Appendix B: Calculating The Correct Test Voltage - Elcometer 266 Manual Del Usuario

APPENDIX B: CALCULATING THE CORRECT TEST VOLTAGE

The Elcometer 266 includes a built-in voltage calculator which will
determine and set the correct test voltage based upon the test
standard and the thickness of coating you are testing, see Section 9.1
'Automatically Setting the Voltage' on page en-17.
Alternatively, the voltage can be set by the user, see Section 9.2
'Manually Setting the Voltage' on page en-18, using the following
guidelines which describe how a safe, but effective, test voltage may
be determined.
OVERVIEW
For effective testing, the test voltage must lie between two limits - the
upper and lower limits.
The upper voltage limit is that at which the coating itself would

breakdown and be damaged. Therefore, the test voltage should
be lower than this value.
The lower limit is the voltage required to break down the

thickness of air equivalent to the coating thickness. If the output
voltage is not greater than this value, then a flaw will not be
detected.
These two limits can be determined and a voltage approximately half
way between them selected as the test voltage.
DIELECTRIC STRENGTH
Whatever the material, if a high enough voltage is applied, it will
conduct electricity. However, for insulators, such as paint, the level of
voltage required to achieve a current flow usually results in irreversible
material damage.
The voltage at which a particular thickness of material breaks down is
termed 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 (AC, DC or pulsed),
temperature and thickness. The graph on page en-39 shows the
relationship between breakdown voltage (DC) and thickness for
materials of different dielectric strengths.
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en-38