Instruction Manual
8
06/23
HV Testing Basic Physics
AS noted above, the principle of this method is that the material being tested has a greater breakdown
voltage (in kilovolts) than the equivalent “thickness” of air. Air has a breakdown voltage of
approximately 3kV/mm (approx. 76kV/inch) so it is easy to calculate the voltage necessary for a spark
to form in a hole through a membrane of any similar thickness. There are other effects that will change
this simple calculation at very small gaps, or much larger ones, but in the range of thicknesses of typical
materials being tested, this approximation is appropriate.
The maximum voltage that can be used is dependent on the thickness, and the dielectric strength (or
breakdown voltage), of the material. Most plastics have breakdown voltages in the range of 10 to 100
kV/mm and some exotic materials are rather higher.
Accordingly, there is generally a significant difference between the voltage required to breakdown the
thickness of the material being tested, and that required to breakdown the air within a flaw in the
material. This means that when a flaw passes between the electrodes, a current will pass, and this
current can be detected by the DCCT.
Test voltages are specified somewhat higher than the minimum limit, in order to ensure detection of non-
perpendicular faults in the material, whilst being somewhat lower than the breakdown voltage of the
material being tested. This is to reduce the risk of the test inadvertently damaging the material being tested.
The ideal approach is to identify an appropriate calculated voltage (Buckleys are happy to provide
advice) and then to test this in a range of scenarios to ensure that the desired detection level is
achieved without causing unwanted material damage.
There are published test standards to which users may refer which provide guidance on test voltages
often based on material thickness. Most standards do not provide guidance on current thresholds.
Publishers include the BSI, NACE, ASTMs and the ISO.
Corona Discharge & Material Current Draw
Whilst we associate air as being an extremely effective electrical insulator, we are also familiar with
sparks travelling through the air. Lightning is an obvious example, but the spark plugs of petrol engines
and piezoelectric gas lighters are perhaps more relevant. In these cases, the design of the system
ensures that the electrode voltage rises very rapidly, and decays almost instantly when the spark
occurs. This is not very useful for a continuous inspection system as we need the voltage to exist at all
times, with the spark to only occurring when a flaw passes between the test electrodes. Accordingly,
the HV system must produce and maintain a significant voltage difference between the test
electrodes.
There are two physical phenomena which counteract the maintenance of this voltage difference.
The first of these is
Corona Discharge
. This is a process by which the air surrounding a charged body,
such as the electrode, becomes ionised, and in so doing, moves away from the charged body. This is
often visible as a blue glow (the corona) around sharp edges. It causes a continuous flow of current
from the electrode, the magnitude of which is largely dependent on the voltage applied. The other
factor which significantly affects corona discharge is the shape of the body. Pointed items will cause a
more significant discharge than spherical ones. Electrodes with metallic brushes are particularly
susceptible to this effect.
The second is
Material Draw
. Highly insulating materials are typically able to act as capacitors, and will
hold a dielectric charge on their surface, particularly if there is a counter charge on the opposite
surface. As the material passes through the electrodes, there will be deposition of charge on the
material surface, which is drawn away from the electrode by the material passing, and this represents a
further flow of current from the electrode. The magnitude of this current is dependent on the material
being tested, the width of the electrode, as well as the velocity of the material passing the electrode.
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