Static Electricity 2000 Edition

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STATIC ELECTRICITY
2000 Edition
FIGURE 4.3.1
Approximate energies of types of discharges compared with minimum ignition energies (MIE) of typical
combustible materials. (Adapted from Walmsley, 1992, p. 26.)
4.3.2 Corona Discharge.
Corona is an electrical discharge in
the microampere range that results from a localized electrical
needle-shaped breakdown of gases by charges on surfaces
such as sharp edges, points, and wires. The charges can arise
on conductors at high voltages or on grounded conductors
that are situated near a charged surface. Corona is accompa-
nied by a faint luminosity. (See Figure 4.3.2.)
FIGURE 4.3.2
Corona discharge. (Pratt, 1997, p. 32.)
4.3.2.1
In most cases, the energy density of corona discharge
is very low. Consequently, the hazard from corona discharge is
small. Where corona discharge is more intense, prebreak-
down streamers called brush discharges occur. These appear as
random filaments of light that make faint hissing or frying
sounds. Brush discharges that originate on needle-like tips
with radii smaller than 1 mm do not, in general, lead to igni-
tion. Discharges from blades, however, can ignite mixtures
that have very low ignition energies, such as hydrogen–air or
carbon disulfide–air mixtures. Gas–air and vapor–air mixtures
can be ignited if brush discharges originate from elements
with edge diameters greater than 5 mm, or from a rod with a
hemispherical end, such as a human finger. (See Figure 4.3.2.1.)
FIGURE 4.3.2.1
Brush discharge. (Adapted from Walmsley, 
1992, p. 27.)
4.3.2.2
Sharp edges, corners, and projections (e.g., those with
an edge diameter of 5 mm or less) that point toward charged
surfaces should be identified. These sharp regions concen-
trate the charge, providing intense, localized stresses that can
lead to electrical corona and sparks.