Static Electricity 2000 Edition

4.3.7 Discharge on the Surface of an Insulator Backed by a
Conductor.
A surface coated with a thin (less than 8 mm)
layer of an insulating material will act as a capacitor to store
charge. At sufficiently high charge levels (i.e., greater than 250
µC/m
2
), a branching discharge will be observed on the sur-
face of the coating. This branching discharge is called a propa-
gating brush discharge. Alternatively, an electrical breakdown
through the layer can occur. The energy stored in the coating
W
1
2
--- CV
2
=
W
1
2
---
Q
2
C
------
=
W = 
¹⁄₂CV
2
W = 
¹⁄₂QV
W = 
¹⁄₂Q
2
/
C
Q = CV
10
4
10
3
100
10
1
0.1
0.01
10
–3
10
–4
Q (
µC)
W (mJ)
10
4
10
3
100
10
1
0.1
0.01
10
–3
10
–4
10
–5
C (pF)
10
4
10
3
10
100
1
0.1
V (kV)
10
4
10
3
10
100
1
0.1

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77–
10
STATIC ELECTRICITY
2000 Edition
can be as high as several joules per square meter, so the energy
of the discharge, however distributed in space, can be suffi-
cient to ignite gas–air, vapor–air, and dust–air mixtures. (See
Figure 4.3.7.)
FIGURE 4.3.7
Propagating brush discharge. (Pratt, 1997,
p. 35.)
FIGURE 4.3.8
Bulking brush discharge during filling of
silos with a bulk powder.