Elcometer 266 Manual Del Usuario página 40

APPENDIX B: CALCULATING THE CORRECT TEST VOLTAGE (cont)
The upper voltage limit is the dielectric strength of the material
multiplied by its thickness and the lower voltage limit is the dielectric
strength of air multiplied by the thickness.
The dielectric strength of coating materials usually lies in the region of
10 kV/mm to 30 kV/mm. The dielectric strength of air ranges from
1.3 kV/mm to 4 kV/mm.
Breakdown voltage against thickness for materials of different
dielectric strengths: This graph is useful if you do not have a
standard to work to and wish to know more about how to establish a
test voltage.
ESTABLISHING THE VOLTAGE LIMITS
The Lower Limit: The lower limit for effective operation is that
required to breakdown the thickness of air equivalent to the coating
thickness. The breakdown voltage of a given thickness of air varies
with humidity, pressure and temperature but is approximately
4 kV/mm (0.1 kV/mil).
If the coating thickness is known, or can be measured, the lower limit
value can be read from the graph given above, using the line marked
AIR. For instance, if the coating thickness is 1.0mm then the lower
limit is approximately 4.5 kV.
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