Spark Ignition Hazards Caused by
Charge Induction
J. E. Owens
CONDUX, Inc., Newark, DE 19711
Unexpected ignition hazards, caused by static charges on nonconductors,
warrant greater attention in industrial plants and laboratories.
The ignition hazard caused by static sparking from ungrounded metal objects is well recognized. Bonding and
grounding procedures, established by many Standards for
eliminating this hazard, are followed by industry. Less
well recognized are hazards caused by static sparking
from semiconductive liquids, or the human body, as a result of a phenomenon called “induction charging.’’ This
phenomenon involves static electrification of a nonconductor which, in turn, creates induced charges and static
potentials in a nearby conductor or semiconductor.
Sparking, created by the induced charges, can cause fires
in flammable gadair or vapor/air mixtures. This paper discusses the following topics:
0 Special characteristics of induction charging.
Six incidents in which induction charging resulted
in a fire.
Protective measures that were taken against this
hazard.
CAUSE OF INDUCTION CHARGING
Induction charging is the redistribution of electrons or
ions in a conductive or semiconductive material, created
by an externally-generated electric field. Figure 1 shows
induced (-) charges on the surface of a conductor, created
by the electric field from charged nonconductive film.
Assume that the conductor is insulated from ground and
is at zero potential before the film is brought near. The
electric field, produced by the (+) charges on the film,
causes electrons at the surface of the conductor to be
“bound,” thereby raising the potential of the conductor to
a (+) value that may cause sparking. Upon removal of the
film, the induced charges are no longer bound, and the
potential of the conductor returns to zero.
If the conductor is grounded, on the other hand, there is
no sparking hazard. Charge induction still takes place,
but charges are able to move to or from ground in order to
keep the conductor at zero potential.
Perhaps the most significant aspect of the induction
charging phenomenon is that it is able to convert a relatively innocuous charge buildup on a nonconductor into a
serious sparking hazard. Through this conversion, energetic sparks can take place from an ungrounded, conductive or semiconductive material instead of from the
charged nonconductor.
IGNITION HAZARDS OF DIRECT SPARKING FROM
NONCONDUCTORS
Industrial experience has confirmed that static charges
on nonconductors are much less likely to cause incendive
Plant/Operotions Progress (Vol. 7, No. 1)
sparking than are static charges on ungrounded conductors. Gibson and Lloyd (1965)pointed out that it is possible to cause fires in flammable vapor/air mixtures by
sparking from charged film, but that it requires a surprisingly high charge density to create this hazard. Sparks
from charged nonconductors have low energies because a
relatively small area (typically less than 3 x
m2) of
the charged surface feeds charges into a spark. Another
factor is the size of the spark-promoting electrode.
Grounded, sharp electrodes wilI not cause hazardous
sparking because they promote corona discharge. Electrodes having a radius of curvature of about 5 x
m are
particularly hazardous because the high electric field
concentration at such surfaces promotes sparking rather
than corona. As the radius of curvature is increased further, the field concentration and the sparking tendency
decrease.
To assess the possible ignition hazard of direct sparks
from charged nonconductors in a plant process, it is necessary to make an on-line static survey, using an electric
field intensity meter (not a static voltmeter). Field intensity values that exceed 5 x lo5 Vim in potentially flammable locations should be considered hazardous, and
steps should be taken to reduce those levels with appropriate types of static neutralizers. Although this value of
electric field intensity is insufficient to cause sparking, it
protects against the increased field intensities that are
produced by curvature of field lines to a spark-promoting
electrode.
IGNITION INCIDENTS CAUSED BY INDUCTION CHARGING
Smoll Plastic Bottle
One-liter plastic bottles were being used during the
collection of small samples (about 0.2 liter) of a methanolbased liquid for analytical testing. Upon the completion
of the tests, the bottles were capped and placed in a plas-
Charged
Nonconductor
E
= electrlc
field
I
+--
E
Conductor
z-
V
= potontlal
C
= capacltance
Figure 1. Charge induction by a charged nonconductor.
January, 1988
37
tic bag. The bag and bottles were then taken to another
location, where the bottles were emptied into a safety
can. A small fire occurred as a technician was starting to
pour the contents of one of the bottles into the safety can.
An investigation concluded that the fire was caused b y
sparking from the liquid to the safety can. The potential of
the liquid had been raised above ground by induction
charging, created by static charges on the outside of the
plastic bottle. In this instance, the bottles had been
charged by contact and separation from the plastic bag.
By wetting the entire inside surface of the bottle, the
small quantity of conductive liquid accumulated induced
charges (Q) equal in number to the so-called “triboelectric” charges on the outside of the bottle. The potential
(V) of the liquid can be expressed as:
V
C
=
=
Q/C
J
=
Q2/2C
Assuming the following values:
Minimum ignition energy of methanol vapor/air =
0.14 x
joule
Electrical conductivity of methanol = 4.4 x
mho/cm, = 4.4 x
siemen/m
Ambient temperature = 23°C
0
Electric field intensity at surface of bottle: E = 8 x
lo5volts/m
Charge density: Q/A = KJ&E = 1 x 1.85 x lo-’’ x 8 x
lo5 = 7.1 x
coulomb/m2
K, = relative dielectric constant of air = 1
I& = permittivity of a vacuum = 8.85 x lo-’’ farad/
m
Charged area: A = 200 cm2 = 0.02 m2
0
=
Q/A x A
=
7.1 x
x .02 = 1.4 X
coulomb
C = lo-” farad
Sparking voltage: V = Q/C = 1.4 x 10-7/10-” = 1.4
x 104
Stored energy: J = Q2/2C = 2.0 x 10-14/2 x lo-’’ =
1.0 x
joule.
The above calculations show that the sparking voltage
and the stored energy are sufficient to ignite the methanol
vaporiair mixture, which is near its most flammable concentration at 23°C.
Induction charging in ungrounded liquids has certain
surprising aspects that differ from the normal concepts of
electrostatic hazards:
0
Very small quantities of liquid can cause hazardous
sparking.
0
“Conductive” liquids (i.e., those having electrical
conductivities above about 10,000 picosiemens/m)
are more hazardous than are “nonconductive”
liquids.
0
Partial filling of a container may be more hazardous
than complete filling.
Steps that can be taken to eliminate the induction
charging hazards with plastic containers for conductive,
flammable liquids include:
0 Avoid transporting these containers in plastic bags
or coat pockets.
0 Neutralize static charges on the containers by
means of blower-ionizers or by wiping the container with a wet cloth before it is used.
38
A metal drum with a rigid plastic liner was being filled
with a “conductive,” flammable liquid b y free-fall from
the top. When the drum was about a third full, a fire occurred in the drum.
The incident investigation concluded that the free-falling liquid did not provide electrical continuity from the
liquid in the drum to ground, and that, b y inducing
charges in the liquid, static charges on the liner had
caused incendive sparking. In this instance, the liner had
been placed in the drum just prior to starting the drumfilling, and the liner had been charged during handling in
the low humidity environment.
For improved safety, the liners should be neutralized
with a blower-ionizer or wiped with a wet cloth before
they are inserted into the drums. Measurements of the
electric field intensity can indicate the presence of static
charges on the liners.
capacitance, liquid to ground, farad
The stored energy (J) in the charged liquid can be expressed as:
Q
Rigid Plastic Drum Liner for Metal Drums
January, 1988
Nonconductive Plastic Bag a5 a Liner for Waste-Collecting
Containers
A plastic bag had been placed in a drum for rags soaked
with a conductive solvent. A fire occurred after several
wet rags had been thrown into the drum. A review of this
incident showed that the plastic bags sometimes had islands of high static charges (field intensities above 5 x
lo5V/m). Contact of the solvent-wet rag with these areas
caused induction charging of the rag, thereby raising its
potential relative to other wet rags that had not been
charged. Ignition was attributed to this difference of potential.
Corrective action required that no plastic bags be used
in the waste containers or that conductive, grounded bags
be used.
+++++++++++++++
FILM
- -\- \- _\ - -E -l -\- \
- -\- \
--
FILM
eiectrlc field exists wlthin film
no stgnificant field In air
normally n o sparklng hazard
Figure 2. Nonconductivefilm with dipolor charges.
M
_-
dlpolar
-
+\+dlpolar+ + +
Figure 3. Sparking caused by coating dipolarly charged film.
Plant/Operations Progress (Vol. 7, No. 1)
Personnel Electrification
The human body is a good conductor for static electricity. Because of its significant capacitance to ground (e.g.,
200 x lo-’’ farad), the body can store energies that are
well above the minimum ignition energies of vapor/air
mixtures (values from about 0.1 to 2 millijoules). Even at
the threshold of shock sensation, the body has a stored
energy of about one mJ. Because stored energies can exceed 20 mT, an ungrounded Derson can be a serious hazard in potentially flammable locations.
This hazard was demonstrated by a fire in a laboratory,
caused by sparking from an ungrounded person who had
just removed his coat. The removal of the coat left charges
on other clothing, which then induced charges in the person’s body, raising its potential to several kilovolts. As the
person approached a laboratory bench, he caused a spark
to ground in a location where flammable gas was leaking
from experimental equipment.
Corrective action required an increased awareness on
the part of laboratory personnel of the hazards of clothing
static, and of the need to dissipate static charges to ground
in a safe location before entering a location where Hammable gases or vapor may be present.
ONE-SIDE-COATING OF FILM
DIPOLARLY-Charged Film
A fire occurred on a film-coating line on which a solvent-based, semiconductive coating was being applied to
one surface of a nonconductive film. Induction static neutralizers and nuclear static eliminators had been positioned properly. On-line measurements of electric field
intensity showed no apparent static electrification as the
film entered the coating station.
Further investigation showed that the uncoated film
carried DIPOLAR charges. (See Figure 2). This type of
charging produces no significant electric field in air and
no sparking hazard; the field is within the film. A potential hazard is created, however, if DIPOLARLY charged
film is one-side-coated with a semiconductive coating.
In this incident, the film was in contact with a
grounded,
when the coating was
(See
Figure 3 ) . Static charges on the roll side of the film were
bound to induced charges in the roll. Charges on the air
side of the film were conducted to ground as the coating
was applied. As the freshly-coated film left the coating
roll, the static charges on the uncoated surface appeared,
temporarily, as MONOPOLAR charges. Electric field
lines from these charges switched from the coating roll to
the coated surface, thereby inducing charges in the
coating. The induced charges caused a flow of electrons
to the coating applicator (ground), and a potential difference along the coating, which was sufficient to cause
sparking.
Where coating processes are subject to this hazard, care
must be taken to avoid the generation of DIPOLAR
charges on the film. This is particularly important because there is no convenient method of removing these
charges, once they have formed. Special electrostatic instrumentation is required for the measurement of DIPOLAR charges.
Nip-Roll Cooting
A fire occurred on a coating line where a semiconductive coating was being applied to film by nip rolls. It
was determined that the film carried no significant level
of MONOPOLAR or DIPOLAR charges into the coating
station. Tests made on the line with no coating showed
Plant/Operations Progress (Vol. 7, No. 1)
that the rubber nip roll was generating high MONOPOLAR charge levels (electric field intensities above lo5
V/m) on the uncoated surface of the film. The electric
field lines from these charges created induced charges
and potential differences in the fresh coating that resulted
in incendive sparking.
This hazard was eliminated by positioning static neutralizers just downstream of the coating rolls to neutralize
most of the MONOPOLAR charges.
SUMMARY
Induction charging can cause ignition hazards even
though Proper bonding and grounding Practices are f d lowed. The increasing use of plastic containers for flammable liquids, Particularly those that are not normally
considered static-prone, warrants an increased awareness
of this hazard. When static surveys are made to identify
locations of high charging in film-coating operations, it is
important that the type of charging be determined. Normal electrostatic meters measure MONOPOLAR charge
buildup (by measuri% the field intensity), but cannot
measure DIPOLAR electrification; special instrymentation is required. In some coating operations, the induction charging phenomena cause hazardous conditions
immediately after coating, where a DIPOLAR charge distribution (nonsparking) is converted, temporarily, to a
MONOPOLAR (sparking) form.
LITERATURE CITED
1. American Petroleum Institute, Protection Against Ignitions
Arzsing Out of Static, Lightning, and Stray Currents, RP
2003, Washington, DC (1982).
2. Eichel, F. G., “Electrostatics,”Chemical Engineering, p. 153
(March 13, 1967).
3. Gibson, N. and F. C. Lloyd, “Incendivity of Discharges from
Electrostatically Charged Plastics,” Brit. J . Appl. Phys., vol.
16, p. 1619 (1965).
4. Haase, H., Electrostatic Hazards: Their Eualuation and Control, English Translation by M. Wald, Verlag Chemie, New
York, p. 108 (1977).
5. Loveland,R, J., “ElectrostaticIgnition Hazardsin Industry,”
J . OfElectrostatics, vol. 11, p. 3 (1981).
6. National Fire Protection Association 30, Flammable and
Combustible Liquids Code (1984).
7. National Fire Protection Association 77, Recommended Practice on Static Electricity (1983).
8. Owens, J. E., “Ignition Hazards of Charged Dielectrics in
Flammable Environments,” ZEEE Trans. Znd. Applic., vol.
IA, p. 1424 (Nov./Dec.1984).
J. E. Owens joined the du Pont company in 1950
and held several technical positions with a pnmary interest in the measurement and control of
electrostatic charges and electrostatic hazards.
Prior to his 1985 retirement he was a senior consultant in the Hazards Engineering Group in du
Pont’s Engineering Department. He is now an independent consultant, specializing in electrostatics and hazards analysis
January, 1988
39