CLC/TR 50404:2003

SLOVENSKI STANDARD
01-november-2003
1DGRPHãþD
SIST R044-001:2002
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Electrostatics - Code of practice for the avoidance of hazards due to static electricity
Ta slovenski standard je istoveten z: CLC/TR 50404:2003
ICS:
13.230 Varstvo pred eksplozijo Explosion protection
13.260 9DUVWYRSUHGHOHNWULþQLP Protection against electric
XGDURP'HORSRGQDSHWRVWMR shock. Live working
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT CLC/TR 50404
RAPPORT TECHNIQUE
TECHNISCHER BERICHT June 2003

English version
Electrostatics -
Code of practice for the avoidance of hazards
due to static electricity
This Technical Report was approved by CENELEC on 2003-04-19.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Czech Republic,
Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Luxembourg, Malta,
Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2003 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. CLC/TR 50404:2003 E
Foreword
This CENELEC code of practice gives information about the product and process properties necessary
to avoid electrostatic hazards as well as operational requirements to be written in the users manual to
ensure safe use of the product or process. It can be used in a risk assessment of electrostatic hazards
or for the preparation of product family or dedicated product standards for machines (i.e. type C
standards in CEN, as defined in EN 414:1992, 3.1).

This CENELEC document is based on a number of documents including two national Codes of
Practice: from the UK, BS 5958: Parts 1 & 2:1991, Control of undesirable static electricity; and from
Germany, ZH 1/200: October 1989, Code of Practice for preventing risks of ignition due to electrostatic
charges: Guidelines in static electricity, and a document published by Shell International Petroleum:
Static electricity - Technical and safety aspects. It gives the best available accepted state of the art
guidance for the avoidance of hazards due to static electricity.

This document is mainly written for designers of processes, manufacturers and test houses.
Appropriate information about the procedures necessary to avoid electrostatic hazards shall be written
in the users manual or on the product to ensure safety. This document can also be used by suppliers
of equipment (e.g. machines) when no product family or dedicated product standard exists or where
the existing standard does not deal with electrostatic hazards.

This CENELEC document was originally prepared by the Technical Committee CENELEC TC 44X,
Safety of machinery: electrotechnical aspects. The text of the first edition approved by CLC/TC 44X on
1997-11-07 and its publication was authorised by the CENELEC Technical Board on 1999-01-01.

Following a decision by CENELEC BT, the maintenance of the document was undertaken by the
Technical Committee CENELEC TC 31, Electrical apparatus for explosive atmospheres - General
requirements, which has delegated the revision to its Working Group 20 dealing with electrostatic
hazards.
The text of the draft was submittted to the National Committees for approval by correspondence and was
approved by CENELEC as CLC/TR 50404 on 2003-04-19.

This Technical Report supersedes R044-001:1999.
____________
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Contents
Clause Page
1 Scope. 5

2 Definitions. 5

3 General. 7
3.1 Standard approaches. 7
3.2 Alternative approaches . 8

4 Static electricity in non-conductive solid materials . 8
4.1 General considerations . 8
4.2 Dissipative solid materials. 8
4.3 The use of conductive or dissipative materials in place of non-conductive ones. 9
4.4 Precautions required when using non-conductive solid materials . 9
4.5 Conveyor belts and transmission belts . 12

5 Static electricity in liquids . 13
5.1 General considerations . 13
5.2 Ignition hazard. 14
5.3 Precautions against ignition hazards during liquid handling operations. 15
5.4 Tanks and Containers. 16
5.5 Pipes and hoses for liquids . 31
5.6 Special filling procedures . 34
5.7 Plant processes (blending, stirring, mixing and crystallisation). 36
5.8 Spraying liquids and tank cleaning. 38
5.9 Glass systems. 39

6 Static electricity in gases. 40
6.1 General. 40
6.2 Grit blasting . 41
6.3 Fire extinguishers. 41
6.4 Inerting. 41
6.5 Steam cleaning . 41
6.6 Accidental leakage of compressed gas . 41
6.7 Spraying of flammable paints and powders . 42
6.8 Extraction systems. 42
6.9 Vacuum cleaners, fixed and mobile. 42

7 Static electricity in powders. 43
7.1 General. 43
7.2 Powders in the absence of flammable gases and vapours. 43
7.3 Powders in the presence of flammable gases or vapours . 50

8 Static electricity when handling explosives and electro-explosive devices. 52
8.1 Explosives manufacture, handling and storage . 52
8.2 Handling of electro-explosive devices. 54

9 Static electricity on persons . 55
9.1 General considerations . 55
9.2 Conducting floor. 55
9.3 Dissipative and conductive footwear. 55
9.4 Clothing. 55
9.5 Protective gloves. 56
9.6 Other items. 56

10 Electric shock. 56
10.1 Introduction. 56
10.2 Discharges relevant to electric shock . 56
10.3 Sources of electric shock . 57
10.4 Precautions to avoid electric shocks. 57
10.5 Precautions in special cases. 57

11 Earthing and bonding . 58
11.1 General. 58
11.2 Criteria for the dissipation of static electricity from a conductor. 59
11.3 Earthing requirements in practical systems . 60
11.4 The establishment and monitoring of earthing systems. 62

Annexes
Annex A (informative) Fundamentals of static electricity . 64
Annex B (informative) Electrostatic discharges in specific situations. 73
Annex C (informative) Igniteability properties of substances . 77
Annex D (informative) Classification of hazardous areas. 79

Bibliography. 80

Figures
Figure A.1 - Equivalent electrical circuit for an electrostatically charged conductor. 67

Tables
Table 1 - Restriction on area or width values of non-conductive solid materials in hazardous
areas containing potentially explosive atmospheres of groups IIA, IIB and IIC. 10

Table 2 - Conductivities and relaxation times of some liquids. 14

Table 3 - Precautions for filling large metal tanks with low conductivity liquids. 18

Table 4 - Maximum filling velocities for loading low conductivity liquids other
than petroleum products into road tankers. 22

Table 5 - Vehicles and compartments suitable for high-speed loading for ADR compliant vehicles. 22

Table 6 - Influence of the sulphur content on vd limits for road tankers. 23

Table 7 - Flow rate limits for road tankers . 23

Table 8 - vd and flow rate limits for loading rail tankers with non-petroleum liquids. 24

Table 9 - Flow rate limits for loading rail tankers with petroleum fuels. 25

Table 10 - Use of different types of FIBC . 49

Table 11 - Summary of maximum earthing resistances for the control of static electricity . 62

Table A.1 - Charge build up on medium resistivity powders. 66

Table A.2 - Values of capacitances for typical conductors. 70

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1 Scope
This document is a code of practice for avoiding ignition and electric shock hazards arising from static
electricity. The processes that most commonly give rise to problems of static electricity are described
in detail. They include the handling of solids, liquids, powders, gases, sprays and explosives. In each
case, the source and nature of the electrostatic hazard is identified and specific recommendations are
given for dealing with them.
Basic information about the generation of undesirable static electricity in solids, liquids, gases,
explosives, and also on persons, together with descriptions of how the charges generated cause
ignitions or electric shocks, is given in the annexes.

This document is not applicable to the hazards of static electricity relating to lightning, to damage to
electronic components, nor to medical hazards.

2 Definitions
Regulations relating to safety and electrostatics make use of many adjectives in order to quantify the
conducting ability of materials. Different regulations and different industries use different adjectives;
even when the same adjectives are used their definitions can vary. In order to avoid confusion, and to
assist with translation, the adjectives normally used to quantify the resistance of a material in this
document are conductive, dissipative and non-conductive (see 2.6, 2.8 and 2.9). However, in parts of
the document liquids are also described according to their conductivities (see 5.1) and powders
according to their resistivities (see 7.2.1).

NOTE 1  More details about electrostatic properties, concepts and terms are given in the annexes
NOTE 2 The values given in the following definitions are the generally accepted ones. However, in parts of the document the
values quoted in the text differ from those in the definitions. This is because the process, the method of handling or the
material being handled is sufficiently unusual that a different (higher or lower value) is required.

For the purpose of this document the following definitions apply:

2.1
volume resistivity
the resistance of a body of unit length and unit cross-sectional area

2.2
surface resistivity
the resistance across opposite sides of a surface of unit length and unit width commonly expressed in
ohms (or ohms/square)
2.3
surface resistance
the resistance expressed in ohms between two electrodes in contact with the surface to be measured
(usually parallel electrodes, each 100 mm long and 10 mm apart)

2.4
leakage resistance
the resistance expressed in ohms between an electrode in contact with the surface to be measured
and earth (usually a circular electrode, 20 cm in area)

NOTE  The resistance depends upon the volume or surface resistivity of the materials and the distance between the chosen
point of measurement and earth.

2.5
conductivity
the reciprocal of volume resistivity

2.6
conductive
an adjective describing a material incapable of retaining a significant electrostatic charge when in
contact with earth and having a volume resistivity equal to or lower than 10 Ωm (for certain items there
are special definitions e.g. conductive hose)

2.7
conductor
a conductive object
2.8
dissipative (electrostatic dissipative)
an adjective describing a material incapable of retaining a significant amount of electrostatic charge
when in contact with earth. These materials have a volume resistivity higher than 10 Ωm but equal to
9 10 9
or lower than 10 Ωm, or a surface resistivity less than 10 Ω (or surface resistance less than 10 Ω)
measured at ambient temperature and 50 % relative humidity

2.9
non-conductive
an adjective describing a material that is neither conductive nor dissipative and on which electrostatic
charges can accumulate and not readily dissipate even when in contact with earth (e.g. most common
plastics)
2.10
non conductor
a non-conductive object
2.11
antistatic (deprecated)
an adjective commonly used as a synonym for conductive or dissipative describing a material that is
incapable of retaining a significant electrostatic charge when in contact with earth. In this context the
word is commonly used to describe a type of footwear and antistatic additives (ASAs) for use with
liquids
2.12
electric shock
pathophysiological effect resulting from an electric current passing through human or animal body

2.13
relaxation time
the time during which the electrostatic charge on a solid surface, in the bulk of a liquid or powder, or in
a cloud of mist or powder, decays exponentially to 1/e (i.e. about 37 %) of its original value

2.14
hazardous area
an area in which flammable or explosive gas/vapour-air or dust-air mixtures are, or can be, present in
such quantities as to require special precautions against ignition

2.15
two-phase liquid
a mixture of two immiscible liquids which, when settled, forms two separate phases with a distinct
interfacial boundary
2.16
dissipative footwear
footwear that ensures that a person standing on a conductive or dissipative floor has a resistance to
5 8
earth of more than 10 Ω but less than 10 Ω

- 7 - CLC/TR 50404:2003
2.17
conductive footwear
footwear ensuring a resistance to earth typically of less than 10 Ω

2.18
dissipative clothing
clothing made from a material with a surface resistivity of less than 5 × 10 Ω (see EN 1149-1) or with
good charge decay characteristics (see prEN 1149-3)

2.19
minimum ignition energy (MIE)
the minimum energy that can ignite a mixture of a specified flammable material with air or oxygen,
measured by a standard procedure

3 General
3.1 Standard approaches
Static electricity occurs commonly in industry and in daily life. Many of the effects are harmless and
either pass completely unnoticed or are simply a nuisance, but static electricity can also give rise to a
hazardous situation. Hazards caused by electrostatic charge include

- ignition and/or explosion,
- electric shock in combination with another hazard (e.g. fall, trip) - see EN 292-1, 4.3 and 4.10,
- electric shock giving rise to injury or death, see EN 292-1, 4.3.

In addition, static electricity introduces operational problems during manufacturing and handling
processes, e.g. by causing articles to adhere to each other, or by attracting dust.

It is generated by
- the contact and separation of solids e.g. the movement of conveyor belts, plastics film, etc. over
rollers, the movement of a person,
- the flow of liquids or powders, and the production of sprays,
- an induction phenomenon, i.e. objects becoming charged due to being in an electric field.

The accumulation of electrostatic charge can give rise to hazards and problems in a wide range of
industries, and to ignition and explosion hazards particularly in chemicals, pharmaceuticals, petroleum
and food processing industries.

The purpose of this document is to provide recommendations for the control of static electricity. In
some cases static electricity plays an integral part of a process, e.g. paint spraying, but more often it is
an unwelcome side effect and it is with the latter that this guidance is concerned.

Because of the large number of industrial processes which could be involved it is not possible to give
detailed information relevant to all of them. Instead, this document attempts to describe the problems
associated with each process and to give a code of practice on how to avoid them. This information
should enable the plant operator to take whatever precautions could be necessary to avoid ignitions of
potentially flammable atmospheres and electric shocks.

For convenience this document is divided into a number of clauses. These deal with problems
associated with the following:

- the handling of solids;
- the storage and handling of liquids;
- the handling of gases and vapours;
- the storage and handling of powders;
- the storage and handling of explosives
- electrostatic problems caused by persons;
- avoidance of electric shock;
- earthing and bonding of plant and machinery.

This document also contains some fundamental information relating to electrostatic charging and its
problems. This is contained in the annexes and it should enable the Reader to better understand the
advice given and also to extend the advice to processes that have not been dealt with in the guidance.

It is very seldom that an electrostatic hazard can be treated in isolation. Precautions against
electrostatic hazards should be in addition to other precautions, e.g. explosion protection. They should
also be consistent with precautions taken to avoid other hazards that may be present, such as ignitions
due to other causes, and toxicity. It is important that all sources of risk in a system of work are
considered and that a balanced approach to safety covering all risks be considered. In particular, care
should be exercised in the provision of earthing systems where they can interfere with other protective
systems, e.g. cathodic protection or intrinsically safe electrical equipment.

3.2 Alternative approaches
If the requirements of this document cannot be fulfilled, alternative approaches can be applied under
the condition that at least the same level of safety is achieved. This may be established by a special
risk assessment carried out by persons having appropriate experience.

4 Static electricity in non-conductive solid materials

4.1 General considerations
Non-conductive solid materials are being used increasingly in equipment and structures in many forms
including pipes, containers, sheets, coatings and liners. Many of these materials have volume
resistivities greater than 10 Ωm and their use in hazardous areas can give rise to the following
electrostatic hazards:
- the material could insulate conductive objects from earth which could become charged and give
rise to sparks;
- charges on the surface of the material could lead to brush discharges;
- a combination of conductive and non-conductive materials in the presence of prolific charge
generators (e.g. pneumatic transfers of powders, spraying of charges) could lead to very energetic
propagating brush discharges.
The use of non-conductive materials needs to be restricted in some hazardous areas. The restrictions
depend on the zone classification of the hazardous area (see Annex D):

- in zone 0, non-conductive solid materials should only be used if charging mechanisms capable of
generating hazardous potentials will not occur either during normal operation (including
maintenance and cleaning) or even in the case of rare malfunctions;

- in zone 1, non-conductive solid materials should only be used if charging mechanisms capable of
generating hazardous potentials will not occur either during normal operation (including
maintenance and cleaning) or in the case of likely malfunctions;

- in zone 2, non-conductive solid materials may be used if charging mechanisms capable of
generating hazardous potentials are unlikely to occur during normal operation (including
maintenance and cleaning).
- in the dust zones 20, 21 and 22 consideration should be given to spark, brush, cone, and
propagating brush discharges (see Annex B). However, practical experience and the absence of
incidents indicate that brush discharges are of low incendivity with regard to powder clouds.

NOTE  Many powders and dusts are non-conductive materials and recommendations for the avoidance of electrostatic
hazards associated with powders are given in Clause 7.

4.2 Dissipative solid materials

A solid material is defined as dissipative if its surface resistance does not exceed 10 Ω. However,
since surface resistance normally increases considerably with decreasing humidity the upper limit will
depend on relative humidity.
- 9 - CLC/TR 50404:2003
When testing materials, this value is acceptable only if it is measured at a relative humidity of less than
30 %. For measurements at 50 % relative humidity the upper limit is 10 Ω.

Providing that materials which meet these values of surface resistance are connected to earth no
further protective measures need to be taken. In processes involving high speed separation (e.g.
conveyor and transmission belts, see 4.5.3 to 4.5.5) other values can be required.

4.3 The use of conductive or dissipative materials in place of non-conductive ones

It is good practice to minimise the use of non-conductive materials in hazardous areas and there are
many materials which used to be entirely non-conductive, e.g. rubbers or plastics, that are now
available in grades which are dissipative, i.e. they comply with the requirements of 4.2. However these
grades normally contain additives such as carbon black and the high proportion of carbon black
required may degrade the physical properties of the material.

In some cases conductive or dissipative coatings are used to make the non-conductive material non-
chargeable. However, the durability of these applications and their suitability for use in hazardous
areas of zone 0 and zone 1 has yet to be proven. In any case, it is important that the conductive
coating is properly earthed.
Fabrics, e.g. filter cloth, can be made dissipative by incorporating stainless steel or other conductive or
dissipative fibres in the fabric. Care has to be taken to ensure that, as a result of washing or
mechanical stress, the overall conductivity of the fabric is maintained and isolated patches of
conductive fibres are not formed.

4.4 Precautions required when using non-conductive solid materials

4.4.1 General
To prevent incendive discharges the precautions given in 4.4.2 to 4.4.9 should be taken in all zones
where the use of non-conductive solid materials is unavoidable.

The precautions given in 4.4.2 relate to avoidance of spark discharges, those given in 4.4.3 to 4.4.8 to
incendive brush discharges, and those given in 4.4.9 to propagating brush discharges.

4.4.2 Bonding of conductive items

All metal and other conductive material should be bonded to earth with the exception of very small
items:
- capacitances below 3 pF need not be earthed provided high charging mechanisms do not occur;
- in zones 1 and 2 where gases or liquids belonging to group IIA and IIB are used the maximum
allowed insulated capacitance may be increased to 10 pF provided high charging mechanisms do
not occur;
- in dust zones the maximum allowed insulated capacitance may be increased to 10 pF provided
either high charging mechanisms do not occur or powders with minimum ignition energies higher
than 10 mJ are handled.
4.4.3 Restrictions on the size of chargeable surfaces

The restriction on the size of chargeable surfaces depends on the ignitability of the gases and vapours
(expressed by the representative groups IIA, IIB and IIC, see EN 50014) and the classification of the
hazardous area:
(a) for sheet materials the area is defined by the exposed (chargeable) area;
(b) for curved objects the area is the projection of the object giving the maximum area;
(c) for long narrow materials, such as cable sheaths or pipes, the maximum size is defined by the
transverse dimension (i.e. the diameter for a cable sheath or pipe); when it is coiled it should be
treated as for a sheet (see item a).

It is essential that non-conductive solid materials used in hazardous areas do not exceed the maximum
area or width values given in Tables 1(a) and 1(b) for the zone within which it is used unless it can be
shown that hazardous electrostatic charges are not to be expected (see A.3.4) or charging
mechanisms will not occur at any time.

Table 1 - Restriction on area or width values of non-conductive solid materials in hazardous
areas containing potentially flammable atmospheres of groups IIA, IIB and IIC

(a) Restrictions on areas
Maximum area, cm
Zone
Group IIA Group IIB Group IIC
0 50 25 4
1 100 100 20
2 No limit (see 4.1) No limit (see 4.1) No limit (see 4.1)

(b) Restrictions on widths of narrow materials (e.g. pipes, cable sheaths)

Maximum width, cm
Zone
Group IIA Group IIB Group IIC
0 0.3 0.3 0.1
1 3.0 3.0 2.0
2 No limit (see 4.1) No limit (see 4.1) No limit (see 4.1)

NOTE  Even smaller diameters can be required for narrow pipes (or tubes) containing flowing liquids or powders.

4.4.4 Avoidance of incendive brush discharges

Layers or coatings of non-conductive solids on earthed conducting surfaces (in particular metal
surfaces) can give rise to brush discharges. Practical experience shows that these discharges are
unlikely to be incendive if
- repeated electrostatic charging processes are avoided (e.g. repeated filling and emptying of a
drum), and
- the layer is of non-fluorinated polymer (e.g. polyethylene), and
- the thickness of the layer does not exceed a value of 2 mm in the case of gases and vapours of
groups IIA and IIB and a value of 0,2 mm in the case of gases and vapours of group IIC.

In those cases no special protective measures are necessary within hazardous areas.

4.4.5 Use of earthed metal meshes

If the restriction on size given in 4.4.3 cannot be met, incendive brush discharges can be avoided by
incorporating an earthed mesh (or metal frame) into the non-conductive solid or by wrapping such a
mesh around its surface. This method of protection is acceptable in hazardous areas providing that

- the mesh size (i.e. the area contained by the wires) is restricted to a factor of four times the values
given in Table 1(a), and
- the layer thickness above the mesh is restricted to the values given in 4.4.4, and
- high charging mechanisms do not occur.

However, an internal mesh does not guarantee protection against propagating brush discharges (see
4.4.9).
4.4.6 Humidification
The surface resistivity of some non-conductive solid materials can be reduced to dissipative levels if
the relative humidity is maintained above about 65 %. Even though damp air is not conductive, a film
of moisture forms on the surface on many materials depending on the hydroscopic nature of the
material. Whereas some materials such as glass or natural fibres form a sufficiently conducting film of
moisture, other materials such as polytetrafluoroethylene (PTFE) or polyethylene do not.

- 11 - CLC/TR 50404:2003
Increasing the relative humidity, therefore, is not effective in all cases and, in general, it should not be
used as the sole protective measure, especially not in zone 0.

4.4.7 Ionising the air
4.4.7.1 General
Ionisation of the air is a method of making the air locally ion-rich so that charges on non-conductive
solid materials can be neutralised. It is particularly useful for discharging plastic sheets or films.
Methods that may be employed include those given in 4.4.7.2 to 4.4.7.4. Neutralisation cannot
succeed if the rate at which charge is generated exceeds the rate at which ions are supplied to the air
or if ions of the wrong polarity are present. Correct installation and regular maintenance is, therefore,
essential for those devices.
4.4.7.2 Passive ionisers
Pointed electrodes such as earthed sharp needles, fine wires or conductive tinsel produce corona
discharges when placed in the electric field from a charged body. These provide ions which neutralize
the charge on the body. This method, however, is limited in its effectiveness (see A.3.3) and may, in
addition, produce discharges. Therefore, it should not be used in zone 0 and should not be used in
zone 1 as the only safety measure.

4.4.7.3 Active ionisers
A more efficient method of producing ions is to apply a high voltage to a number of corona points.
Commercial systems commonly use alternating voltages in the range 5 kV to 10 kV supplied to a row
of points. The currents from the corona points are limited either by high resistance or capacitive
coupling. Active ionisers should not be used in zone 0 and should not be used in zone 1 as the only
safety measure.
4.4.7.4 Radioactive sources
Radioactive sources ionise the surrounding air and can be used to dissipate the charges from a
charged body. Radioactive ionisation itself does not present an ignition hazard; however, it is limited in
its effectiveness and should not be used in zone 0 and 1 as the only safety measure.

4.4.7.5 Ionised air blowers
Ionised air blowers using either high voltages or radioactive sources are used mainly for dissipating
charges from awkwardly shaped objects. However, the ion concentration can rapidly decrease
downstream due to recombination or adsorption of the ions by the walls. As a result, it is difficult to
convey the ionised air over large distances. Unless it is approved for use in a hazardous area, the
parts containing the high voltages should be placed outside. This method of protection should not be
used as the only safety measure in zone 0.

4.4.8 Antistatic agents
Antistatic agents are frequently used on clothing and floors and to increase the conductivity of liquids
and materials. Care must be taken to guarantee the presence of a sufficient concentration of these
agents. For example, antistatic agents may become diluted or washed out. Therefore, their
effectiveness needs to be monitored and maintained.

4.4.9 Avoidance of propagating brush discharges

High or repeated electrostatic charging processes acting on non-conductive layers or coatings can lead
to propagating brush discharges (see B.3.9.). These discharges can be prevented by the following
measures:
(a) avoid having thin non-conductive coatings on metals or other conductive materials. Propagating
brush discharges tend to occur with thin coatings; they can normally be prevented by having
thicknesses greater than about 10 mm;

(b) increase the surface or volume conductivity of the coating. It is not known exactly what value of
surface resistance will prevent the occurrence of propagating brush discharges but the values of
surface resistances quoted in 4.2 and a leakage resistance less than 10 Ω are sufficiently low;

(c) use a coating with a low dielectric strength (breakdown voltage less than 4 kV, see A.3.5) instead
of one with a high dielectric strength. Coatings with a low dielectric strength tend to electrically
break down before a propagating brush discharge can develop. Due to their slight porosity, layers
of paint usually show a low breakdown voltage so that propagating brush discharges are difficult
to obtain from such layers.
NOTE  Polymer films which are wound on to a reel or are lifted from a conductive or non-conductive surface can acquire
bipolar charges i.e. equal and opposite charges on the two surface of the film. This can lead to brush discharges and
occasionally even to propagating brush discharges.

4.5 Conveyor belts and transmission belts

4.5.1 General
Due to the continuous separation of the contacting surfaces, e.g. a driving shaft and a belt, the moving
surface can acquire a considerable amount of charge and become an ignition hazard. The amount of
charge acquired depends on the material of the conveyor belt as well as the materials of the driving
shaft and the rollers. It will increase with the velocity and tension of the belt and the width of the area
of contact.
4.5.2 Conveyor belts
Conveyor belts are endless belts which run over rollers and transport materials. Usually the conveyor
belt is made of non-conductive material whereas the driving shaft and the rollers are made of metal.

The charge acquired by the belt can only be safely dissipated to earth via the earthed dissipative rollers
if the conveyor belt is sufficiently dissipative (see 4.5.3).

4.5.3 Conductivity criteria for conveyor belts

A belt is considered to be dissipative if the surface resistances on both sides of the belt are below 3 x
10 Ω. In cases where the belt consists of layers of different materials it is considered to be dissipative
if the resistance through it does not exceed 10 Ω (resistance measured at 23 °C and 50 % relative
humidity). Care should be taken to ensure that repairs do not increase the values given.

4.5.4 Conditions of use for conveyor belts

In zone 0, conveyor belts which meet the criteria of 4.5.3 may be used providing the belt velocity is
restricted to 0,5 m/s and belt connectors are not used.

In zone 1, explosion group IIC, the requirements for zone 0 apply.

In zone 1, explosion groups IIA and IIB, and zones 20 and 21, conveyor belts may be used if the belt
velocity is restricted to 5 m/s; belt connectors are permitted. If the belt velocity exceeds 5 m/s the
criteria for transmission belts apply (see 4.5.6).

In zone 2 and 22 protective measures are not necessary unless experience shows that frequent
discharges occur.
4.5.5 Transmission belts
Transmission belts are V-belts and flat belts which drive rotating parts or machines. Sometimes the
belt materials are non-conductive whereas the pulleys are normally of metal. The amount of charge
acquired by the belt due to the continuous separation of the contacting surfaces depends on the
material of the belt and pulleys and increases with the velocity and tension of the belt and the width of
the contact area.
- 13 - CLC/TR 50404:2003
4.5.6 Conductivity criteria for transmission belts

The belt material is sufficiently dissipative if:

R x B ≤ 10 Ωm
where R is the resistance measured at the inner side of the mounted transmission belt between an
electrode halfway between the two pulleys and earth and B is the width of the flat belt or double the
width of the side face of the V-belt.

In cases where the belt consists of layers of different materials the belt is considered to be dissipative if
the resistance across it does not exceed 10 Ω (resistance measured at 23 °C and 50 % relative
humidity). Care should be taken to ensure that repairs do not increase the value given.

4.5.7 Conditions of use for transmission belts

In zone 0, transmission belts shall not be used.

In zone 1, explosion group IIC, transmission belts shall not be used.

In zone 1, explosion groups IIA and IIB and in zone 21, transmission belts may be used if the following
criteria are met:
(a) the belt velocity does not exceed 30 m/s (no information is available for higher velocities);

(b) the belt material meets the conductivity criteria defined in 4.5.6;

(c) the belt is earthed via conductive pulleys.

In zone 20, with dusts having a minimum ignition energy greater than 10 mJ, the requirements for
zone 21 apply.
In zone 20 with dusts having a minimum ignition energy of less than 10 mJ, transmission belts shall not
be used unless the ignition or explosion hazards are dealt with by alternative means.

A layer of wax or dirt on the belt could increase the adhesiveness of the belt and also its resistance and
this could increase the charging hazard. It is essential that layers of non-conductive adhesives used to
connect the belt do not interrupt the conductive path. Belt connectors should not be used.

For low speed transmission belts, the criteria given in 4.5.4 for conveyor belts can be used.

5 Static electricity in liquids

5.1 General considerations
The following requirements apply to common flammable liquids, such as organic solvents,
hydrocarbons, fuels and alcohols. Additional precautions may be required where the atmosphere
above the liquid is very sensitive to ignition, e.g. carbon disulphide.

Liquids can become electrostatically charged when there is relative movement between the liquid and
adjacent solids or there is a second immiscible phase. Spraying of liquids can also create a highly
charged mist or spray. Further details of charge generation and charge accumulation in liquids are
given in A.1.3 and A.2.2. The level of charge accumulation in a particular liquid (and therefore the
electrostatic hazard that can be created) is strongly dependent upon the electrical conductivity of the
liquid. To describe the possible hazards and associated means of prevention the conductivities of
liquids have been classified as follows:

- high conductivity > 1 000 pS/m;

- medium conductivity  between 50 pS/m and 1 000 pS/m;

- low conductivity < 50 pS/m.

In general, hazardous levels of charge accumulation are associated with liquids of low conductivity.
However, exceptions include processes that produce mists or sprays, high flow velocities of medium
conductivity liquids in non conductive pipes and two-phase mixing operations.

The conductivities and relaxation times for a number of liquids are given in Table 2.

NOTE  For further details on the mechanisms of charging in liquids see Annex A.

Table 2 - Conductivities and relaxation times of some liquids

Liquid Conductivity Relaxation time
pS/m s
Low conductivity
Highly purified paraffins -2 2 000
Typical paraffins -1 2 – 200
10 – 10
Purified aromatic compounds -1 2 – 200
10 – 10
(toluene, xylene etc.)
Typical aromatic compounds 5 – 50 0,4 – 4
Gasoline -1 2 0,2 – 200
10 – 10
Kerosene -1 0,4 – 200
10 – 50
Gas oil 2 0,2 – 20
1 – 10
White oils -1 2 0,2 – 200
10 – 10
Lubricating oils -2 3
...