Over the years, there have been numerous reports of explosions
in grain silos, of oil tankers blowing up during tank washing,
of patients being killed during an operation by a pressure wave
set off by an ignition of the anesthetic gas, of everything
from minor accidents in the laboratory or kitchen to disasters
in space vehicles. It's an interesting story in itself how the
number of static-caused explosions seems to have dwindled over
the last two decades, but we will leave that one to another
discussion. Instead, we will look into what a discharge is and
what sometimes makes it incendivethat is, capable of causing
Decay and Discharge
A charged body may lose its charge in two ways. First, let's
suppose the body is a conductor. If it is connected to ground
by a path containing mobile charge carriers, the charge will
apparently leak away in a current. This is what happens in any
wrist strap or surface layer of a topical antistat. If, by contrast,
the charged body is a true insulator, this process can only
take place if the body is totally immersed in a conductive fluidin
practice, always ionized air. In this case the body doesn't
really lose its charge. Rather, the field is neutralized by
oppositely charged ions attracted from the fluid. This is called
The decay current is driven by the field from the charge to
be neutralized, but all the field does is move existing charge
carriers. The only effect of a decay current (apart from neutralizing
the charge and field) is a dissipation of heat, as given by
The other way by which a body may "lose" its charge, totally
or partly, is through an electrostatic discharge. A discharge
happens if the field from a charge is high enough to cause ionization
in the surrounding medium. The difference between decay and
discharge is primarily that, in the discharge process, the charge
carriers are created by the field, and the development of the
process may be much more dramatic than in decay.
In a casual context, electrical discharges are often called
sparks. It is, however, more practical to reserve this name
for a special kind of discharge, namely that taking place between
well-rounded conductors at different potentials.
Types of Discharge
Bowing to tradition and convenience, we may divide electrical
discharges into three sometimes-overlapping groups: corona,
spark, and brush discharges.
Corona Discharge. If the field strength in front of a sharp
point of a conductor exceeds the breakdown field strength for
the medium (air, for instance), a corona discharge will take
place. This may happen if a conductor with sharp protrusions
is given a high voltage, the critical value of which depends
upon the geometric conditions, like distance to grounded surroundings.
But it may also happen if a grounded, sharp conductor (at zero
voltage) is brought near a charged object, like a piece of plastic
that has been rubbed. This event demonstrates that it does not
take a high voltage to cause a discharge, only a high field
strength (see Figure 1).
|Figure 1. Corona discharge.
In a corona discharge, the ionization is limited to a small
region around the electrode, where the breakdown field strength
is exceeded. In the rest of the field, we have just a current
of slow-moving ions and even slower-moving charged particles
finding their way to some suitable counter electrode, such as
the walls of the room.
A corona discharge is also called a silent discharge. It may
be maintained as long as the breakdown field strength is exceeded
in some regionthat is, as long as the voltage of the electrode
or the charge density of the charged insulator is high enough.
Spark Discharge. At the other extreme of the discharge range,
we have the spark. This kind of discharge may take place between
two well-rounded conductors at different potentials, one of
them often grounded (see Figure 2). Again, the discharge starts
at a point where the breakdown field strength is exceeded. But
in contrast to the corona discharge, in a spark the ionization
takes place all the way between the two electrodes.
|Figure 2. Spark discharge.
If the electrodes are connected to a voltage supply, the discharge
may turn into a continuous arc, but in the normal case of a
spark from an insulated conductor, the discharge is a very fast
process, where energy given by the equation
is dissipated in the narrow discharge volume.Here C is the
intercapacitance of the two electrodes, V their potential difference.
|Figure 3. Brush discharge.
Brush Discharge. In between the corona discharge and the spark
is the brush discharge, which may take place, for example, between
a charged material and a normally grounded electrode with a
radius of curvature of some millimeters. If a brush discharge
is maintained over longer periods, it may appear as irregular
luminescent paths (see Figure 3).
Almost all discharges from insulators are brush discharges,
like the crackle that you hear when you pick up a charged photocopy
or that you feel when you pull a sweater over your head. Only
if the discharge comes from a heavily charged, thin sheet of
an insulator backed by a grounded conductor (stemmed branched
brush discharge) can the discharge have something close to the
properties of a spark.
For our purposes here, the difference between the various types
of discharges, as just described, lies primarily in their different
incendivity that is, the ability of a discharge to cause ignition
or combustion. If we have a mixture of, say, oxygen (O2)
and diethyl ether ([C2H5]2O),
the molecules may react with each other if they get into a close-enough
encounter, forming water and carbon dioxide. For this to happen,
a certain amount of energy has to be delivered in a sufficiently
small volume and in a sufficiently short time. The amount of
energy depends strongly upon the gas mixture, both in terms
of the types of components as well as their relative concentrations.
Figure 4 shows the ignition energies for diethyl ether vapor
mixed with either pure oxygen or atmospheric air. For a concentration
of approximately 16% ether vapor in pure oxygen, it takes only
about 1 ÁJ to start an explosion. For ether vapor in atmospheric
air, the minimum ignition energy is about 0.2 mJ for a concentration
of about 6% ether vapor.
Although the curves in Figure 4 are developed specifically
for diethyl ether, they are fairly typical for a wide range
of vapors of organic compounds, aliphatic as well as cyclic.
Consequently, the value of 0.2 mJ may be regarded as a rule-of-thumb
lower-energy limit for vapor-air mixtures. Thus, whether an
electrostatic charge may cause an ignition in a given environment
depends on whether the discharge may deliver an energy of more
than 0.2 mJ (or the relevant specific value) in a small-enough
volume and in a sufficiently short time.
How incendive, then, are the various types of discharge we've
discussed? The rate and density of the energy dissipated in
corona discharges will always be too low to initiate an ignitionin
other words, they are not incendive under any circumstances.
In brush discharges, the total energy may easily be high enough,
but in most cases either the rate or the density of the energy
dissipation is too low to cause an ignition. It is nonetheless
possible to create such charging and discharging conditions
that a brush discharge may cause ignition in a mixture of common
organic vapors and atmospheric air. But it should be stressed
that such conditions are very rarely, if ever, encountered by
accident. Therefore, we may conclude that brush discharges,
and thus discharges from insulators, have very low incendivity.
It's a completely different story with sparks. Again, sparks
are discharges between rounded conductors (one of them, often,
a grounded object) at different potentials. As already suggested,
such a system may be characterized electrostatically by the
intercapacitance (or partial capacitance) C of the electrodes.
If the voltage difference between the electrodes is V, an energy
W given by the equation
will be stored in the system. If a spark occurs, almost all
of this energy will be rapidly dissipated in the narrow discharge
volume. If the discharge occurs in an explosive atmosphere,
ignition may result.
|Figure 4. Ignition energy for diethyl ether mixtures.
By way of example, let's examine a fairly ordinary situation.
A person with a capacitance of, say, 200 pF walks across an
insulating carpet or takes off a sweater (or does both). She
hereby gets charged to a voltage of 2000 V and is loaded with
an electrostatic energy of 0.4 mJ. She then starts to remove
her nail polish using a solvent that is mainly acetone, (C2H5)2CO.
This solvent has a minimum ignition energy like that of diethyl
ether, around 0.2 mJ in atmospheric air. If she next touches
a grounded item and causes a spark in the vicinity of the open
bottle of polish remover, will she cause an explosion?
Most likely not. If we look again at Figure 4, we notice that
the curve corresponding to atmospheric air is very narrow. This
means that as soon as you move just slightly outside the most
easily ignited mixture (6% ether), the necessary energy is much
higher. It is therefore possible only in a very small region
to cause the acetone vapor to ignite by a 0.2-mJ spark.
On the other hand, somewhere between the surface of the acetone,
where the mixture is too rich, to perhaps a couple of feet away,
where the mixture is too lean, we'll find the most volatile
mixture. If our polish-removing person is very unlucky, that's
where she may draw a spark.
It is fairly safe to assume that an electric discharge disseminating
an energy less than the minimum ignition energy Wmin ~ 0.2 mJ
in atmospheric air is not incendive, no matter what explosive
vapors are present. For a capacitive systemthat is, an insulated
conductorwith the capacitance C, we may thus define an "explosion-safe
voltage" Vex as
In the case of our friend with the polish remover, we find
the theoretical safe voltage to be 1400 V.
The concept of a safe voltage level refers only to explosion
risks. When dealing with electronics, the acceptable voltage
levels are often considerably lower. And needless to say, the
safe voltage concept can also not be applied to charged insulators.
Why not? Simply because there is no such thing as the voltage
of an insulator.
Niels Jonassen, MS, DSc, retired from
the Technical University of Denmark, where he conducted
classes on static electricity. After retiring, he divided his time among the
laboratory, his home, and Thailand, writing on static
electricity topics and pursuing cooking classes. He passed away in 2006.