with
the requirements of this standard.
NOTE – If the electrical apparatus has to withstand particularly adverse service conditions (for example rough
handling, humidity effects, ambient temperature variations, effects of chemical agents, corrosion), these should be
specified to the manufacturer by the user and are not the responsibility of the testing station.
4.2 Enclosures for use in zone 20 or 21 which can be opened more quickly than the time
necessary
• to allow incorporated capacitors, charged by a voltage of 200 V or more, to discharge to a
value of residual energy of 0,2 mJ; or
• to allow the cooling of enclosed hot components to a surface temperature below the
temperature class of the electrical apparatus
shall be marked with the following or equivalent warning:
“AFTER DE-ENERGIZING, DELAY X MINUTES BEFORE OPENING”
“X” being the value in minutes of the delay required.
Alternatively the apparatus may be marked with the warning:
“DO NOT OPEN WHEN AN EXPLOSIVE DUST ATMOSPHERE IS PRESENT”.
4.3 Where the apparatus has to meet other environmental conditions, for example, protection
against ingress of water and resistance to corrosion, the method of protection used shall not
adversely affect the integrity of the enclosure.

– 10 – 61241-1-1 © IEC:1999(E)

5 Enclosure materials
5.1 Non-metallic enclosures and non-metallic parts of enclosures

The following requirements apply to non-metallic enclosures and non-metallic parts of
enclosures on which the type of protection depends. In addition, the requirements of 20.4.7

apply to enclosures for use in zone 20 or 21.

5.1.1 Documents submitted to the manufacturer shall specify both the material and the

manufacturing process of the enclosure or part of the enclosure.

5.1.2 The specification for plastic materials shall include
a) the name of the manufacturer;
b) the exact and complete reference of the material including its colour, percentage of fillers
and any other additives if used;
c) the possible surface treatments, such as varnishes, etc.;
d) the temperature index “TI” corresponding to the 20 000 h point on the thermal endurance
graph without loss of flexural strength exceeding 50 %, determined in accordance
with IEC 60216-1 and IEC 60216-2 and based on the flexing property in accordance with
ISO 178. If the material does not break in this test before exposure to the heat, the index
shall be based on the tensile strength in accordance with ISO 527 with test bars of type 1.
The data by which these characteristics are defined shall be supplied by the manufacturer.
5.1.3 The testing station is not required to verify compliance of the material with its
specification.
5.1.4 Thermal endurance
5.1.4.1 Plastic materials shall have a temperature index “TI” corresponding to the 20 000 h
point of at least 20 K greater than the temperature of the hottest point of the enclosure or the
part of the enclosure (see 20.4.7.1), having regard to the maximum ambient temperature in
service.
5.1.4.2 The endurance to heat and to cold of the enclosures, or parts of enclosures, of plastic
materials shall be satisfactory (see 20.4.7.3 and 20.4.7.4).
5.1.5 Electrostatic charges on enclosures or parts of enclosures of plastic material for use in
zone 20 or 21.
5.1.5.1 The following requirements apply only to plastic enclosures, to plastic parts of
enclosures and to other exposed plastic parts of electrical apparatus for
• non-fixed electrical apparatus;
• fixed apparatus with plastic parts that are likely to be rubbed or cleaned on site.
5.1.5.2 Enclosures of plastic material with surface area projected in any direction of more
than 100 cm shall be so designed that under normal conditions of use, maintenance and
cleaning, danger of ignition due to electrostatic charges is avoided.

61241-1-1 © IEC:1999(E) – 11 –

5.1.5.3 This can be achieved by the use of plastic material having one or more of the

following characteristics:
• Insulation resistance ≤10
Ω (resistance against electrostatic discharge to earth through or

across the surface of insulation, measured according to the method described in IEC 60093
with an effective area of the circular electrode of 20 cm ).

• Breakdown voltage ≤4 kV (measured across the thickness of the insulating material

according to the method described in IEC 60243-1).

• Thickness ≥8 mm of the external insulation on metal parts. (External plastic layers of 8 mm

and greater on metal parts such as measurement probes or similar components make

propagating brush discharges unlikely to occur. When evaluating the minimum thickness of
the insulation to be used or specified, it is necessary to allow for any expected wear under
normal usage.)
5.1.5.4 If, however, the danger of ignition cannot be avoided in the design, a warning label
shall indicate the safety measures to be applied in service.
NOTE – When selecting electrical insulating materials, attention should be paid to maintaining a minimum insulation
resistance to avoid problems arising from touching exposed plastic parts that are in contact with live parts.
5.2 Enclosures containing light metals
5.2.1 Materials used in the construction of enclosures of electrical apparatus to be used in
explosive dust atmospheres shall not contain, by weight, more than 6 % in total of magnesium
and titanium.
5.2.2 Threaded holes in enclosures for fasteners which secure covers intended to be opened
in service for adjustment, inspection and other operational reasons may only be tapped in the
material of the enclosure when the thread form is compatible with the material used for the
enclosure.
6 Fasteners
6.1 Parts necessary to achieve a standard type of protection or used to prevent access to
uninsulated live parts shall be capable of being released or removed only with the aid of a tool.
6.2 Threaded holes for fasteners which secure covers intended to be opened in service for
adjustment, inspection and other operational reasons may only be tapped into the plastic or
light metal material when the thread form is compatible with the plastic or light metal material
of the enclosure.
7 Interlocking devices
Interlocking devices used to maintain a type of protection shall be so constructed that their
effectiveness cannot readily be defeated by the use, for example, of a screwdriver or pliers.
8 Bushings
8.1 Bushings in enclosures used as connection facilities, and which may be subjected to a
torque while the connection or disconnection is being made, shall be mounted in such a way
that all parts are secured against turning.
8.2 Bushings in enclosures for use in zone 20 or 21 shall comply with the relevant torque test
as specified in 20.4.4.
– 12 – 61241-1-1 © IEC:1999(E)

9 Materials used for cementing

9.1 The manufacturer’s documents submitted according to 20.2 of this standard shall testify

that for the intended operating conditions, the materials used for cementing, and on which

safety depends, have a thermal stability adequate for the minimum and maximum temperatures
to which they will be subjected, within the rating of the electrical apparatus.

9.2 The thermal stability is considered adequate if the limiting values for the material are

below or equal to the lowest working temperature and at least 20 K above the maximum
temperature in service.
NOTE – If the cementing has to withstand adverse service conditions, appropriate measures should be agreed
between user and manufacturer.
9.3 The testing station is not required to verify the characteristics listed in the documents
mentioned in 9.1.
10 Connection facilities and terminal compartments
10.1 Electrical apparatus which is intended for connection to external circuits shall include
connection facilities, except if the electrical apparatus is manufactured with a cable
permanently connected to it. All apparatus constructed with permanently connected
unterminated cables shall be marked with the symbol “X” to indicate the need for appropriate
connection of the free end of the cable.
10.2 Terminal compartments and their access openings shall be dimensioned so that the
conductors can be readily connected.
10.3 Terminal compartments shall be so designed that after proper connection of the
conductors, the creepage distances and the clearances comply with the requirements, if any, of
the specific standard for the type of apparatus concerned.
11 Connection facilities for earthing or bonding conductors
11.1 A connection facility for the connection of an earthing or equipotential bonding conductor
shall be provided inside the terminal compartment of electrical apparatus and near the other
connection facilities.
11.2 Electrical apparatus with a metallic enclosure shall have an additional external
connection facility for an earthing or equipotential bonding conductor. This external connection

facility shall be electrically in contact with the facility required in 11.1. The external connection
facility is not required for electrical apparatus which is designed to be moved when energized
and is supplied by a cable incorporating an earthing or equipotential bonding conductor.
NOTE – The expression “electrically in contact” does not necessarily involve the use of a conductor.
11.3 Neither an internal nor external earthing or bonding connection facility is required for
electrical apparatus for which earthing (or bonding) is not required, such as electrical
apparatus having double or reinforced insulation, or for which supplementary earthing is not
necessary.
11.4 Earthing or equipotential bonding connection facilities shall allow for the effective
connection of at least one conductor with a cross-sectional area as shown in table 1.

61241-1-1 © IEC:1999(E) – 13 –

Table 1 – Minimum cross-sectional areas of protective conductors

Cross-sectional area of phase conductors of Minimum cross-sectional area of the

the installation corresponding protective conductor
S Sp
2 2
mm mm
S
S ≤ 16
16 < S ≤ 35
S > 35 0,5 S
11.5 In addition to meeting this requirement, earthing or bonding connection facilities on the out-
side of electrical apparatus shall provide for effective connection of a conductor of at least 4 mm .
11.6 Connection facilities shall effectively be protected against corrosion. They shall also be
designed so that the conductors are secured against loosening and twisting and so that the
contact pressure is maintained.
11.7 Contact pressure of electrical connections shall not be affected by dimensional changes
of insulating materials in service, due to temperature or humidity, etc.
11.8 Special precautions shall be taken if one of the parts in contact consists of a material
containing light metal. One example of a means of connecting to a material containing light
metal is to use an intermediate part made from steel.
12 Cable and conduit entries
12.1 The manufacturer shall specify in the documents submitted according to 20.2, the
entries intended for use with cable or conduit, their position on the apparatus and the maximum
number permitted.
12.2 Cable and conduit entries shall be constructed and fixed so that they do not alter the
specific characteristics of the type of protection of the electrical apparatus on which they are
mounted. This shall apply to the whole range of cable dimensions specified by the
manufacturer of the cable entries as suitable for use with those entries.
Cable and conduit entries may form an integral part of the apparatus, i.e. one major
12.3
element or part forms an inseparable part of the enclosure of the apparatus. In such cases, the
entries shall be tested and certified with the apparatus.

NOTE – Cable and conduit entries, which are separate from, but installed with the apparatus, are usually tested and
certified separately from the apparatus but may be tested and certified together with the apparatus if the apparatus
manufacturer so requests.
12.4 Where the design of a cable entry is such that twisting of the cable can be transmitted to
the connections, then an anti-rotation device shall be fitted.
12.5 Entry by conduit or cable entries shall be either by screwing into threaded holes or by
locking in plain holes
• in the wall of the enclosure; or
• in an adaptor plate designed to be fitted in or on the walls of the enclosure; or
• in a suitable stopping box, integral with or attached to the wall of the enclosure.

– 14 – 61241-1-1 © IEC:1999(E)

12.6 Blanking elements intended to close openings in the walls of electrical apparatus when
they are not fitted with cable or conduit entries, shall, together with the enclosure wall of the
apparatus, satisfy the requirements of the specific type of protection concerned. The means

provided for this shall be such that the blanking element can be removed only with the aid of a

tool.
12.7 When the temperature under rated conditions, including any manufacturer’s installation

requirements, is higher than 70 °C at the cable or conduit entry point, or 80 °C at the branching

point of the conductors, the outside of the electrical apparatus shall be marked as a guide for

the selection by the user of the cable or of the wiring in the conduit, in order to ensure that the

rated temperature of the cable is not exceeded (see figure 1).

IEC  790/99
Key
1 Branching point of the conductors
2 Sealing ring
3 Cable entry body
4 Clamping ring with curved rim
5 Cable
Figure 1 – Illustration of entry points and branching points

13 Supplementary requirements for electrical apparatus for practice B
for use in zone 20 or 21
13.1 Joints
13.1.1 Plain joints (see figure 2) shall have a minimum contact width from inside to outside of
the enclosure and a maximum allowable clearance between the surfaces as shown in table 2.

61241-1-1 © IEC:1999(E) – 15 –

Table 2 – Plain joints
Minimum contact width of the plain joint (mm), W 522

Maximum allowable clearance between the surfaces of the joint (mm), G 0,05 0,22

NOTE – For widths of joints between 5 mm and 22 mm the maximum clearance may be increased by 0,01 mm for

each millimetre increase in joint width greater than 5 mm.

G
W
IEC  791/99
Figure 2 – Plain joints
Spigotted joints (see figure 3) which have axial, , and radial, , paths neither of
13.1.2 L W
which is less than 1,2 mm, may have diametrical clearances as shown for plain joints in
table 2. The radial section, , of the spigotted joint shall have a maximum allowable clearance,
W
G, as shown for plain joints in table 2.
L
W
G
IEC  792/99
Figure 3 – Spigotted joints
– 16 – 61241-1-1 © IEC:1999(E)

13.1.3 For gasketed joints (see figure 4) requirements are given in table 3.

O
W
IEC  793/99
Figure 4 – Gasketed joints
Table 3 – Gasketed joints
Maximum opening dimension (mm), O 305 915 >915
Minimum required effective width of gasketed joint (mm), W 3 4,8 9,5
NOTE – For maximum opening dimensions between 305 mm and 915 mm, the
effective width of the gasketed joint shall be increased by 0,003 mm for each
millimetre increase in the maximum opening dimension greater than 305 mm.
13.2 Operating rods, spindles or shafts
13.2.1 Equipment meeting the requirements of “Dust ignition protected apparatus type
DIP B20 or DIP B21” and the test requirements of 20.4.3 of this standard shall not depend on
running contact seals to ensure dust-tightness.
13.2.2 If running contact seals are used, the equipment shall meet the design details of
table 4 and table 5 and the dust tests of 20.4.3 without the running contact seals installed.
13.2.3 Power shafts for transmitting power at rotational speeds of 100 r/min or more (see
figure 5) shall meet the requirements for the length of path from the inside to the outside of the
enclosure as given in table 4.

Running contact seal
D
D
1 2
L
IEC  794/99
Figure 5 – Power shafts for speeds of 100 r/min. or more

61241-1-1 © IEC:1999(E) – 17 –

Table 4 – Power shafts for speeds of 100 r/min or more

Minimum length of path for power shafts (mm), L 12,5 38,5

Maximum allowable diametrical clearance (mm), D – D 0,26 0,57
2 1
NOTE 1 – For lengths of path for power shafts between 12,5 mm and 38,5 mm the maximum diametrical clearance

may be increased by 0,012 mm for each millimetre increase in the length of path greater than 12,5 mm.

NOTE 2 – The minimum length of path may include both the inner and outer lips of the bearing housing.

13.2.4 Operating rods, spindles or shafts having rotary motion of less than 100 r/min or axial

motion, shall have either a threaded joint with three full threads engaging, or have a minimum

length of path from the inside to the outside of the enclosure as given in table 5.
Table 5 – Power shafts for speeds of less than 100 r/min
Minimum length of path for operating rods, spindles or shafts (mm), L 12,5 25,5
Maximum allowable diametrical clearance (mm), D – D 0,13 0,21
2 1
NOTE – For lengths of path for operating rods, spindles or shafts between 12,5 mm and 25,5 mm, the maximum
diametrical clearance may be increased by 0,006 mm for each millimetre length of path greater than 12,5 mm.
13.3 Clearance of bolts
Bolts passing through the enclosure wall shall have a maximum diametrical clearance between
the unthreaded shank of the bolt, N, and the clearance hole in the enclosure, DS-DH, of not
more than 0,26 mm and a length of path, , of not less than 12,5 mm (see figure 6).
L
L
N
IEC  795/99
Figure 6 – Clearance of bolts
14 Rotating electrical machines
14.1 External, shaft-driven cooling fans of rotating electrical machines shall be enclosed by a
fanhood which is not considered to be part of the enclosure of the electrical apparatus. Such
fans and fanhoods shall meet the following requirements.

– 18 – 61241-1-1 © IEC:1999(E)

14.2 Ventilation openings for external fans

The degree of protection (IP) of ventilation openings for external fans of rotating electrical

machines shall be at least
• IP20 on the air inlet side,
• IP10 on the air outlet side,

according to IEC 60034-5.
14.3 For vertical rotating machines for use in zone 20 or 21, foreign objects shall be

prevented from falling into the ventilation openings.
14.4 Construction and mounting of the ventilating systems
Fans, fanhoods and ventilation screens shall be constructed so as to meet the requirements
of the resistance to impact test according to 20.4.2.1 and the required results according
to 20.4.2.3.
14.5 Clearances for the ventilating system for use in zone 20 or 21
In normal operation the clearances, taking into account design tolerances, between the
external fan and its hood, ventilation screens and their fasteners shall be at least 1/100 of the
maximum diameter of the fan, except that the clearances need not exceed 5 mm and may be
reduced to 1 mm if the opposing parts are manufactured so as to have dimensional accuracy
and stability. In no case shall the clearance be less than 1 mm.
14.6 Materials for external fans and fanhoods for use in zone 20 or 21
14.6.1 External fans, fanhoods, ventilation screens, etc., shall have an electrical insulation
resistance measured according to 5.1.5.3 not exceeding 10 Ω.
14.6.2 The thermal stability of plastic materials shall be considered adequate if the
manufacturer’s specified operating temperature of the material exceeds the maximum
temperature to which the material will be subjected in service (within the rating) by at least 20 K.
14.6.3 The external fans, fanhoods, ventilation screens, of rotating electrical machines,
manufactured from materials containing light metals shall not contain by weight more than 6 %
of magnesium.
15 Switchgear
15.1 Switchgear with contacts immersed in flammable dielectric is not permitted.
15.2 Disconnectors (which are not designed to be operated under the intended load) shall:
• be electrically or mechanically interlocked with a suitable load breaking device; or
• be marked at a place near the actuator of the disconnector, with the warning
“DO NOT OPERATE UNDER LOAD”.
15.3 Where switchgear includes a disconnector, the latter shall disconnect all poles and shall be
designed so that the position of the disconnector contacts is visible, or their open position is
indicated in accordance with the requirements for the isolation function specified in IEC 60947-3.
Any interlock between such disconnector and the cover or door of the switchgear shall allow this
cover or door to be opened only when the separation of the disconnector contacts is effective.

61241-1-1 © IEC:1999(E) – 19 –

16 Fuses
Enclosures containing fuses shall

• be interlocked so that insertion or removal of replaceable elements can be carried out only
with the supply disconnected and so that the fuses cannot be energized until the enclosure

is correctly closed; or
• alternatively, the apparatus shall be marked with the warning

“DO NOT OPEN WHEN ENERGIZED”.
17 Plugs and sockets
17.1 Plugs and sockets shall comply with either a) or b) below:
a) be interlocked mechanically, or electrically, or otherwise designed so that they cannot be
separated when the contacts are energized and the contacts cannot be energized when the
plug and socket are separated; or
b) be fixed together by means of special fasteners according to 9.2 of IEC 60079-0 and the
apparatus marked with the warning:
“DO NOT SEPARATE WHEN ENERGIZED”.
17.2 In the case where bolted types cannot be de-energized before separation because they
are connected to a battery, the marking shall then state:
“SEP
...