Static Electricity 2000 Edition

4.3.3 Sparks Between Conductors.
4.3.3.1
Sparks from ungrounded charged conductors, includ-
ing the human body, are responsible for most fires and explo-
sions ignited by static electricity. Sparks are typically intense
capacitive discharges that occur in the gap between two
charged conducting bodies, usually metal. The energy of a
spark discharge is highly concentrated in space and in time.
4.3.3.2
The ability of a spark to produce ignition is governed
largely by its energy, which will be some fraction of the total
energy stored in the system.
4.3.3.3
The energy of a spark can be determined from the
capacitance of the conductive system and the electrical poten-
tial or from the quantity of charge separated from the conduc-
tors and is expressed by the following equations:
10
4
10
3
10
2
10
1
1
10
–1
10
–2
10
–3
Discharge types
Equivalent energies of discharge (mJ)
Equivalent energies
Corona
Brush discharge
Materials
Bulking brush
discharge
Propagating
brush discharge
Sparks
Dusts, gases in
oxygen-depleted
atmospheres
Alkanes in air, distilate fuels, hybrid
mixtures, extremely sensitive dusts,
some vapors (e.g., methanol)
Explosives, hydrogen, unsaturated
hydrocabons, alkanes in oxygen
10
4
10
3
10
2
10
1
1
10
–1
10
–2
10
–3
MIE ranges (mJ)
+ + + + + + + + + + + + + + + + +
+
+
+
+
+
–
–
–
–
–

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FUNDAMENTALS OF STATIC ELECTRICITY
77
–9
2000 Edition
where:
W = energy (joules)
C = capacitance (farads)
V = potential difference (volts)
Q = charge (coulombs)
The relationships are shown in Figure 4.3.3.3.