Buckleys PST-100, Instruction Manual, Page: 9 / 16

PST-100
9
Calculating the test voltage
The test voltage needs to be high enough to locate a fault but not so high as to
cause one. With reference to BS EN 60052:2002, (measurement of voltage with
Sphere-Gaps) it can be seen that 32,000 volts will
jump a gap of 1cm between spheres of 5cm diameter, however the same voltage
will jump a gap of nearly 3cm between needles. This is because the shape of the
electrode affects the point at which corona discharge starts. A spark or arc over,
will occur when the voltage is high enough such as to cause localised breakdown
of the air between the electrodes, a spark then forms, ionising the air drastically
reducing its electrical resistance.
For materials with a thickness in the range of 1mm to 30mm, the NACE Standard SP -
02-74 voltage calculation formula is normally used.
SP-02-74 test voltage formula
Alternatively
After the test voltage is calculated it is necessary to check that the voltage is not
so high as to damage the material. The dielectric strength is the voltage at which
the material starts to breakdown, this is expressed in volts per mm, normally with
D.C. As an example, a 2mm thick sheet of P.V.C. would require a voltage of
11.18kV using the above formula.
Referring to the manufacturer’s technical data, the dielectric strength is 8.4kV per
mm. Therefore, 2mm of such material has a dielectric strength of 16.8kV and is safe
to test at 11.18kV.