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Managing Electrostatic

Discharges (ESD) in the

Manufacture of Explosives

Part 1: Technical Guide

by

SAFEX GEP Workgroup

SAFEX Good Explosives Practice Series GPG 03(1) Rev 1

SA

FE

X I

nternatio

nal

SAFEX Good Explosives Practice Series

Good Explosives Practice (GEP) is a generic term used to describe a

method of working with explosives. GEP ensures that explosives

hazards are understood and practical measures applied to reduce:

• Likelihood of initiation; and

• Potential consequences of an initiation if it occurs.

The use of the term “Best Explosives Practice” has been avoided for a

number of reasons such as:

• What is best for one explosive may not be best for another

• There may be financial or process constraints as to the practices

that can be implemented

• It is often difficult to get people to agree on an absolute best

The GEP initiative is an attempt to capture the expertise that is still

available in our business. This expertise is under threat due to

influences such as the transfer of classical processes to newer

technologies; the concentration of companies inside the explosive

business; the retirement of experienced colleagues grounded in the

fundamentals of explosives; and shorter time explosives practitioners

have available to acquire an appreciation of the basis of safety in

their operations.

To assess hazards and risks systematically without such a basic

understanding of explosives practices is almost impossible. Hopefully

the GEP approach will grow over time to a database of good practices

in our business and address this need

GEP’s can be regarded as the recommended practices the explosives

industry follows. They can be captured in a number of ways:

Standard Operating Procedures, Statutes, Maintenance Manuals,

Operating Manuals, etc. A Good Practice Guide (GPG) is one of the

ways SAFEX has elected to document some of the essentials that

result in GEP. GPG is synonymous with a “Code of Practice”. The

latter may suggest everyone concerned has signed-off on it which is

not necessarily the case with a GPG.

Managing Electrostatic Discharges (ESD) in the Manufacture of Explosives

Part 1: Technical Guide

by

SAFEX Good Explosives Practice Workgroup

SAFEX Good Explosives Practice Series GPG 03(1) Rev 1

Published: September 2012

This Revision supersedes the previous version, GPG 03(1). The previous version is not recommended for ESD protected area

design and certification The information in this publication is based on the authors’ experience and general industry practices at the time of publication. The views expressed are those of the authors and do not necessarily represent the official position of SAFEX International. The publication should be read in conjunction with regulatory and statutory requirements as applicable. All recommendations contained in this publication are made without guarantee. The author and SAFEX International cannot accept any liability for consequences arising (whether directly or indirectly) from the use of such advice.

i

CONTENTS

GLOSSARY OF ACRONYMS AND ABBREVIATIONS 1

1 INTRODUCTION 2

1.1 What is static electricity? 2

1.2 What causes static electricity? 2 1.2.1 Contact-induced charge separation (Triboelectric charging) 3 1.2.2 Flow induced charge separation 4 1.2.3 Pressure-induced charge separation (Piezoelectric charging) 4 1.2.4 Heat-induced charge separation (Pyroelectric charging) 4 1.2.5 Charge-induced charge separation (Electrostatic induction) 4 1.2.6 Corona discharge 4

2 ELECTROSTATIC DISCHARGE (ESD) 5

2.1 Causes of ESD 5

2.2 Types of ESD 5 2.2.1 Corona discharge 6 2.2.2 Brush discharge (insulator discharge) 8 2.2.3 Bulking brush discharge (cone discharge) 9 2.2.4 Spark or capacitor discharge 10 2.2.5 Propagating brush discharge 11 2.2.6 Lightning-like discharge 13

2.3 Conditions for Ignition by ESD 13

3 ESD PREVENTION METHODS 13

3.1 General measures 15

3.2 Bonding and Grounding 16

3.3 Electrostatic Discharge (ESD) Process Controls 16

3.4 Management of ESD-protected areas 18 3.4.1 Standards for the explosives industry 19 3.4.2 Static dissipative work surfaces 22 3.4.3 Tools, instruments, materials and controls 23 3.4.4 Maintenance matrix for an ESD-protected area: 27

3.5 Electrostatic Discharge (ESD) Operator Control 28

3.6 Electrostatic Discharge (ESD) Risk Analysis 30

4 CONCLUSION 31

ii

5 ACKNOWLEDGMENTS 32

6 REFERENCES 32

7 ANNEXURES 33

Annexure 1: The Triboelectric Series 33

Annexure 2: Conductivity Testing Schedule 35

Annexure 3: MIE of Various Explosives 36

Annexure 4: MIE of Various Solvents 40

Annexure 5: Electrostatic Notions 42 Electrostatic Charging Modes 42 Accumulation and dissipation of charges 43 Breakdown 43 Discharge energy 44 Capacitance 44

Annexure 6: Comparison of MIE and ESD Energy 46

Annexure 7: MIE vs Discharge Mode 47

Annexure 8: Nomogram to Assess Ignition Risk with Capacitive Discharge 48 The Nomogram 49 Alternate nomogram 51

Annexure 9: ESD Risk Analysis Table 53

Managing ESD in Explosives Manufacture – Part 1: Technical Guide Rev.1 1

GLOSSARY of ACRONYMS and ABBREVIATIONS ANSI American National Standards Institute BS IEC British Standards Institution issue of an IEC standard cm centimetre dia diameter DA PAM USA Department of the Army Pamphlet DMM Digital multimeters DoD USA Department of Defense e.g. For example Earth Another term for “Ground”, the word used in this guide EIDAS Explosives Incidents Database Advisory Service EPA ESD Protected Area EOS/ESD Electrical overstress and Electrostatic discharge Forum ESD Electrostatic discharge F farad GPG Good Practice Guide GEP Good explosives practice Hazop Hazard and Operability Study HSL UK Health and Safety Laboratory Hz Hertz IEC International Electrotechnical Commission (Geneva) INRS Institut National de la Recherche Scientifique (Paris) J joule kV kilovolt kΩ kilo-ohm l litre LCL Lower Concentration Limit m metre MIE Minimum Initiation Energy mJ millijoule mm millimetre MV megavolt MΩ megohm NASA-STD National Aeronautics and Space Administration Standard PAM Pamphlet RA Risk assessment Rev Revision RH Relative humidity UK United Kingdom US United States (of America) V volt VAC volts alternating current VDC volts direct current Ωm ohm-metre µm micrometre µJ microjoule


1 INTRODUCTION The GPG for managing electrostatic discharges (ESD) in the manufacture of explosives comprises two parts: a Technical Guide and an Operating Guide. This GEP, which has been prepared using materials provided by a number of explosives companies and independent experts, constitutes the Technical Guide. The Operating Guide will follow in due course. The GPG should be regarded as a review of currently applied good practices in the industry. This Guide is unlikely to be complete and readers are encouraged to record and share any additional topics they feel should be included. 1.1 What is static electricity? Static electricity refers to the build-up of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Although charge exchange can happen whenever any two surfaces come into contact and separate, a static charge will only remain when at least one of the surfaces has a high resistance to electrical flow (an electrical insulator). The effects of static electricity are familiar to most people. We can see, feel and even hear the spark as the excess charge is neutralized. This happens when a charged body is brought close to another body of opposite charge or grounded (e.g. a charged finger approaching a grounding plate). The familiar phenomenon of a static 'shock' is caused by the charge transfer for example, when combing one's hair or taking off a sweater. Through contact and separation between the comb and the hair, the comb loses some electrons while the hair gains some (or vice-versa) in a process termed “triboelectric” generation. This is described in Clause 1.2.1 below. 1.2 What causes static electricity? The materials we observe and interact with from day-to-day are formed from atoms and molecules that are electrically neutral. They have an equal number of positive charges (protons in the nucleus) and negative charges (electrons in shells surrounding the nucleus). Static electricity is produced when there is a loss or a gain of


charges between bodies (one loses charges while the other gains those charges). This can happen in a number of ways:

1.2.1 Contact-induced charge separation (Triboelectric charging)

See Triboelectric Material Chart in Annexure 1.

Electrons can be exchanged between materials on contact; materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells tend to gain them. It results in one material becoming positively charged and the other negatively charged. This triboelectric effect is the main cause of static electricity as observed in everyday life. Some of its characteristics are: • Contact and separation can be between the same material or

different materials. • Higher pressure or speed of separation increases the charge. • In mechanical systems, rougher surfaces generally reduce

contact area, slipping and consequent tribogeneration, but highly-polished surfaces are commonly used in explosives powder handling to promote free flow by minimizing friction and hence tribogeneration.

• In an insulator-conductor or insulator-insulator situation, the insulator prevents charges from moving on the surface. Hence in a contact-separation situation the charges remain at the point of contact in insulators and will distribute evenly in electrically-isolated conductors. During a discharge event, the insulator will transfer only the charges at the point of contact, or close enough to the point of contact for charges of sufficient potential to discharge across the air gap between insulator and conductor1. As the discharge only originates from a small portion of the area of the insulator surface, the discharge energy is limited. (The other charges on the insulative body remain in place). Conversely, a charged, electrically-isolated, conductor transfers all of its charges at the point of discharge resulting in much more energetic sparks.

1 The maximum permitted areas of insulators on IS-rated equipment are given in Table 4 of IEC 60079-0:2004 Electrical apparatus for explosive gas atmospheres. The hazards with very sensitive primary explosives are considered in Clause 2.2.1.


1.2.2 Flow induced charge separation Liquid flow will induce charges in liquids. If the liquid is conductive the charges will dissipate through the conductive, grounded piping. If the liquid is an insulator the charges will build up in the liquid not in contact with the pipe.

1.2.3 Pressure-induced charge separation (Piezoelectric charging)

Applied mechanical stress generates a separation of charge in certain types of crystals, ceramics and polymer molecules. For example, during powder pressing, a charge may develop as result of piezoelectric effect.

1.2.4 Heat-induced charge separation (Pyroelectric charging) Heating generates a separation of charge in the atoms or molecules of certain materials. All pyroelectric materials are also piezoelectric, although the converse is not always true. The atomic or molecular properties of heat and pressure response are closely related.

1.2.5 Charge-induced charge separation (Electrostatic induction)

This occurs when an electrically charged object is placed near a conductive object isolated from ground. The presence of the charged object creates an electric field that causes electrical charges on the surface of the other object to redistribute. Even though the net electrostatic charge of the system has not changed, the object now has regions of excess positive and negative charges.

1.2.6 Corona discharge A pointed electrode will concentrate the electric field at its tip if charged relative to a nearby object to the extent of creating an ionic wind or at higher charge, a luminous corona discharge surrounding the point. If the object is a grounded plate the charge will dissipate; if the object is not grounded it will accumulate charge.


2 ELECTROSTATIC DISCHARGE (ESD) Electrostatic discharge is the sudden and momentary electric current that flows between two objects at different electrical potentials. Conduction is caused by direct contact or induced by an electrostatic field causing a low-impedance ionized conduction path. 2.1 Causes of ESD A cause of ESD events is static electricity generated in one of the ways outlined above. An ESD event may occur when the charged object approaches or comes into contact with a conductive path. For static electricity to be a hazard, four conditions must be met: • There must be a means for a static charge to develop. • The charges have no path to ground zero and hence

accumulate. • Enough energy must build up to cause ignition. • There must be a discharge of the accumulated charge (a

spark). • The spark must occur in an ignitable gas, vapour or dust

mixture or sensitive explosive material. 2.2 Types of ESD The most spectacular form of ESD is the spark, which occurs when a strong electric field creates an ionized conductive channel in air. This can cause minor discomfort to people, severe damage to electronic equipment, and fires and explosions if the air contains combustible vapours or particles. However, many ESD events occur without a visible or audible spark. A person carrying a relatively small electric charge may not feel a discharge that is sufficient to damage exposed sensitive electronic components. A small discharge energy between 0.1 mJ and 0.3 mJ will ignite most flammable hydrocarbon vapour/air mixtures2 whilst some sensitive primary explosives may be initiated with discharges as low as 10 µJ. (See Annexure 3) The human body barefooted on a highly static generating carpet may, at suitably low humidity levels,

2 See Annexure 4 of this manual and Gexcon online Gas Explosion Handbook (GexCon AS, P.O. Box 6015, NO-5892 Bergen Norway) Clause 4.5. http://www.gexcon.com/handbook/GEXHBcontents.htm


be charged to 15 kV and will thus generate a spark discharge energy3 of almost 17 mJ.

When the Minimum Initiation Energy (MIE) and lower concentration limits or flammable limits are known, the energy leg of the fire triangle must be studied. Experimental MIE values for various types of explosives and solvents are listed in Annexures 3 and 4. This requires some knowledge of the types of discharge and their energy levels. The fire triangle or combustion triangle

depicted in Figure 1 is a simple model for understanding the ingredients necessary for most fires. A fire requires three elements: energy or ignition source, fuel, and an oxidizing agent (usually oxygen). The fire is prevented or extinguished by removing any one of them. A fire naturally occurs when the elements are combined in the right mixture. An explosive already combines fuel and oxidizer either on a molecular base or as a component mixture. There are six types of discharge:

• Corona discharge • Brush discharge • Bulking brush discharge (cone discharge) • Propagating brush discharge • Spark or capacitor discharge • Lightning-like discharge

2.2.1 Corona discharge 4 Corona discharge occurs when the field strength in a small region around the front of the pointed conductor in Figure 2 on the next page exceeds the breakdown strength of air5, typically 3 MV/m. For

3 IEC 61000-4-2 Electromagnetic compatibility (EMC) – Testing and measurement techniques – Electrostatic discharge immunity test. Clause A7 The equivalent capacitance of the human body is given as 150 pF. 4 Glor, Martin. 1988. Electrostatic Hazards in Powder Handling. Research Studies Press Ltd., Letchworth, Hertfodshire, UK. ISBN 047192024X 5 Rigden, John S. Macmillan Encyclopedia of Physics. Simon & Schuster, 1996: 353.

Figure 1: Fire triangle


a continuous discharge, ionization of the air around the tip is visible as a luminous “corona” in a darkened room. Further away from the electrode point the field strength decreases and conduction is via slow-moving ions and charged particles.

Figure 2: Corona discharge The energy density of the discharge in the corona region6 is 39.8 J/m3. Corona discharge energy is the product of the corona volume and energy density. The shape of the corona region around the point is typically a spherical segment. Corona discharge in flammable gas mixtures: Initiation energies of most common flammable gas/air or vapour/air mixtures are between 0.1 mJ and 0.3 mJ. There is no history of corona discharge being an ignition hazard with these mixtures, indicating that the discharge energy is <0.1 mJ. (Notable exceptions7 are hydrogen, acetylene and carbon disulphide in air where mixtures have initiation energies an order magnitude lower). Corona discharge and sensitive primary explosives: Annexure 3 gives the spark initiation energy of lead styphnate as low as 10 µJ, 6 The discharge energy density in air is given by: 0.5 x (3 MV/m)2 x (absolute air permittivity = 8.854x10-12 F/m) = 39.8J/m3 7 Gexcon online Gas Explosion Handbook (GexCon AS, P.O. Box 6015, NO-5892 Bergen Norway) Clause 4.5. http://www.gexcon.com/handbook/GEXHBcontents.htm


and thus it could be initiated by corona discharge. Other less-sensitive primaries should not be initiated by corona discharge. A “thistle” electrode is generally an effective and safe method of dissipating static charge, but for the most sensitive gas mixtures and explosives, use should be avoided and stringent measures to counter static build-up practised.

2.2.2 Brush discharge (insulator discharge) 8

Figure 3: Brush discharge If the electrode has a radius of curvature between 5 mm and 50 mm, brush discharge is more likely to occur9. It results when a charged insulating material is brought to the edge of a grounded piece of equipment (e.g. a charged polyethylene bag touching the edge of a mixing tub, a vent pipe, a rivet head, a cable, a casing edge or a fingertip). The brush discharge energy level is between 0.67 to 3.6 mJ as per the Glor study10.

8 Pratt, Thomas H. 2000. Electrostatic Ignitions of Fires and Explosions. Center for Chemical Process Safety (CCPS) on AICHE Industry technology alliance, New York, NY. ISBN 0816999481 9 Jones, Thomas B. and Jack L. King. 1992. Powder Handling and Electrostatics : Understanding and Preventing Hazards. Lewis Publishers Inc., Chelsea, MI. ISBN 0873714881 10 Glor, Martin. 1988. Electrostatic Hazards in Powder Handling. Research Studies Press Ltd., Letchworth, Hertfodshire, UK. ISBN 047192024X


2.2.3 Bulking brush discharge (cone discharge)11

Figure 4: Cone or bulk discharge Bulking brush discharge is the type of discharge observed on the cone of a bulked heap of powder. This discharge occurs when large containers are filled with powders that have:

- coarse particles (1 to 10 mm dia.) - high resistance (> 1010 Ω-m) - high charge to mass ratio (e.g. pneumatic conveying, or sliding

down chutes) - high filling rate

The energy level that can be reached is 10 mJ.

11 Glor, Martin. 1988. Electrostatic Hazards in Powder Handling. Research Studies Press Ltd., Letchworth, Hertfodshire, UK. ISBN 047192024X


Usually powders with sensitivity greater than 10 mJ will not be ignited by a cone discharge except if there is a hybrid mixture. A hybrid mixture is a combination of solvent vapours and dust. Such a mixture can create a situation where even if the lower concentration limit of the vapour is not reached the mixture can ignite. Hence hybrid mixtures must always be considered at risk with cone discharge even if the powder electrostatic sensitivity is greater than 10 mJ. INRS12 suggest cone discharge can attain 100 mJ which seems very high considering no ignition has been observed with powder having MIE's greater than 10 mJ.

2.2.4 Spark or capacitor discharge 13

Figure 5: Spark discharge

A Spark discharge occurs between two isolated conductive objects (people, products, machinery) when:

12 Berger, N., M. Derrozière, J.C. Bilet, D. Guionnet and H. Romat. 2004. Électricité statique. INRS, Paris, France 13 Glor, Martin. 1988. Electrostatic Hazards in Powder Handling. Research Studies Press Ltd., Letchworth, Hertfodshire, UK. ISBN 047192024X

capacitor or on the person

The discharge takes place when non-grounded


- one of them is charged to high potential and the other to a lower potential or to ground

- the distance between the conductors is not too large compared to the radii of curvature of their surfaces and

- the electric field between the conductors exceeds 3 MV/m, the dielectric strength of air (an insulator). Breakdown of the air then generates a plasma or spark discharge. The approach of a charged finger to a grounding plate is a good example of this phenomenon. As the distance between the finger and the grounding plate decreases, the electric field between the finger and the plate increases due to reduced distance. When the electric field exceeds 3 MV/m or 30 kV/cm, breakdown occurs in the form of a spark.

When a spark occurs, all the charges of one conductor are rapidly discharged onto the other conductor. It is unacceptable to continue operations where electrostatic sparks are known to occur around flammable atmospheres or in explosives environments whatever their energy content may be. Measures to minimize the risk of ESD are considered in Clause 3 of this manual.

2.2.5 Propagating brush discharge 14

This type of discharge is illustrated in Figure 6 on the next page and occurs when breakdown occurs in an insulating film that is applied against a grounded conductive surface (like an interior coated vessel or pipe).

Opposite polarity charge builds on the surface of the film and the surface of the conductor. When the breakdown voltage of the film is attained, all the charges on the conductor are precipitated at the breakdown zone creating a large discharge. A table of dielectric strengths of solids15 lists the values for bulk thermoplastic polymers ranging from polycarbonate at 15 kV/mm to polypropylene at 23.6 kV/mm. Insulating films can exhibit far higher dielectric

14 Pratt, Thomas H. 2000. Electrostatic Ignitions of Fires and Explosions. Center for Chemical Process Safety (CCPS) on AICHE Industry technology alliance, New York, NY. ISBN 0816999481 15 L. I. Berger. Dielectric strength of insulating materials. CRC Handbook of Chemistry and Physics, 2000 edition. CRC Press - 6000 NW Broken Sound Parkway, Suite 300, Boca Raton, FL 33487. Page 15-44, Table 3. Dielectric strength of solids.


strengths, for example, low-density polyethylene film 40 µm thick has a dielectric strength of 300 kV/mm or a breakdown voltage of 12 kV.

Figure 6: Propagating brush discharge The following are examples of propagating brush discharges: a) high velocity pneumatic transfer of powder through insulating

pipes or conductive pipe with insulating internal coating or grounding wire wrapped around a pipe made of insulating material

b) inspection windows made of glass or plexiglass in pneumatic transfer pipe

c) high velocity transportation of highly insulating liquids through insulating pipes or conductive pipe with insulating interior

d) continuous impact of powder particles onto an insulating surface (coated dust deflector particles in cyclone, filling of large containers or silos made from insulating material)

e) flexible intermediate bulk containers f) silos and containers internally coated with an insulating layer of

high dielectric strength.

Several joules of energy can be expected from a propagating brush discharge and for large object like silos it can reach 100 joules.


2.2.6 Lightning-like discharge Lightning-like discharges have been observed in dust clouds generated by volcanoes. Such discharges are not expected to occur in an industrial environment except in silos of a volume not less than 60 m3 or of not less than 3 m in diameter. It is more likely that a cone discharge would occur before a lightning-like discharge.

2.3 Conditions for Ignition by ESD The flow chart below (Figure 7) shows the conditions that have to be present for an ESD ignition to occur. This discharge energy then has to be higher than the minimum initiation energy (MIE) of the system/substance being considered. ESD controls have to break this chain using methods discussed in the following clauses.

Figure 7: Flowchart illustrating conditions necessary for an ignition by ESD 3 ESD PREVENTION METHODS ESD prevention methods depend on the type of discharge and can be summarized in Table 1 on the next page:

IGNITIONSTART

NoIgnition

NoNo

NoNoNo

Explosive Atmosphere or Dust

Yes ChargeGenerated

Yes ChargeAccumulate

Yes BreakDown

Yes DischargeEnergy

High Enough?

Yes


Table 1: ESD Prevention Methods 16

Discharge Type Typical Prevention Method

Spark

Bonding17 and grounding18 of all conductive equipment

Use of conductive shoes on conductive flooring that has been verified regularly (see Clause 3.4 and table in Annexure 2.)

Brush discharge

Prohibit insulating materials Use antistatic additives to make some

material conductive (e.g. graphite, conductive additives for highly resistive solvents, etc.)

Limit the speed of the process (e.g. extrusion speed of press for propellant)

Propagating brush discharge

Use conductive pipe (e.g. vacuum pipes) Limit usage of composite materials (e.g.

conductor-insulator which collects large charges like a capacitor)

Do not use insulating material having a breakdown voltage greater than 4 kV

Cone discharge

There isn't any practical method to prevent this type of discharge

Filling rate can play a role - charge accumulation increases with increase in speed

Conductivity plays a role - additives can reduce charge build-up

16 Berger, N., M. Derrozière, J.C. Bilet, D. Guionnet and H. Romat. 2004. Électricité statique. INRS, Paris, France 17 A Bonding system connects various pieces of conductive equipment together to keep them at the same potential. Static sparking cannot take place between objects that are the same potential 18 Grounding is a special form of bonding in which conductive equipment is connected to a grounding electrode or to the building grounding system in order to prevent sparking between conductive equipment and grounded structures.


3.1 General measures The following general ESD prevention measures can be considered:

a) Reducing contact and friction between the particles of the material or between the material and the equipment will reduce accumulation of charges. For example, in paper fabrication reduction in pressure in the calendar rolls or increased roughness will reduce contact between the product and the equipment.

b) Reduction in speed in a propellant press will reduce static generation in the propellant.

c) Grounding and bonding (see Clause 3.2 below) are necessary for all conductive and semi-conductive material surfaces or equipment and should be checked regularly. Typical applications include conductive floors, work surfaces / tables and conductive packaging. Examples of grounding for personnel include conductive wrist straps, footwear, gloves, clothing or hairnets

d) Additives can be added to leather, cardboard, rubber, textiles, plastics and liquids. Additives such as polyols, sulphur organic compounds or nitric and phosphoric compounds need humidity to render the article to have conductive properties. On the other hand, ionic conducting additives such as metals, graphite lamp black and organic semiconducting polymers do not require humidity to provide the article with conductive19, dissipative20 or anti-static21 properties.

e) When possible, relative humidity above 65% will reduce the risk of electrostatic discharge considerably.

f) Charge neutralization techniques may also be used. Thistles can be used on packaging as when plastic separators are used.

Forced air plasma or photo-ionizers may also be used. This will be for localized areas typically up to a meter from the ionizer. Their application is where controlled or increased charge neutralization is required, or areas where high humidity levels

19 In the realm of electrostatics, conductive refers to resistances of typically < 106 ohm. 20 Static dissipative typically covers the range 105 to 109 ohms. 21 Anti-static typically covers the range 109 to 1012 ohms. Materials with higher values are regarded as insulators.


are problematic. Depending on the application, explosion-proof ionizers may be required.

g) Gas inerting can be used in extreme conditions especially when the material such as solvents requires oxygen in order to ignite. As most explosives do not require oxygen to ignite, gas inerting is ineffective.

The efficiency of whichever prevention method is adopted must be established and documented at start-up. This must be verified periodically to ensure it has not degraded.

3.2 Bonding and Grounding

All metal parts are to be grounded and/or bonded. Some practitioners prefer grounding one piece of equipment and bonding the other equipment in series. Grounding straps can be grounded in the four corners of the room and go around the room. Grounding individual equipment is preferred because if one bonding strap is loose several pieces of equipment could end-up with faulty grounds if they are bonded to it. In severe conditions where solvent vapours or powders with high electrostatic sensitivity are involved, grounding equipment individually and bonding should be the standard. The maximum resistance to ground should be 2 ohms. See Annexure 2 of DA PAM 385-64 for grounding specs of different components and inspection frequency. ANSI/ESD 20.20:2007 uses a maximum ground resistance of 1 ohm. (See Clause 3.4, Table 2)

3.3 Electrostatic Discharge (ESD) Process Controls Clause 3.2 noted the importance of metal articles or equipment being properly grounded in order to prevent the generation of high-energy spark discharges. The following guidelines apply for processes in an ESD Protected Area (EPA):

a) As a general rule, hoses and belts should have a maximum electrical resistance of 250 kΩ. Table 2 in Clause 3.4.1 and Annexure 2 list other resistance limits to be observed in EPA’s.

b) Plastic guides and shields should be avoided because they accumulate charge. This will, in turn, create electrical fields that can displace charges on ESD sensitive articles, powders and conductors.


c) Plastic bags can generate a brush discharge originating from the contact of an insulating material and a grounded conductor with energy between 1.6 and 3.6 mJ. A hybrid mixture of dust and vapour clouds or explosives can have MIE‘s much lower than these values. In such circumstances plastic bags should be prohibited.

d) Anti-static plastic or conductive bags or equivalent should be used on any powder that has an electrostatic sensitivity less than 25 mJ.

e) Wheeled carriages, carts or tables should have conductive castors that make effective contact to a conductive floor. The table top of wheeled tables should also be covered with a conductive or static dissipative covering.

f) A grounding chain can also be considered for mobile equipment.

g) For highly sensitive ESD material or in the presence of solvent vapours in a poorly ventilated room, dissipative or anti-static plastic containers should be used. Grounded steel can promote brush discharges whereas static dissipative plastics drain charges in a more gentle way.

h) Spring loaded grounding straps may have to be installed on rotating shafts because of the poor conductivity of grease in the bearings.

i) The recommended relative humidity (RH) in the literature for ESD environments is 65%. However, as this may compromise some powders with moisture sensitivity, an RH of 55% can be considered. This may also be the case for older buildings. Use of explosion-proof ionizers may be considered if high relative humidity is not acceptable for the process.

j) If water atomizer or steam nozzles are used to humidify the room, the jet should not be directed towards the ESD sensitive powder or devices. If droplets form due to malfunction, they can carry a charge.

k) The triboelectric effect caused by contact and separation of the same or different materials generates electrostatic charges. It should be remembered that an increase in production rate, which increases the contact and separation of particles, would increase the charge generation. If charge dissipation or


drainage is not improved in those circumstances, the charges will accumulate and may discharge in several ways.

l) Non-conductive solvents such as Hexane can create particular problems because they don’t drain off electrical charges through the piping. Special additives can be added to increase conductivity of these solvents. In these cases the electrical characteristics of the solvents used should be verified.

m) Brooms and brushes used in EPA's are traditionally made of horse or hog hair or corn fibre; brushes should be horse or camel hair. Brushes made from antistatic synthetic materials may also be considered22. Dustpans should be of semi-conductive materials. Such dustpans are not readily available and may be of wood or vacuum formed from static-dissipative plastic sheets. Low-cost moulds for vacuum forming are made of wood or composites. Semi-conductive vacuum formable plastic sheets are readily available.

n) Ventilation systems blowing on exposed electrostatic sensitive powders can be a problem and should be avoided. In the case of forced air-drying, filtration of the air is of the outmost importance. Dust pollen will generate static in airflow. Use of explosion-protected ionizers to reduce static build up may be considered.

o) Compressed air blowing off electrostatic sensitive powders should be prohibited. The contact and separation of the powder particles may generate sufficient static for ignition. SAFEX Investigation Report IR812 (record # 16500 in the SAFEX-EIDAS Database) describes such an incident.

p) MIE’s and lower concentration limits (LCL) and lower and upper flammable limits of solvents should be known in order to determine what level of electrostatic discharge would ignite the product. See Annexures 3 and 4.

3.4 Management of ESD-protected areas Safe operation and maintenance of an ESD-protected area (EPA) is strongly dependent on the execution of a Risk Assessment (RA) or Hazop of the process and environment. Control measures and other 22 “How to choose the right anti-static brush”. Gordon Brush, 6247 Randolph Street, Commerce, CA 90040-3514. http://www.gordonbrush.com/news.php?articles=News Articles&newsid=21


parameters determined in the assessment should be implemented in the EPA. Reference to RA and Hazop procedures may be found in the following: • Clause 3.6 of this Guide where the Risk Assessment procedure

is briefly discussed. • Hazard and Operability Studies (HAZOP) Application Guide

BS IEC 61882:2001. • Review of hazard identification techniques - report HSL/2005/58

from the UK Health and Safety Laboratory.

Improved ESD management technology has to a large extent come about as a result of the requirements of the electronics industry: Here the continuous push towards smaller and ever denser electronic circuitry has placed very strict requirements on ESD management. This clause only looks at ESD controls that may be considered in explosives manufacture. ESD control parameters have been drawn primarily from US Defense and Aerospace explosives standards: No detailed codes of practice for commercial explosives manufacture were found.

3.4.1 Standards for the explosives industry The following standards and guidelines were referred to: • Safety standard for Explosives, Propellants and Pyrotechnics.

NASA-STD-8719.12 Change 2, 2011-12-12. • DoD Contractor’s Safety Manual for Ammunition & Explosives.

(United States) DoD 4145.26-M, March 13, 2008. • Ammunition and Explosives Safety Standards. (United States)

Department of the Army Pamphlet 385–64, 24 May 2011. Table 17-1 (see Annexure 2) gives grounding system inspection and test requirements.

• ANSI/ESD 20.20:2007 For the Development of an Electrostatic Discharge Control Program for – Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices) Note: The ANSI 20.20:2007 standard was included as it offers a good cross-reference of management and measurement techniques for EPA’s, but test limits are mostly not in line with explosives practice.


Table 2 below provides a summary of control parameters from the above standards.

Table 2: Some EPA control parameters

Tec

hn

ical

re

qu

irem

ent

Sta

nd

ard

&

refe

ren

ce

Imp

lem

enti

ng

p

roce

ss o

r m

eth

od

Tes

t m

eth

od

-st

and

ard

or

advi

sory

Rec

om

men

ded

ra

ng

e

Grounding or bonding systems

PAM 385-64 Table 17-1

& ESD 20.20 8.2 table 1

Ground electrode ground rods, loops etc.

ANSI EOS/ ESD

S 6.1 <25 Ω

PAM 385-64 Table 17-1

Equipment and machinery grounding

<2.0 Ω

ESD20.20 8.2 Table 1

Equipment grounding conductor

ANSI EOS/ESD

S 6.1 <1.0 Ω

Personnel ground

PAM 385-64 Table 17-1

Wrist strap to ground. See note i.

<1 MΩ

DoD 4145.26 Clause

C6.4.7.2

Wrist strap (with conductive cream, if necessary, opposite hand to ground)

>250kΩ to <1.2 MΩ

NASA-STD-8719.12 Clause 5.13.2.4

Wrist strap wearer’s hand to ground (Hand nearest wrist strap, no conductive cream)

<10 MΩ

PAM 385-64 Table 17-1

Conductive footwear or heel grounder on wearer.

>25 kΩ to <1 MΩ (See below for electrocution limits).

DoD 4145.26 Clause

C6.4.7.5.1

Total resistance of conductive footwear and floor. See note ii.

<1 MΩ

DoD 4145.26 Clause

C6.4.7.5.2

Minimum resistance to prevent electrocution

110 VAC 40 kΩ

220 VAC 75 kΩ


EPA (ESD protected area)

DoD 4145.26 Clause

C6.4.7.5.3 Work surface <1 MΩ

DoD 4145.26 Clause C6.6.1 Relative humidity >60%

DoD 4145.26 Clause C6.6.1

and ANSI/ESD

20.20 Table 3

Localized ionization systems CAUTION: See note iii.

ANSI EOS/ESD S3.1

<±50 V offset

ANSI/ESD 20.20 Table 3

Garment. See note iv.

ESD TR53 1 x 105 to 1 x 1011

Ω

Notes for Table 2: i. Where wrist straps with resistance <1 MΩ are unavailable, wrist

straps for the electronics industry with a resistance range of 0.8 – 1.2 MΩ, complying with ANSI/ESD 20.20, could be considered as replacements. Assess the risk of any deviations from recommended resistance values in Table 2.

ii. Flooring resistance in EPA’s should be included with personnel grounding resistance to ensure the maximum operator to ground resistance does not exceed 1 MΩ. At the low end of the resistance scale, the flooring should be considered as part of the minimum operator to ground resistance for protection against electrocution, as indicated in Table 2.

iii. Ionization systems (cautionary note) - Electrical blower ionizers will generally not be permitted inside hazardous areas. Intrinsically-safe alpha-particle ionizers or photo-ionizers could be considered for localized static control where relative humidity levels of over 60% are unacceptable for process, product or plant equipment. In the case of the alpha-particle ionizers, the radiation health aspects must be complied with and the risk assessed of possible radiation interaction with process materials.

iv. Garment conductivity: Cotton garments have excellent anti-static properties, particularly in high-humidity conditions. Checks on the anti-static properties are often regarded as unnecessary.

Where cotton garments may be unsuitable e.g. for clean process areas due to high particulate generation, anti-static synthetic garments may be considered. Proper controls are


essential to ensure these garments maintain their anti-static properties. ANSI/ESD 20.20 lists three levels of garment static control: • Static control garment <1011 Ω • Groundable static control garment <109 Ω • Groundable static control garment <107 Ω

Test method ESD TR53 is applicable.

3.4.2 Static dissipative work surfaces Static dissipative work surfaces are manufactured for the explosives industry23 and have been successfully used for some two decades in the manufacture of electronic delay detonators. The dissipative work surfaces were both for the protection of the exposed electronics and to minimize the risk of ignition of the extremely static-sensitive lead styphnate primary explosive used. No ignitions occurred in this period.

The dissipative work surface mats consist of a low resistivity lower layer, which forms a ground plane that is electrically grounded, and a higher resistivity upper layer for the work surface. Typical properties are: • Low tribogeneration properties and low charge accumulation. • Rapid charge dissipation and decay. • Good solvent resistance. • Heat resistant. • Abrasion resistant.

Static dissipative surfaces have a typical resistivity range from 105 to 109 ohm per square24. The relatively high ground resistance means that in the event of a discharge to the sensitive material on the work surface, most of the discharge energy will be dissipated in the resistive path of the mat, considerably reducing the energy of the air discharge. This results in a similar reduction in the possibility of electrostatic ignition of a primary explosive or damage to electronics, if present. Mats at the lower end of this resistivity range should meet

23Messrs Static Solutions, Inc., 43 Broad Street #A103, Hudson MA 01749 USA, manufacture rubber static dissipative work surfaces for explosives environments. 24 ANSI EOS/ESD S1.11-1993, EOS/ESD Association Standard for Protection of Electrostatic Discharge Susceptible Items - Surface Resistance Measurement of Static Dissipative Planar Materials.


the requirements of DoD 4145.26 Clause C6.4.7.5.3 for resistance to ground of <1 MΩ. As a general rule, use of mats with resistance to ground of > 1 MΩ should be treated with caution and the associated risks and benefits of use carefully assessed.

3.4.3 Tools, instruments, materials and controls a) The Static Field Meter is used to ensure the work area is

essentially static free. Picture 1 shows a typical hand-held unit. A grounded operator will hold the instrument 25 mm from a surface to indicate the potential in volts or kilovolts. Garments, furniture and work surfaces may be tested by rubbing (triboelectric generation) and the meter used to indicate if charge is generated and the rate at which it decays. A static field meter with an alarm may be used for monitoring critical areas.

b) A Surface Resistivity Meter is shown in Picture 2. It is a versatile hand-held instrument with electrodes on the underside. It is simply placed on the surface to be checked and the test button depressed. The wide range of the instrument will determine whether resistance values of, for example, a conductive floor, a dissipative work surface, anti-static garments or packaging, are within specified limits.

Picture 1: Static Field Meter held at a fixed distance from a charged surface will read the potential in kV. (Photo courtesy Monroe Electronics)

Picture 2: A surface resistivity meter is placed on the surface to be tested and reads the value in “ohms per square” (Photo courtesy Monroe Electronics


c) A Humidity and Temperature Recorder, as shown in Picture 3, can be used to monitor humidity and temperature in critical areas. Such an insrtument may also interface with a computer for long-term data logging. An advantage of electronic temperature and humidity sensors is their rapid response which may show inadequacies in air-conditioning humidity control missed by traditional slow-response ”horsehair” humidity recorders.

d) An ionizer performance tester is

shown in picture 4. Where ionizers are used for ESD control, it is important to monitor performance regularly as there is normally no direct indication of ionizer function. Such an instrument may also provide useful measurements in areas held at higher humidities without ionizers present.

Picture 4: Ionizer performance tester. (Picture courtesy of Monroe Electronics Inc.)

Picture 3: An example of an electronic temperature and humidity recorder. Transfer of temperature and humidity data to a computer is possible with this type of instrument. (Graphic courtesy of Extech Instruments Corporation)


e) Ground resistance measurement meter - Low ground resistance of fixtures and equipment in an EPA environment is most important. Purpose-designed resistance meters with a resolution of better than 0.1 Ω may be used to measure grounding resistance. As electrolytic effects or other stray currents may lead to erroneous readings using direct current measurement, alternating current is regarded as more reliable. The instrument shown in Picture 5 below measures at 820 Hz, a frequency anharmonically related to utility frequencies.

f) Personnel grounding testers – two versions are commonly

used in EPA’s to ensure acceptable grounding. The first version is a wall-mounted unit used to check footwear grounding by all entering the protected area, and the second version is used at a workstation by an operator to ensure that grounding via a wrist strap, say, is acceptable. The instruments should be procured to give go/no-go indications suited to the EPA resistance controls in place. A typical instrument is shown in Picture 6 above.

g) Digital multimeters (DMM) are an inexpensive and essential tool in maintaining the EPA and apart from regular checks on personnel grounding systems, as most can measure from 0.1 Ω to 40 MΩ, DMM’s are useful in locating stray voltages and leakage currents.

Picture 6: Personnel ground system tester. (Courtesy Monroe Electronics Inc.)

Picture 5: A ground resistance checking kit. (Courtesy Extech Instruments Corporation)


h) High-voltage insulation testers (sometimes referred to by the commercial name “Megger”) are used to ensure that isolation resistance is adequate and that insulator breakdown should not occur. It is an instrument commonly used to check plant wiring, cabling and instrument isolation. Depending on the model, test voltages ranging from 500 VDC to 5 kVDC may be selected.

i) Anti-static tools, handling and packaging materials: There are many products for handling and protection of ESD-sensitive devices for the electronics industry. Anti-static tools and containers for handling in-process products and for equipment repair are available. Anti-static packing tubes and plastic bags are widely used for stillage, storage and transport. Resistance values of <1011 Ω per square are typically quoted, achieved by surface vacuum metallization, other surface treatment or by inclusion of stable bulk additives25. If such resistance values are deemed too high for sensitive explosive materials, semi-conductive natural-fibre or graphite-loaded polymers may be considered. Wood is also regarded as a good anti-static container material.

j) Cleaning agents and methods: • Cleaning agents and methods used on ESD-protective items

(e.g., work surfaces and floor coverings) should not reduce the effectiveness of these items. They should not cause leaching or leave insulating or conductive residues. (Many household cleaning agents contain silicones for improved appearance and may deposit an insulating layer.)

• In addition to other required properties (e.g. solvency) cleaning agents used on ESD sensitive items should be chosen for a low electrostatic charging property.

• Natural fibre materials are normally used for cleaning ESD sensitive items. Specialized synthetic cleaning materials are available for EPA’s26 where, for example, ultra-clean areas or process intolerance to cellulose are considerations.

25 “Hostastat” manufactured by Clariant International Ltd. Pigments & Additives Division, Marketing Plastic Business, Rothausstrasse 61, 4132 Muttenz, Switzerland. http://www.pa.clariant.com 26 ITW Texwipe, 1210 South Park Drive, Kernersville N.C. 27284, U.S.A.


k) Labelling of static sensitive materials or devices: Two symbols, shown in Picture 7, are frequently used to indicate ESD sensitivity and are commonly used in work areas or on packaging.

3.4.4 Maintenance matrix for an ESD-protected area: Table 3 below gives an example of the periodic control measures that might be required to maintain an EPA. Such a matrix should be developed during the risk assessment and design phase of the EPA facility prior to establishment of the manufacturing process. Table 3: A sample inspection matrix for certification and periodic inspection

of an EPA. (The inspection periods are illustrative only).

Par

amet

er

Init

ial

Cer

tifi

cate

Co

nti

nu

ou

s

Eac

h s

hif

t

Mo

nth

ly

Six

mo

nth

ly

An

nu

ally

Facility grounding and bonding X X

Floor grounding X

Work surface grounding X X

Picture 7: Symbols used to indicate ESD-sensitive devices, operations or materials. The text is adapted to address the particular hazard and may thus be customized for explosives manufacture.


Par

amet

er

Init

ial

Cer

tifi

cate

Co

nti

nu

ou

s

Eac

h s

hif

t

Mo

nth

ly

Six

mo

nth

ly

An

nu

ally

Work surface resistivity X X

Static charge dissipation X X

Wrist/heel grounder strap resistance

X X

Tool and equipment grounding X X

Stool/chair grounding X X

Facility entry doors/curtains X X

Temperature and humidity X X

Static test equipment certification X X

Monitoring static levels X X

Air ionizer effectiveness X X

Soldering iron tip & other tool ground

X X

Cart/wagon/tram grounding X X

Personnel garments X X

Conveyor belting X

3.5 Electrostatic Discharge (ESD) Operator Control As we have already seen, people can also build up an electrostatic charge. Therefore, it is equally important to take the necessary precautions in minimising the risks associated with operating personnel. The following are some of the practices that should be considered for people operating in an EPA. Operators should: a) Not remove overcoats in the workroom.


b) Touch a grounding plate before entering an EPA. It should be remembered that this precaution is only a momentary one as charge can be collected on further walking.

c) Not comb their hair in an EPA. d) Wear cotton or high cotton under-garments. (There is, however,

debate on whether synthetic under-garments might be an ESD hazard).

e) Wear cotton or anti-static work clothes. See note iv in Clause 3.4.1 on the controls for anti-static garments. Where there is a flame or fire hazard, use garments treated with a flame retardant. (Nomex / Nomostat or equivalent).

Besides these precautions, other ways of controlling operator electrostatic charge generation include: a) Conductive wrist straps. b) Conductive shoes, shoe covers, heel grounders and legstats. For maximum resistance of these devices see Table 2, Clause 3.4.1. and Annexure 2.

Care should be taken to wear the wrist straps, legstats and heel grounders correctly ensuring that they are well adjusted to make maximum skin contact. Testing of these items should be performed with frequency in accordance with Table 3, Clause 3.4 and values may be logged according to plant protocol. Out of specification devices must be replaced immediately. Dry foot skin condition can hinder good conductivity. Orthotic inserts for shoes may also increase resistance. Conductive socks, some with silver threads, will improve conductivity of some operators. Some specialized creams will also help but care should be taken to choose the proper cream since many hydrating products contain ingredients that increase the electrical resistance of the skin.

Conductive floors and conductive shoe soles should be kept clean to ensure good electrical continuity. This implies that if an operator goes outside, he should change shoes, wear rubber galoshes or clean the shoe soles with a tack mat or brushes before re-entering the EPA. Binders in priming compositions are notorious for dirtying shoe soles especially those made of porous rubber. 3M static control surface mark remover 8001 or equivalent can be used to maintain the cleanliness shoe soles as well as table tops.


The DoD Contractor’s safety manual27 states that contractors shall use conductive table tops, floors and shoes for grounding personnel at operations involving exposed explosives with electrostatic sensitivity of 0.1 J or less. In addition, in clause C6.4.7.5.1, it is noted that the footwear and floor ground resistance should not exceed 1 MΩ. If the shoe or heel grounder or legstat is 500 kΩ then the floor resistance may not exceed 500 kΩ. The floor grounding resistance should be measured at least once every 6 months.

To prevent electrocution the minimum floor resistance plus operator shoe or ground resistance should not be lower than 40 kΩ for 110 VAC and 75 kΩ for 220 VAC utilities. (DoD Contractor’s safety manual, clause C6.4.7.5.2). Correspondingly higher values of minimum resistance should be used for higher utility voltages.

3.6 Electrostatic Discharge (ESD) Risk Analysis An ESD risk analysis requires knowledge of the different types of discharge and their associated energy levels as well as the MIE's of the powders, solvents or hybrid mixtures involved. Such an ESD risk analysis will determine the type of discharge that will ignite the process and the measures necessary to prevent such an ignition. The ESD Risk Analysis table in Annexure 9 is a tool for the risk analysis process, which entails the following steps: a) List the equipment or processes; b) For each equipment or process list schematic reference

numbers or process steps; c) List minimum ignition energies (MIE) of the dangerous

atmosphere or the electrostatic sensitive powders or devices involved;

d) Identify the probable discharge modes that apply to the process step;

e) Compare the MIE with the discharge mode energy and decide whether there is ignition risk;

f) If there is ignition risk, identify the preventive measures. g) Perform measurement of process conditions.

27DoD Contractor’s Safety Manual For Ammunition & Explosives. (United States) DoD 4145.26-M, March 13, 2008. Clause 6.4.7.


Most explosives manufacturers will have a risk assessment process making use of a risk matrix for determining risk acceptability. The risk assessment on a particular process will determine the likelihood and consequences of an event. When these are checked in the risk matrix, the box that the process risk occupies will provide a risk rating. If the rating for a process is in a red box, the risk is unacceptable and mitigation is necessary. A process with risk in a green box in the matrix is acceptable.

4 CONCLUSION This Guide intends to provide, to more senior levels of factory staff, an understanding of the fundamentals of static charge generation and the issues surrounding electrostatic discharge. The application of this knowledge to the management of ESD in explosives manufacturing facilities is covered in chapter 3.

Practical methods of reducing ESD hazards are covered in this chapter, together with the importance of good grounding, process controls, tools and measures for managing ESD protected areas, operator control measures and risk analysis of ESD.

With the information given it should not only be possible to apply the principles of ESD management in explosives manufacture, but to provide a basic understanding of the subject and necessary control measures to operators on the line.

Figure 8: A typical risk matrix


5 ACKNOWLEDGMENTS SAFEX acknowledges with thanks: • The SAFEX GEP Workgroup for their initiative and ground work

in the preparation of this Guide. Maurice Bourgeois from General Dynamics OTS (Canada) is a member of the Workgroup and was responsible for large parts of this publication.

• Dr. Stafford Smithies (Expert Panel) of Victor Solomon & Associates, South Africa for preparing Clause 3.4 and revising sections of the text, as well as for his professional advice on the practical implementation of these concepts in an explosives environment.

• Ron J. Zezulka of TB&S Consultants, Canada who is a specialist in the field, for acting as technical advisor in the preparation of this Guide. In addition, he has generously offered to share his vast experience in the development of an Operations Guide for explosives manufacturing personnel as a supplement to this Technical Guide.

6 REFERENCES References have been added as footnotes at the relevant point in the text for ease of reading.


7 ANNEXURES Annexure 1: The Triboelectric Series 28 The triboelectric series is a list that ranks various materials according to their tendency to gain or lose electrons. It usually lists materials in order of decreasing tendency to charge positively (lose electrons), and increasing tendency to charge negatively (gain electrons). Somewhere in the middle of the list are materials that do not show strong tendency to behave either way. Note that the tendency of a material to become positive or negative after triboelectric charging has nothing to do with the level of conductivity (or ability to discharge) of the material. Due to complexities in experiments that involve controlled charging of materials, different researchers sometimes get different results in determining the rank of a material in the triboelectric series. One of the reasons for this is the multitude of factors and conditions that affect a material's tendency to charge. The triboelectric series in the table below extends over two pages and is a collation of several widely-used triboelectric series published on the web.

Most Positive (+) Air

+

Human Hands, Skin Asbestos Rabbit Fur

Glass Human Hair

Mica Nylon Wool Lead

Cat Fur Silk

Aluminum Paper

28 Copyright © www.SiliconFarEast.com by courtesy of Silicon Far East


Cotton

Steel

-

Wood Lucite

Sealing Wax Amber

Rubber Balloon Hard Rubber

Mylar Nickel

Copper Silver

uv Resist Brass

Synthetic Rubber Gold, Platinum

Sulfur Acetate, Rayon

Polyester Celluloid

Polystyrene Orlon, Acrylic

Cellophane Tape Polyvinylidene chloride (Saran)

Polyurethane Polyethylene

Polypropylene Polyvinylchloride (Vinyl)

Kel-F (PCTFE) Silicon Teflon

Silicone Rubber Most Negative (-)


Annexure 2: Conductivity Testing Schedule 29

29 Department of the Army Pamphlet 385-64. 2011. NFPA77 Recommended Practice on static electricity. Ammunition and explosives safety standards, Washington DC


Annexure 3: MIE of Various Explosives 30 The MIE’s quoted in this table are approximate and should be used as a guide only Source of data – column 4:

30 Paquet, Mario. 2005. Technical Guidelines Electrostatic Propellant Explosives. Expro Tech., Montreal, Canada

(1) US Bureau of Mines (2) Report S (G) 1 Birmingham University, Dr E.G.Cox

(3) Ministry of Supply, UK (4) A.R.D. Explosives Report 411/43, UK


Propellant explosives


(Continued)


(Capacitance 500 pF and Gap Setting 0.005”)


Annexure 4: MIE of Various Solvents 31 LMIE = lowest minimum ignition energy measured at optimum concentration Stoi = stoichiometric concentration Opt = optimum concentration at which LMIE was measured LFL = lower flammable limit UFL = upper flammable limit Su = fundamental burning velocity LOC = limiting oxygen concentration of fuel-oxygen-nitrogen mixture n/a = not applicable

Gas/Vapour LMIE= G(mJ)

Stol (vol%)

Opt=Ca (vol%)

LFL (vol%)

UFL (vol%)

Su (cm/s)

LOC in N2 (vol%)

Furan 0.22 4.44 2.3 14.3

n-Heptane 0.24 1.87 3.4 1.05 6.7 46 11.5

n-Hextane 0.24 2.16 3.8 1.1 7.5 46 12

n-Hextane (in O2) 0.006 9.25 1.2 52* n/a

Hydrogen 0.016 29.5 28 5+-1 75 312 5

Hydrogen (in O2) 0.0012 66.7 4.0 94 1400 n/a

Hydrogen (in N0) 8.7 50.0 n/a

Hydrogen sulfide 0.068 4.0 44 7.5

Isooctane 1.35 1.65 0.95 6.0 41

Isopropyl alcohol 0.65 4.44 2.0 12.7 41

Isopropyl chloride 1.08 2.8 10.7

Isopropyl amine 2.0 31

Methane 0.21 9.47 8.5 5.0 15.0 40 12

Methane (in O2) 0.0027 33.3 23 5.1 61 450 n/a

Methane (in NO) 8.7 n/a

Methanol 0.14 12.24 14.7 6.0 36.0 56 10

31 Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 p.226


Methyl acetylene 0.11 4.98 6.5 1.7

Methyl acrylate 2.8 25 8.7

Methylene chloride >1000 14 22 19

Methylene chloride (in O2)

0.137 11.7 68 n/a

Methyl ethyl ketone 0.53 3.66 5.3 2.0 12.0 11

Methyl butane (Isopentane)

0.21 2.55 3.8 1.4 7.6 43 12

Methyl cyclohexane 0.27 1.96 3.5 1.2 6.7 44

Methyl formate 0.4 4.5 23

n-Pentane 0.28 2.55 3.3 1.5 7.8 46 12

2-Methyl pentene 0.18 2.72 4.4 47

Propane 0.25 4.02 5.2 2.1 9.5 46 11.5

Propane (in O2) 0.0021 16.7 15 n/a

Propionaldehyde 0.32 2.6 17 58

n-Propyl chloride 1.08 2.6 11.1

Propylene 0.28 4.45 2.0 11.0 52 11.5

Propylene oxide 0.13 4.98 7.5 2.3 36.0 82 7.8

Isopropyl mercaptan 0.53

Styrene 2.05 0.9 6.8 9.0

Tetrahydrofuran 0.54 2.0 11.8

Tetrahydropyran 0.22 4.7 48

Thiophene 0.39

Toluene 0.24 2.27 4.1 1.1 7.1 41 9.5

Trichloroethane 7.5 12.5 14

Trichloroethane (in O2)

0.092 5.5* 57* n/a

Trichlorosilane 0.017 7.0 83

Triethylamine 0.75 2.10 1.2 8.0

Vinyl acetate 0.7 4.45 2.6 13.4

Vinyl acetylene 0.082 1.7 100 0

Xylene (s) 0.2 1.96 1.0 7.0


Note 1: It is possible that the fuel concentration most easily ignited by capacitive spark discharge, as reported in this table, might differ from that most easily ignited by other types of static discharged (e.g. brush discharge) Note 2: LFL and UFL values in air taken primarily from NFPA 325 Note 3: LMIE, LFL and UFL values in oxygen taken primarily from NFPA 53(*denotes elevated temperature). Note 4: Su values taken from [223]. Note 5: LOC values taken primarily from NFPA 69.

Annexure 5: Electrostatic Notions

Electrostatic Charging Modes

Tribocharging

Contact and separation between same materials or different materials.

The higher the pressure or the speed of separation the higher the charging.

The rougher the surface the lower the charging.

In an insulation-conductor or insulator-insulator situation, the insulator prevents charges from moving on the surface, hence in a contact-separation situation the charges lay still even if two surfaces are in contact therefore there is a high charge build-up; on the other hand, when two conductors are involved, the electrons will move toward the other object with positive charge through the last point of contact, this explains why conductors pick up less charges in a tribocharging situation

Induction

Electric fields will displace charges in bodies under the influence of the field, for example a positively charged object can displace negative charges on a neutrally charged object

Piezo

Charges can be generated under the effect of pressure on a body, for example, during powder pressing, a charge can develop from piezo effect


Charge transfer

A charged body can transfer its charge to another body simply by contact; For example, an operator handling negatively charged plastic trays can become negatively charged

Flow charging

Liquid flow will induce charges in liquids; if the liquid is conductive, the charges will dissipate through the grounded piping; if the liquid is an insulator the charges will build up

Corona charging

A needle (1/8 inch or less) will concentrate charges at its tip if charged by a source to the extent of creating an ionic wind or corona discharge onto a nearby object. If the object is a grounded plate the charge will dissipate; if the object is not grounded it will accumulate the charges

Accumulation and dissipation of charges Charges can accumulate on bodies as per the charging modes described above. There are essentially two strategies one can adopt: Reduce the charging by use of conductive materials, reduced speed of contact-separation, reduced flow, etc. Promote dissipation with high relative humidification (55% RH min, 65% RH preferred), grounding, bonding, use of conductive packaging, ionizers (static and forced air) conductive shoes, conductive floors, conductive wheels for carts, conductive leg and wrist straps. Use conductive additives in low conductivity liquids.

Breakdown Breakdown is tested with two oppositely charged plates with the insulating material to be tested between the plates (like an electric capacitor). As the voltage differential between the plates is raised, the electric field on the insulating material is raised. Opposite charges then accumulate on each face in contact with the plates. When the voltage reaches the breakdown voltage of the insulating material a spark (plasma) occurs. This results in one plate


discharging to the other thereby reducing voltage differences and charge accumulation on both plates. Each insulating material and gas can withstand a voltage up to a certain level above which the arc forms. This voltage is called breakdown voltage. The distance between the plates makes a difference. The greater the distance, the higher the voltage must be to generate an arc. Therefore, the electric field and dielectric strength is expressed in MV/m or kV/cm. For example, the dielectric strength for air is 30 kV/cm

Discharge energy Discharge energy equation for a capacitive discharge is W = ½ CV2

where: W : Energy in joules C : Capacitance in farads V : volts

Capacitance The capacitance is the amount of charge in coulomb necessary to raise the voltage of an object by 1 volt; hence 1 Farad, which is the unit for capacitance, is 1 coulomb per volt. A coulomb is the amount of electrons moving per second for a current of 1 ampere (or 6.25 x 1018 electrons/sec). Hence bigger objects need more coulombs to raise their voltage by 1 volt and have larger capacitance. Examples of objects and their typical capacitance are shown in the table on the next page:


Object Capacitance pF (10-12 F)

100 mm flange 10

Bucket or metal scoop 20

Person wearing thick sole sandals 100

Person wearing thick rubber soled shoes

200

Person wearing worn leather shoes Up to 400

205 l drum 375

Road tanker at 15 kV 5 000

50 l container 100

Filter bag holder 50

Small lined vessel 10 000

Typical energy levels of charged objects are illustrated as follows:

Object Energy mJ

Flange at 10 kV 0.5

Shovel at 15 kV 2

Funnel at 8 kV 0.6

Drum 205 l at 20 kV 40

Person at 10 kV 10

Road tanker at 15 kV 100


Annexure 6: Comparison of MIE and ESD Energy 32

32 Glor, Martin. 1988. Electrostatic Hazards in Powder Handling. Research Studies Press Ltd., Letchworth, Hertfodshire, UK. ISBN 047192024X


Annexure 7: MIE vs Discharge Mode 33

33 Berger, N., M. Derrozière, J.C. Bilet, D. Guionnet and H. Romat. 2004. Électricité statique. INRS, Paris, France

Several 100 µJ

extremely sensitive

Several mJ

Several 10 mJ

Several 100 mJ

Several J

from a few µJ to a few J

a few J

a few 100 J

a few mJ

spark d

ischarg

e

pro

pag

ating

disch

arge

con

e disch

arge

bru

sh d

ischarg

e

coro

na d

ischarg

e

MIE


Annexure 8: Nomogram to Assess Ignition Risk with

Capacitive Discharge 34 The nomograms provided in this Annexure may be used to assess the ignition risk of flammable vapor/air mixtures or suspensions of flammable dust by a capacitive electrostatic discharge. They are based on the well-known relationship between the voltage V (in volts), capacitance C (in Farads), charge Q (in Coulombs), and electrostatic energy Ue (in joules). In SI units, these relationships are:

Q = CV and Ue = CV2/2 The quantity Ue should be thought of as the maximum possible energy released (and converted to heat) if the capacitor is fully discharged in a spark. If the minimum ignition energy or MIE (in joules) of a flammable atmosphere of vapor, gas, or dust is known, then one important requirement for an ignition is: Ue > MIE Despite a number of factors complicating the physical circumstances of ignition, this inequality is accepted nevertheless as the criterion for assessment of ignition risks associated with capacitive discharges. Measurements of MIE for suspended dusts are not straightforward, and there is evidence that capacitive discharges slowed by series resistance actually yield lower MIE values than earlier published values obtained by standard ignitability tests. A very important restriction on the use of these relations is that the capacitance C must be reasonably well-identified by the geometry. For example, the capacitance between two conductors is well-defined and measurable, and distinct, capacitive spark discharges will occur between them for the right set of conditions on voltage and charge. Use of the nomograms in this bulletin is restricted to the assessment of ignition risks for such discharges between conducting bodies35. When a charged, insulating surface is involved, other types 34 Jones, Thomas B. 2004. Nomograms for assessment of ignition risk associated with capacitive discharges. Dept. Elec. Eng., University of Rochester, Rochester, New York, USA 35 The capacitance between a metallic conductor and a charged, insulating surface is ill-defined and difficult to estimate. Furthermore, discharges between such surfaces will be of either the brush or propagating brush type. Such discharges represent recognized ESD ignition hazards, but quantitative assessment of the inherent risks can not be based on a capacitive discharge model.


of electrostatic discharges occur requiring other, more qualitative methods of evaluation.

The Nomogram Nomogram 1 on the next page is based on that of Bodurtha36 and also Jones and King 37. It allows for quick graphical determination of any one of the parameters C, V, or Ue, as long as the other two are already known. The two known points are marked on the appropriate log scales and a line is drawn through them. The unknown quantity is given by the intersection of this line with the third log scale. For convention's sake, the nomogram uses the following units:

• capacitance C in picofarads (= 10-12 Farads) • voltage V in volts, • electrostatic energy Ue in millijoules (10-3 joules)

Commonly accepted values for the capacitance of implements and plant components are marked on the capacitance log scale. On the energy log scale, some commonly accepted values for the MIE of flammable mixtures and airborne dusts are superimposed. Likewise, the voltage scale has some important values indicated, the most important of which are the minimum sparking potential of air at STP (~350 V) and the recommended safe upper limit of 100 V for the potential drop between adjacent ungrounded conductors. In hazard assessment, there are several ways to use the nomogram. For example, consider the human body which has a typical capacitance of 200 pF (cf. footnote reference 2 Clause 2.2 where the value of the human body capacitance used by the International Electrotechnical Commission in ESD testing is 150pF). A person wearing shoes with insulating soles can readily become charged to ~104 volts walking across a carpet or insulating mat on a dry winter day. Using the nomogram, the electrostatic energy storage Ue is found to be 10 mJ. This value is sufficient to ignite hydrocarbon (HC) vapors mixed with air; therefore, operators in a plant where

36 F.T. Bodurtha, Industrial explosion prevention and protection, (McGraw-Hill, New York) 1980. 37 T.B. Jones and J.L. King, Powder handling and electrostatics, (Lewis Publishers (CRC Press), Boca Raton, FL) 1987.


flammable vapors or liquids may be present must wear safety shoes with conductive soles or ankle straps. The value of 10 mJ is considered marginal for ignition of the airborne dusts of polymer compounds, thus conductive soled shoes might be recommended but not required where the only flammable material is dust.

Nomogram 1: Standard nomogram An alternative way to employ the nomogram is to use the capacitance C and minimum ignition energy (MIE) values to determine the maximum safe voltage. As an example, consider a rubber-wheeled tanker truck with capacitance of 1000 pF and figure out the voltage that the truck would have to acquire in order to create an ignition risk for a flammable mist. Connecting the points C = 1000


pF and Ue = 1 mJ, the extended line intersects the voltage scale at V ~ 1300 V. The risk assessment issue then reduces to an estimate of the maximum voltage that could be developed between the truck and ground potential. In fact, V = 1300 V is rather easy to achieve due to rolling friction if conditions are dry. We can thus conclude that provision for grounding for a tanker truck during loading or unloading operations is vital. Note that if the line defined by the given values of C and Ue intersects the voltage scale below V = 350 V, no spark should occur. The recommended value of 100 V provides an appropriate margin of safety for ESD risk assessment, given the inevitable uncertainty about actual capacitance values.

Click http://www.ece.rochester.edu/~jones/demos/nomogram.html to experiment with the on-line nomogram.

Alternate nomogram Another way to assess the ignition risk of a capacitive electrostatic discharge is to determine the maximum amount of charge transferred by the spark. This maximum transferred charge cannot exceed the charge Q (in Coulombs) stored by the capacitance. It is found that spark discharges involving the transfer of less than 0.1 microCoulomb (10-7 Coulombs) will not ignite flammable gases or vapors. Therefore, Nomogram 1 may be modified by the addition of a log scale for the stored electrostatic charge Q as in Nomogram 2 on the next page. This additional scale provided on Nomogram 2 is marked off in units of microCoulombs (10-6 Coulombs). If the line defined by any two of the three parameters (C, V, Ue) intersects this scale below Q = 0.1 microCoulomb, then the risk of ESD ignition is minimal. For example, a metal scoop with capacitance C = 20 pF charged to V = 2000 V is unlikely to ignite flammable mixtures, because the line defined by these two values intersects the Q scale below 0.1µC(oulomb).

A usable definition of MIE depends on clear identification of both the charged object's capacitance and the path of the electrical discharge. If there happens to be significant effective resistance in the circuit, then the apparent minimum ignition energy is altered.


One example where discharge path resistance becomes important is the human body, which does become charged readily, but discharges more slowly across a gap than a metallic conductor due to the finite conductivity of the body. One study, performed with acetone vapor, reported that the effective MIE is increased by a factor of approximately 4 for the case of an electrically charged person 38.

Nomogram 2: Nomogram with the charge scale.

38 R.W. Johnson, "Ignition of flammable vapors by human discharges," Loss Prevention (AIChE), vol. 14, 1981, pp.29-34.


A blank copy of the nomogram in pdf format may be downloaded by clicking on http://www.ece.rochester.edu/~jones/demos/blanknomo.pdf .

An interactive tool for creating custom nomograms is available on-line by clicking

http://www.ece.rochester.edu/~jones/NomoDevel/nomogram.htm Annexure 9: ESD Risk Analysis Table

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About SAFEX International

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objective of improving the safety of operations and their impact

on people and the environment. Operations cover the

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