Electrostatics: what safety folk
Wolfson Electrostatics 1971-2006
Department of Electrical Engineering, University of Southampton
Wolfson Electrostatics Limited 2006 –
32 Church Lane, Highfield, Southampton
Consultancy Research Development Instrumentation
1 day to 3 years duration
Aerospace/avionics, agricultural, automotive,
chemicals, construction, electronics, energy
production, explosives & pyrotechnics, fine
chemicals, food production, fuel handling,
marine, military, mining, packaging, paint
manufacture and application, petrochemicals,
pharmaceuticals, printing and coatings, textiles.
•Flame & glowing combustion
•Friction & impact
•Electrical equipment sparks/arcs
Fuel (electrostatic ignition)
•Solvent above flashpoint eg. acetone FP=-20C
•Solvent below flashpoint as mist, droplets, froth etc. eg white spirit
which can be ignited at 18C (20deg below its FP of 38C)
•Fine dispersed powder and dust
•Hybrids of the above
‘In only about 50% of accident investigations
involving fires or explosions is the source of
ignition determined with any degree of
certainty. It is because of this uncertainty in
many cases that static electricity is blamed.’
From the book ‘Sources of Ignition’ - John Bond
Electrostatic Hazard Analysis Flowchart (5 conditions)
No Are flammable Flammable vapours, gases, aerosols & fine dusts.
atmospheres present? Explosives & pyrotechnics.
Is static electricity Friction, contact, separation.
generated? High voltage supplies, etc.
No electrostatic fire & No Can electrostatic Insulating materials, isolated conductors, the human
explosion hazard. charge accumulate? body.
No Can electrostatic Spark, brush & propagating brush discharges.
No Is the discharge Compare discharge energy with minimum ignition
sufficient to cause ignition? energy of flammable atmospheres.
Electrostatic fire &
explosion hazard present.
Consider each condition in turn
Are flammable atmospheres present?
(sensitive to ignition by static electricity)
material Vapour LEL/MEC UEL Minimum Ignition Kst/Kg
Flashpoint Ignition Temp
Acetone -19C 2% 7% 1.2mJ 535C 80-100
methanol 11C 6.7% 36% 0.14mJ 455C 64
Epoxy powder coating 10g/m3 6kg/m3 15mJ 430C 200
Instant coffee powder 150g/m3 no data 120mJ 490C 30
kerosene 38C 0.70% 5% no data 210C no data
styrene 30C 1.10% 8% 0.2mJ 490C 96
LEL UEL %conc.
LEL UEL % Concentration
Electrostatic ignition of solvent
Aerosol of narrow-cut kerosene (FP=78C) igniting at room temperature
Electrostatic ignition of surface froth
Frothy surface of narrow-cut kerosene (FP=78C) igniting at room temperature
(a) Low energy spark causes ignition (b) fire spreads across surface
Dusts and powders
In general, dust clouds are less sensitive to ignition than flammable solvents
Airborne dusts and powders are of particular concern when considering
electrostatic hazard because:
•Most industrial powders and dusts are electrically insulating in nature
•A given mass of powder represents a large total surface area and therefore
a high capacity for the retention of electrostatic charge
•Many industrial processes handling powders (eg pneumatic conveying)
also generate high levels of electrostatic charge.
Flammable dust concentrations
Industrial hygiene explosive range dust deposit
1-10 mg/m3 20-2000 g/m 3 500kg/m3
A cloud of 40g/m3 of coal dust in air is so dense
that a glowing 25W light bulb can hardly be seen at 2m
Flammable dust concentrations
Illustration of the potential hazards of even thin dust layers. A 1mm layer of dust
of bulk density 500kg/m3 (a) will generate a cloud of average concentration 100 g/m3
if dispersed in a room of height 5m (b). Partial dispersion of up to only 1m (c)
(a) (b) (c)
Is static electricity
Is static electricity generated?
•The contact and separation of solid surfaces
such as moving webs over rollers.
•The movement of personnel.
•The flow/movement of liquids.
•The production of mist or aerosols.
•The flow or movement of powders.
•Charging by induction in an electric field.
Charging of powders
•The nature of the material comprising the particulate.
•Mass flow rate/density (kg/m3)
•Composition of duct walls.
•Turbulence due to bends, constrictions etc.
•Temperature and humidity.
Charging of liquids
Unlike industrial powders most liquids are in electrostatic terms relatively
Electrically conductive. For this reason it is non-polar liquids such as paraffin,
gasoline, purified aromatics (toluene, xylene, etc), diesel and light oils,
which exhibit significant charge generation.
Electrostatic charge is generated due to flow, agitation and droplet generation.
In pipelines the level of charge generated depends principally upon:
•The conductivity of the liquid
•The flow velocity
•The nature of the pipe wall
•The presence of filters, constrictions etc.
•The presence of an immiscible phase eg. water in solvent
Note: The flow of gas produces little or no electrostatic charging.
Can electrostatic charge accumulate?
n Non-conductive surfaces eg. plastic
n Electrically isolated conductive media
n Unearthed metal components
n Electrically insulating (resistive) powders
n Low conductivity liquid in tanks
Metals Soil Natural Concrete Plastics
Charge decay time 10μs 10ms 10s 3h
10-9 10-6 10-3 1 103 106 109 1012 1015 ohm.metres
Tap water Petrol
Conductive Dissipative Insulating
Charge on the human body
n Capacitance is usually in range150-300pF
n Operators may be unearthed due to
footwear, flooring, chairs etc.
n Skin is conductive and may produce sparks
n People are mobile and may carry hazardous
potentials into sensitive areas
n It may be difficult to apply grounding
n Maximum resistance 108 ohms if practical
Can electrostatic discharge (ESD) occur?
n Spark discharge from ungrounded conductor
- energy of discharge given by ½CV2
- plant, product/material being handled, personnel, tools
n Brush discharge from plastic/insulator
- may be produced by surface voltages in excess of -20kV
- very high voltages (>50kV) may give incendive ESD
n Propagating brush discharge
- plastic sheet, plastic lined conductor, pipe wall
n Cone discharge (bulking brush)
-hoppers, silos, bins
Spark and brush discharges
+++ + +
+ + + + +
+ + plastic
energy =1/2CV2 energy = 4mJ max.
Propagating brush discharge
Thin plastic sheet with earthed Plastic pipeline
Note: Surface must become highly charged for PBD to occur
Looking down onto top of powder heap in silo
Note: Equivalent ignition energy up to 10mJ
Is the discharge
sufficient to cause ignition?
Is ESD energy sufficient to cause ignition?
n Brush discharge <4 mJ
n Cone discharge 10mJ
n Spark discharge 1mJ (single bolt) – 100 mJ (large
n Propagating brush discharge >100 mJ
n Solvent vapours and gases 0.2mJ
n Foam/froth ignition energy 1mJ
n Mist ignition energy <10mJ
n Dust/powder ignition typically 5-500mJ
Minimum ignition energy
The minimum ignition energy of a flammable atmosphere can be defined as
the minimum electrical spark energy required to ignite and propagate flame
either partially or totally throughout the volume of that atmosphere
•Coarse powders : 20-200mJ
•Fine powders (<50μm) : 3-30mJ
•Metal dusts & fine aerosols : 0.5-5mJ
•Flammable hydrocarbons : 0.1-2mJ
•Acetylene : 0.017mJ
•Hydrogen : 0.011mJ
•Oxygen-rich vapours : 0.005mJ
Table of MIE values for common
industrial solvents and gases
Typical dust data chart
illustrating effect of particle size
Note MIE values against particle size distribution for fructose
Examples of Hazardous Situations
Electrostatic potential developed on plastic pipe during and
after solvent flow
(Solvent with sufficient O2)
Insufficient oxygen Insufficient fuel
Are flammable atmospheres
Dust cloud 20-2000g/m3 in air
Vapour 2-13% in air
Is static charge generated?
Can electrostatic charge accumulate?
Type A FIBC
Mobile plant - Containers
Trolleys - Drums
- - - +
- -- FIBC - + Tools
- - surface- - +
Electrostatic discharges (ESD)?
brush - -
spark - - spark
Is the discharge energy sufficient to
15mJ - - 0.5mJ
Minimum ignition energy vapour = 0.2mJ, dust cloud >5mJ