5.0 TEST VOLTAGE AND SENSITIVITY.
For effective testing, the output voltage must lie between lower
and upper limits.
The lower voltage limit us the voltage required to break down
the thickness of air equivalent to the coating thickness. See
Section 5.2.1.
The upper voltage limit is the point at which the coating would
break down. See Section 5.2.2.
The following describes how the band of safe, but effective,
voltage outputs may be determined.
5.1 Dielectric Strength.
All material will conduct electricity if a high enough voltage is
applied. However, with insulators, the voltage level required to
achieve a current flow is usually destructive.
The voltage at which a thickness of material breaks down is
called the dielectric strength. This is usually expressed as the
voltage per unit distance, e.g. kV/mm.
Its value depends on the type of applied voltage, e.g. AC, DC
or pulsed, temperature and thickness.
Figure 5 shows the relationship between breakdown voltage
and thickness for materials of different dielectric strengths. The
dielectric strength of coating materials usually lies in the region
of 10 - 30kV/mm.
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