Static Electricity (static
for short) has been blamed for many fires and explosions,
sometimes correctly. Sometimes, however, the investigator
has failed to find any other source of ignition. So
he assumes that it must have been static even though
he is unable to show precisely how a static charge could
have been formed and discharged.
Whenever two dissimilar materials come in contact, a
transfer of electrons from one material to the other
takes place. As a result, one material acquires as excess
of negative charges and the other material an equal
excess of positive charges.
If the two materials
are new separated, each will possess an excess of charge,
and electrostatic charging has taken place. If both
materials are conductors (e.g. 2 metals), the high conductivity
allows an equalization of charges to take place along
the contact surface during the separation, so that no
measurable excess charge is present on complete separation.
Therefore, measurable charges will only be created if
one of the materials involved is a non-conductor. The
human body may be regarded as a conductor in such processes.
All solids having a surface resistance (measured according
to DIN 53'482) higher than 1011 ohms are regarded as
Many static charges flow rapidly to earth as soon as
they are formed. But if a charge is formed on a nonconductor
or on a conductor, which is not grounded, they can remain
for some time. If the level of the charge, the voltage,
is high enough the static will discharge by means of
a spark, which can ignite any flammable vapors, which
may be present. Examples of nonconductors are plastics
and non-conducting liquids such as most pure hydrocarbons.
Most liquids containing oxygen atoms in the molecule
are good conductors.
When a liquid with low conductivity
(more precisely: with a conductivity below 10~8 ohm-1.m-1)
flows through the pipe, only the diffuse accumulation
of ions will move along with the liquid. As a consequence,
the liquid accumulating in the receiving container will
be charged. In the case of a conductive liquid, a balance
of charges will be reached while the liquid is still
within the pipe.
Whenever particles of
dusts or fluids move in a gas phase, the generation
of charges must be expected regardless of their conductivity.
A special type of charge separation takes place when
a liquid (especially a polar one) is atomized (Lenard
effect), e.g. in the case of water, negative charge
will dominate among the smallest droplets, while the
large droplets will mostly carry a positive charge.
Depending on the geometrical situation, on the materials
involved and on certain other conditions, five different
types of discharges are distinguished :
- Spark discharge
- Brush discharge
- Corona discharge
- Lichtenberg discharge
- Lightning - like discharge
the different types of discharge are illustrated by
sketches of the model experiments used to demonstrate
The Spark Discharge
> Discharge in a more or less
> Discharge between two conductors.
> Luminous phenomenon bridges the entire gap
between the conductors.
> Discharge occurs in a sudden burst.
> Typical radius of curvature of the conductors:
> Discharge in the non-homogenous field.
> Discharge between a conductor and a charged conductive
or non-conductive surface or a charged cloud.
> Luminous phenomenon is emitted from the conductor
at the location of the highest field strength and ends
in the gap.
> Several discharges in rapid succession give the
overall impression of a "brush1.
> Typical radius of curvature: ca. 0.5 cm.
Corona discharge :
> Discharge in the highly non-homogenous field.
> Discharge between a conductive point and a charged
conductive or non-conductive surface or a charged cloud,
> Luminous phenomenon in the shape of a "crown"
(corona) only immediately around the conductive point.
> Continuous discharge.
> Typical radius of curvature: <1 mm.
The Lichtenberq discharge:
A further form of discharge became known as the Lichtenberg
discharge. It can be observed along the surface of very
highly charged non-conductive surfaces and is accompanied
by luminous effects.
The lightning - like discharge:
It has never been proved beyond doubt that this type
of a discharge can occur in industrial operations. It
is thought that - similar to the phenomenon observed
in nature - a lightning -like discharge could occur
e.g. between a highly charged dust cloud in a large
silo and the wall of the silo, or may be even between
two dust clouds of different polarity created by sedimentation
in a high silo. According to the present state of knowledge,
this type of discharge cannot occur in vessels smaller
than 60 (m)3, or in silos of any height when the diameter
is below 3 meters.
THE HAZARD CREATED BY AN ELECTROSTATIC DISCHARGE
An electrostatic discharge is capable of causing ignition
when the energy released is greater than the minimum
ignition energy of a combustible mixture present at
the time. For a gas/air or vapor/air mixture to be flammable,
the ratio of the concentration of the flammable material
to the concentration of air (oxygen) must lie between
the lower and upper explosion limits. The vapor mixture
present above a liquid is only flammable if the liquid
temperature exceeds the flash point. However, attention
must be paid to the fact that, due to their small heat
capacity, the temperature of liquid droplets in aerosols
can rise very rapidly above the flash point. Although
the explosion limits are known for gas mixtures, the
corresponding data for dusts are largely unknown. The
lower explosion limit for dusts lies normally in the
range of 20 - 50 g/m3, though for highly ignition-sensitive
dusts it may be around 5 g/m3. The upper limits are
difficult to determine.
Hybrid mixtures (i.e. mixtures of combustible dust,
flammable gas or vapour and air) may be flammable even
if the concentration of both the dust and the vapour
or gas are below the lower explosion limit valid for
the individual component. The minimum ignition energy
of a hybrid mixture lies somewhere between the minimum
ignition energy of the dust and that of the gas or vapour.
But for the time being, no further generally valid statements
can be made.
In general, the probability of the simultaneous occurrence
of an explosive atmosphere and a discharge with release
of sufficient energy to cause ignition, is rather low.
Therefore it is not uncommon that operations to which
electrostatic charges are hazardous go on for years
without difficulties before, all of a sudden, an accident
THE PROBABILITY OF IGNITION CAUSED BY ELECTROSTATIC
|Types of Discharges
+ Ignition to be expected
ignition to be expected •
Some dusts have lower values
From the fundamentals explained on above, five principles
for the application of protective measures are derived.
In industrial operations, these principles have to be
applied selectively, and commensurate to the prevailing
circumstances. The most common applications are described
in the practical section.
Principle: Earth all conductors
A large number of hazardous caused by electrostatic
charging can be eliminated by careful earthing of all
> Earth all pieces
> Interconnect building structures with conductors.
> Earth isolated metal parts, e.g. flanges and metal
sheeting on insulation material on plastic pipelines,
> Place metal drums on conductive floors, or connect
Earth the personnel
The human body, which is a conductor with regard to
static electricity, can carry charges around.
> Install conductive
> Wear conductive footwear.
Prevent and reduce charging by using conductive materials
> If possible, use
aqueous solutions, alcohols, polar solvents.
> If possible, avoid highly insulating plastic containers
> Use conductive drive belts.
> Use conductive pipes.
> Use conductive filter materials for dust filters
if flammable vapours may be present.
Keep Velocities low
> Limit the flow
velocity of liquids and of gases containing dust and
> Avoid turbulence in chargeable liquids.
Avoid Explosive mixtures and aerosols
> Remove gases,
vapours and dusts by suction at the source.
> Work with flammable liquids at temperatures at
least 5 deg.C below their flash point (if combustible
dusts or powders are present simultaneously, the phenomena
explained must be considered).
> Charge flammable liquids to vessels by suction,
and discharge (blow) by nitrogen pressure.
> Work with flammable liquids under inert gas.
It should be noted that working at concentrations above
the upper explosive limit cannot be regarded as a protective
measure, since the hazardous range between lower and
upper limits must be passed through.
Measurement of various Electrostatic parameters
in the Plant:
It is difficult to elaborate in details about the measurement
of various electrostatic parameters in the plant in
this paper. However some data is presented here as ready
The various parameters like Voltage, Field Strength,
Discharges, Resistances (general), Earthing resistance
and Leakage Resistance (discharge resistance) are to
be studied to take the right measures to avoid the hazard
due to static electricity.
Surface resistance, specific resistance or conductivity,
charging current/ magnitude of charge, lowest minimum
ignition energy for flammable materials, chargeability
of dusts and relaxation behavior of charged powder are
studied and measured in laboratory.
The following examples are intended to indicate the
order of magnitude of values found in practice.
||Person, walking on
||Up to 1 kv
with rubber soles on a carpet
||Up to 20 kv
belt, speed 3 to 15 m/sec
||Up to 80 kv
|Surface of a
light fuel in a large container that has been filled
|Flange on a glass
line when Toluene flows through line
||Up to 15 kv
|Flange of a steam
||Up to 15 kv
|Roll of paper,
unwinding at a speed exceeding 10 m/s
||Ca. 1 pF
|Flange, 100 mm
||1m, @ = 1 m)
c) LEAKAGE RESITANCE (discharge
|Human skin, dry
|Human skin, wet
|PVC floor without
|Flange on glass
d) SPECIFIC RESISTANCE OF CERTAIN
||Ca. 5 (10) 9
||Ca. (10) "
||Ca. 6. (10) "
||Ca. 2. (10)12
||Ca. 2. (10)13
||Ca. 2. (10) 1d
e) SPECIFIC CHARGE OF DUSTS
AFTER PNEUMATIC TRANSFER
"Non conductive" liquids
In this category we find : aliphatic and aromatic hydrocarbons,
ethers, esters of higher acids, C1 - derivates
||(10) 1b ohms
petrol and white spirits cyclohexane, benzene, Toluene,
Xylene, Mesitylene, Diethyl Ether
|1, 4- dioxane
||( 10) 11
dibutyl ester bromobenzene, chlorobenzene dichloromethane
chloroform propionic acid
Examples of this category: hydrocarbons with "polar"
groups like alcohols, aldehydes, ketones, acids, esters,
nitrites, amides, amines, nitro compounds.
benzoic acid ethyl ester
n-propaol, n-butanol acetic acid ethyl ester cis-1
|Acetic acid pyridine
acetonitrite, propionitrite, benzonitrite acetone,
butanone, cyclohexanone isobutanol
t-butanol formic acid ethyl ester anhydrous acetic
acid propionaldehyde nitrobenzene
ether dimethyl formamide acetaldehyde
f) FIELD STRENGTH
|Dust cloud (at periphery)
||Ca. 400 kv/m
g) LOWEST MINIMUM IGNITION ENERGY
h) SENSITIVITY OF HUMANS AGAINST
DISCHARGES OF STATIC ELECTRICITY
i) RELAXATION TIMES FOR STATIC
DISCHARGE OF SOME COMMON SOLVENTS
||46 h 42 min
||32 h 26 min
The relaxation time, or amount of time
needed to discharge static charge, varies widely as
shown above. Heptane is preferred over hexane due to
heptane's shorter relaxation time.
ORIGIN OF ELECTROSTATIC CHARGES
1. A non conducting solvent flows out of a metal pipe(separating
effect).liquid is charged, excess charges on the metal
pipe flow down to earth.
2. Powder pours out of a plastic bag (separating
effect), and also the dust cloud. Charges drain away rather
3. Transfer of non conductive
liquid or powder in glass or plastic pipe (separating
effect in wall zone).Pipe and contents are charged,
also the flanges. Charges drain away slowly.
4. Plastic or paper film is unwounded(separating effect).
The film and the roll are charged. Charges drain away
5. Liquid is atomized through a metal(separating effect
when droplets are formed).The droplets of liquid and
the nozzle are charged. The excess charge on the metal
nozzle disputes via earth connection.
6. Stirring a non-conducting
liquid in a vessel (separating effect on the wall).
The liquid, the stirrer & the kettle are charged.
The charges on the vessel& on the agitator flow
down to earth.
7. A man walking with insulating sloes on a nylon carpet
or plastic floor (separating effect between soles and
floor). The man is charged. The charges cannot readily
flow away because of the insulating soles.
8. Power flows through a plastic pipe. Powder and pipe
are charged (separating effect between dust particles
& pipe). The charges on the pipe will cause a separation
of charges on the nearby metal housing, resulting in
an accumulation of charges there. This process is called
During crystallization process in non-conducting
liquids, local zone with extremely high charges may
be formed. Therefore, crystallization process in flammable,
on-conductive liquids should always be kept under inert
Improving the electrical conductivity of flammable
liquids by means of additives
The specific resistance of white sprit of 2*1011 ohrn.m
can be reduced to 2,52*109 ohm.m by adding 0.5% methanol.
The specific resistance of the petrol is in the range
of 1013 ohm.m. It can be reduced to approximately 108
ohm.m. by adding 1 or 2 grams of an antistatic additive
per m3 (e.g. Teepol 530,Lissapol N, AerosolOT, ASA-3).
Keep velocities low. For only practically filled pipes
or pipes which discharges into containers, the velocity
is to be limited as follows (irrespective of the pipe
• For chargeable esters: max. 10 m/s
• For mineral oil products (e.g. gasoline, petrol,
kerosene, paraffin, jet fuel)& for other
chargeable liquids. (Excluding carbon disulphide and
ether), depending on line size.
Rule of thumb for all
homogenous liquids (except ether & carbon disulphide)
& all pipelines: at velocities below 1m/s no dangerous
charges will be generated.
Rule for ail type of flexible tubes or houses
The electrical resistance between the two couplings
must not be higher than 106ohm. This resistance is to
be measured at regular intervals.
Details on earthing
Earthing wires laid loosely around a non-conductive
line for bonding & earthing of flanges will not
create a hazard. ing of flanges!
An earthing wire should not be drawn through the interior
of pipe. If the wire breaks, sparking is possible.
Charging or emptying of drums & small containers
Liquids with a specific resistance > 106ohm.m may
become charged. A dangerous potential builds up on the
surface of the liquid. When a conductive object (funnel,
stirrer, hose) is approached, inductive brush discharges
Take the filling pipe & tube or funnel right down
to the bottom of the vessel, to avoid flashing, whirling
Conductive containers, funnels, nozzles, must be bonded
Earth operating personnel by means of conductive shoes
& conductive floor.
P: insulating plastic material M:
Transfer of flammable liquids by means of drum
Turbulence in the pump & flow in the connecting
pipe lead to charging of the liquid and of the pipe
or tube material. Sparking is possible between metal
parts e.g. between the drum pump & the drum, especially
when the pump is going in or out.
Connect the drum pump with a conductor both to the container
being emptied & to the one being filled ,& earth
it. The connections should not be clamped directly into
the bung-hole or manhole, since combustible vapors are
most likely to be present there.
The two balancing conductors are best
fixed permanently to the drum pump so that the necessity
for bonding & earthing is immediately apparent whenever
the pump is used.
Operations at the open manhole of a reaction
kettle containing a flammable liquid. Measures:
Avoid manipulations at the open manhole, use devices
that permit sampling, pH & level checks on the closed
If manipulations at the open manhole
are unavoidable, use sample beakers(or at least rod
of sample beaker) & measuring rods made of non-conductive
material; do not use devices made of metal, measuring
rods made of wood are acceptable, on-conductive measuring
devices must not be wiped immediately before opening
large glass vessels(Measuhnq vessels, charge tanks,
• Charging of the liquid and of the vessel due
to flow and whirling up. Discharge of sparks possible
between conductive parts, e.g.
between metal flange and operating personnel.
• Earthing of metal flanges larger than 50mm
• Inerting of glass vessel or
• Charging with low velocities or
• Charging by means of closed vacuum
Additional measures if possible:
- Take filling pipes right down the bottom of the vessel.
If PTFE pipes (polytetrofluor ethylene, Teflon) are
to be used, they should be of conductive quality and
Loading & unloading of road
tankers; filling the fuel tank of vehicle, filling plastic
cans (chargeable liquids)
• Use conductive hose material.
• Wear conductive shoes.
• The material drain cock is earthed via petrol
hose & must be firmly rest on the filler cap while
the vehicle is being filled up, so that the latter is
• Filling up plastic cans: Use only small plastic
cans, when filling containers made of non-conductive
material, charges accumulated in the liquid can drain
off only very slowly. However on containers having a
volume of up to 5 litres, no dangerous
charges will accumulate.
Handling of powders with flammable qasses/vapors
For the handling of powders in the presence of gasses
or vapors, e.g. for
• Charging powders into flammable solvents
• Handling of solids being moist with solvents
The following applies:
• Due to the movement of the material during charging,
handling, separation processes are taking place continuously,
and he powder as well as the container (plastic bag)
and pieces of equipment like pipelines, chutes, funnels,
charge ducts will be charged; electrostatic charges
already present in the powder from handling can even
amplify this effect. Thereby discharges can occur. Without
spark discharges can occur. Without earthing, even sparks
jump between metal parts. Brush discharges occur where
a plastic bag is shaken out, when a non conductor is
pulled back from the funnel in the manhole.
• e.g. a charged surface area on a drum is touched
with the hand. Since, during material charging operations,
such discharges of static electricity will always occur
near the filling part of the apparatus where explosive
vapor/air mixtures are most likely to the present, such
operations are particularly hazardous.
• Note: This type of handling in a chemical works
is one of the most frequent causes of accidents, which
are traced back to electrostatic charges!
• However, experience accumulated so far does
not indicate that incendiary discharges can occur if
conductive and earthed containers, funnels, charge ducts
etc. are used exclusively.
- Closed handling of the product, under an inert atmosphere.
As a matter of principle, inerting (i.e. primary explosion
protection) is recommended since it is the most effective
and most reliable measure. Bhut this requires closed
handling so that an inert atmosphere can be maintained.
Thereby it must be borne in mind that especially in
powders of low bulk weight, considerable quantities
of air and thus oxygen may be entertained.
- Closed charging of solids into an inerted apparatus
(two possible alternatives)
for solids free from flammable solvents -all containers
or packing materials permissible including' plastic
|for solids containing flammable
solvents' only conductive, grounded containers without
plastic bags or liners
For solvent free powders: any type of drums or charge
container is permitted.
With this type of handling, solvent free powder may
be poured even out of non-conductive containers or out
of plastic bag into the charging de-
For powders containing flammable solvents: only conductive,
earthed containers are permitted.
Powders containing solvents must not be poured out of
non-conductive plastic containers or out of plastic
bags into the charging device.
of powders into flammable liquids or "open"
charging of powders containing flammable liquid into
an empty apparatus, when and inert atmosphere cannot
be maintained(see hybrid mixtures page....)
Decisive for the hazard situation and
consequently for the type of measures to be taken are
the following criteria:
- The flash point of the flammable
liquid is pre-charged to the apparatus or being present
in the powder.
- The working temperature, i.e. the
temperature of the flammable liquid during the charging
operation or the highest temperature it can reach
during that operation
- The combustibility and the minimum
ignition energy of the solid being charged.
The powder is either not combustible
or its minimum ignition energy (measured for the
fine fraction of the powder) lies in the range abouve
50mj*, and (*This limit was arbitrarily decided
for the time being to verify it, extensive investigations
are being carried out)
- The working temperature
- Is, with certainty, at least 5 deg.
C below the flash point (closed up) of the flammable
liquid (normal scatter of flash point determinations
is +/- 5 deg. C); in addition, possible quality variations
of the liquid have to be considered, they could cause
flash point fluctuations of several deg C.
Measures for Case-A
The choice of materials of construction
for the containers, funnels chutes, etc. is not
restricted, plastic bags are also permitted
- Any parts made of conductive materials
must be bonded and earthed.
- The powder to be charged is combustible
and its minimum igintion energy (measured for the
fine fraction of the powder) is in the range <=
50 mj or unknown.
- Adherence to the flash point/temperatuere
condition stated for the case A cannot be guaranteed.
Measures for Case-B
- Drum or container, funnel, chute,
etc. must be of conductive material* and must be earthed.
("Conductive" plastic is acceptable only
if the conductivity has been verified by measurement.)
The floor must be conductive for static
electricity & operation personnel's must wear conductive
- The use of containers (Whether conductive
or not) with plastic liners or of containers, packing,
etc. with non-conductive coating is forbidden.
All external ignition sources (viz. Open flames, sparks
from grinding or percussion) must be excluded.
Shovels used for charging must either be of wood and
have a wooden handle, or be of
metal and earthed.
The charging of bulk quantities must be done in lots
of not more than 50 kgs, with an
interval of approximately 1/2 minute between charges.
Open charging in the presence of carbon disulphide,
hydrogen, and acetylene is
This procedure makes great demands on the reliability
of the operating personnel and places upon the supervision
and the manager a particularly high degree of responsibility.
SPECIAL PROBLEMS PERTINENT TO CERTAIN TYPES
Entrainment of electrostatic charges
- When highly charged product is filled into a conductive
container, the latter will also be charged if it is
When an earthed object is approached - this may be an
operator wearing conductive shoes, a charge can jump.
If the operator handling such a conductor wears non-conductive
shoes, sparking is still possible when he touches an
earthed conductive object (e.g. earthed pipeline).
Use transport containers with conductive wheels.
- Wear conductive shoes.
- Keep wheels and shoes clean, check conductivity periodically
Intensive charging of the conveyed material as well
of the pipeline due to the separation processes constantly
occurring at high speeds between the conveyed material
and the pipeline.
Sparks between conductive parts, e.g. between metal
flanges and a part of the steel structure of the building.
Entrainment of considerable charges into receiving Containers
Wherever possible have pipeline used for pneumatic transfer
made of metal # throughout and ensure good earthing.
# (Plastic material is acceptable only when sufficient
conductivity has been verified by measurement.)
No insulating coatings on the inner surfaces of metal
containers and metal pipelines.
For plastic transfer lines, plastic flanges should used.
If metal flanges are used, they must be earthed. Individual
bolts on plastic flanges need not be earthed.
On pipelines constructed of insulating material, no
coating or sheathing (e.g. for reinforcement) of conductive
material should be applies, since this will lead to
an increased accumulation of charges on the inside;
berg discharges may occur.
If possible, pneumatic transfer lines should not be
directly connected to containers of silos; instead,
the powder should be separated first in a cyclone of
conductive material (charges will drain off, velocities
will be reduced).
SPRAY DRYING, FLUID BED DRYING
High charging of the substances (powders and possibly
granulates) owing to the high speed separation of the
particles in very dry atmosphere both in the drier itself
and in the dust separator (cyclone) or filter. Charging
of parts of apparatus that come into contact with the
powder, spark discharges with ignition, especially if
flammable dust and flammable vapors or gases are present
simultaneously. Flammable gases may be ignited with
1 /100 to 1 /10 of the energy required to ignite dusts.
Mixtures of combustible dusts with flammable gasses
or vapors in air may be explosive even when the concentration
of the individual components is below their respective
lower explosion limits.
All metal parts of the casing of the apparatus must
be. bonded and earthed; internal metal installation
such as sieves, prop rings, spray nozzles, filter support
baskets, receiving containers and transfer troughs must
be earthed. Individual bolts and nuts need not be earthed.
If conductive filters
are used (necessary only if combustible dust and flammable
vapours or gases can be resent simultaneously) the filter
cloth must contain the metal fibres (> 5 % in homogenous
distribution) which are spun into the individual threads
of the fabric. Filter cloth with separate leads or wires
is dangerous if such a lead or wire breaks, a spark
gap is created.
Inerting of dries. If, for inerting purposes, combustion
gases are used instead of nitrogen, their composition
(oxygen concentration, concentration of flammable components)
must be monitored and control by suitable instruments
ON MACHINES AND APPARATUS
Drive belts, conveyor belts, rollers, cylinders, etc.;
- The continuous separation of two touching surface
generates electrostatic charges depending on the material
breadth and speed of the rotating parts, which can lead
to discharges with sparks.
Use drive belts, belt pulleys, conveyor belts, cylinders,
bearing pulleys, etc. made of conductive material.
On stirred reaction vessels, kneaders, driers, etc.
as a rule the shafts and drive pulleys need not be earthed
since they are sufficiently connected to earth via bearings
and stuffing boxes. Lubricating agents with admixtures
of graphite or molybdenum help to improve the conductance.
In doubtful cases check the moving parts for earthing
(measure resistance to earth).
Charges on paper, textiles or plastic webs are best
conducted away by means of ionized air, by applying
points, metal reeds, conductor brushes, metal wedges,
or by greatly humidifying the air locally.
It is most unlikely that explosive mixtures will be
ignited when electrostatic charges are drained off by
means of earthed points or reeds.
grounded conductive wedge
MAN AS A CARRIER OF ELECTROSTATIC CHARGES Hazard:
Charging of the human body; discharge with sparks when
earthed object is approached (e.g. reaction kettle metal
Wear conductive shoes. (Resistance through soles <
108 ohms Correct: Safety shoes with conductive soles;
shoes with leather soles.
Wrong: Plastic shoes, crepe-rubber soles. Have
conductivity of footwear measured periodically.
When a piece of clothing made of synthetic fibre is
removed, high charges and sparks may be caused (separation
effect). Therefore, clothes must not be changed when
an explosive atmosphere is present.
Hints for planning & construction
On a non-conducting floor persons can become charged
by walking, owing to the constant separation effect
between their shoes & the floor. A discharge with
sparks is possible on approaching an earthed object.
Keep floors made of asphalt or plastic material damp
In special case, a floor can be made sufficiently conductive
by washing it with aqueous glycerine (1%glycerine in
water). However, such treatment will have to be repeated
every few weeks.
When a floor is to be made of epoxy resin, the antistatic
araldite floor material (long lasting antistatic effect)
with araldite BY 157 & hardener HY2987 or 355 should
be used. *
Humidity of the air
The conductivity of the air is scarcely improved by
being greatly humidified. Static charges cannot be sufficiently
dissipated via moist air. Increasing the humidity of
the air, as such, is therefore not a recognized protective
measure. However ,the surface resistance of solid insulating
materials can be reduced by if the relative humidity
of the air is above 65%,because the surface of such
materials always has a certain layer of impurities on
which a conductive film of liquid will be formed the
humidity of the air has a great influence on the chargeability
of substances such as textiles, paper etc. The humidification
is best applied locally & specifically, e.g. by
spraying or moistening (e.g. steam) the paper or fabric
webs. General humidification of rooms above 65% is expensive,
especially in ventilated rooms, and can also cause the
atmosphere to become intolerable for the personnel.
Suggested values for Surface & Resistance
For the prevention or dissipation of charges, the following
should be adhered to:
The earthing resistance of individual conducting parts
should be less than 106ohms(smaller parts up to 108ohms).
The surface resistance of insulating materials should
be less than 1011 ohms.
The resistance of drive belts(ohms) multiplied by their
supporting width(m) should be less than 105ohms Practical
• Iron grids of the building structure count as
• Metal pipes which are clamped to the building
structure count as earthed.
• Completely(also on the outside)enamelled valves
& flanges have to be bridged by an earthing conductor
& must be earthed
• Apparatus such as agitator vessels, receivers,
measuring vessels, box filters etc. made of metal which
are firmly connected to metal pipes as earthed.
• Covers of apparatus (agitator vessels etc.)
which are firmly screwed to the lower part, even if
there is an insulating gasket (e.g. Teflon ring) in
between, count as earthed. In the case of enamelled
vessels, it is advisable to check by measurements.
• Agitators, thermometer tubes, kneading arms
inside apparatus require no special
• Metal storage tanks with fixed metal pipes count
as earthed. Use conductive material for hoses.
• Drive belts (flat belts & V-belts) must
be conductive. In cases of doubt have them checked.
• Transmission has the earthing resistance checked.
• In metal silos & pipes for products in powder
from do not apply an insulating coating inside.
• Glass pipes: Plastic flanges
for small glass pipes, despite the somewhat higher purchase
price compared with metal flanges, work out cheaper
since they do not have to be earthed. Metal flanges,
valves, flaps throttles etc. having a nominal diameter
above 50 mm (pipeline above 2") are to be earthed.
• Apparatus made of glass and porcelain (stills,
stirrer vessels, steam ejectors): Metal flanges and
metal parts are to be earthed. Often they are already
earthed via the metal supports.
• Plastic apparatus and pipes: Metal parts, i.e.
flanges with a nominal diameter above 50 mm (pipeline
larger than 2"), valves, flaps, supporting shells,
protective pipes etc. are to be earthed. Individual
metal bolts need not be earthed since their capacity
is too small to generate a spark capable of being an
• Conductive outer coating or casings on plastic
pipes or apparatus increase the accumulation of charges
on the opposite side of the plastic wall and are thee
fore to be omitted.
Temporary cables, which are used only occasionally for
earthing movable apparatus can be accommodated, to save
space, with the help of cable rollers that wind up automatically.
The paint on metal pipelines can have an insulating
effect, if the individual parts are completely painted
prior to assembly.
Approved conductive paints are available.
FOR FIXED EARTH CONNECTIONS
In order to prevent the earthing in the plant to become
ineffective by damage, the conductor must have an adequate
cross-section. The wire must be adequately fastened
or supported and where necessary, inserted locally in
a protective tube.
Recommendation for earthing conductors:
Copper wire or braid, cross section 1, 5 to 2, 5 mm2,
colour of insulation as specified in local rules or
- Bare copper, diameter not less than 4 mm.
Bridge connectors across flanges should be made of flat
copper, 10X1 mm and firmly screwed to one flange so
that they will not get lost during inspections.
Connecting terminals on flanges, valves etc. have to
be in perfect contact with the metallic object that
is to be earthed:
On enameled parts, if necessary, the enamel is to be
ground off a small area, or a hole is to be drill to
the metal body.
On painted or powder-coated parts, fan-type lock should
be used on bolts and nuts for installation the part.
For large pipelines (e.g. ventilation lines) the use
of bare copper rod as earthing conductor is recommended.
Metal flanges on glass line: It is not advisable to
connect a large number of flanges for earthing purposes.
If the earthing conductor is broken or torn off, the
total capacity of the flanges lined up in parallel)
can assume dangerous values. For each pair of flanges
it sufficient to earth one flange, provided that
The flanges are painted with conductive paint or
- Special washers with points penetrating through paint