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Calculating the dielectric strength of a material
The dielectric strength is the maximum working voltage a material can withstand without breaking down.
It is normally expressed in Volts/mm. The material manufacturer should be able to supply this information but if not an approximate value can be found using a holiday detector.
Calculating the dielectric strength
- Obtain a sample of material with a uniform thickness of about 1mm applied to a sheet of metal.
- Connect the holiday detector to the sample with the earth lead connected to the metal and the high voltage probe (via a pointed probe) to the surface of the material.
- Starting with the output voltage set to minimum, slowly increase the volts until the material breaks down and the alarm on the holiday detector sounds.
- Lift the high-voltage electrode off the surface of the material and note the output voltage.
- Repeat this test a number of times on a new area of the sample at least 20mm from where any previous breakdowns have occurred, noting the breakdown voltage each time.
- Take an average of the voltages and then 75% of that is approximately the dielectric strength of the material.
So now you have a value for the dielectric strength we can look at how this relates to the test voltage calculated previously.
It is important to check, before you start testing, that the test voltage you have selected is not so high that it will actually create faults in a coating. This would rather defeat the object of holiday detection. To demonstrate this let’s look at a worked example.
Say we have a coating 2mm thick which has a dielectric strength of 8400V/mm. Using the NACE formula the test voltage is:
We know the dielectric strength is 8,400V/mm so for 2mm the maximum voltage before breakdown occurs is 2 x 8,400 = 16,800V. In this example then the test voltage of 11,180 V can be used since it is less than the breakdown voltage of the material (16,800V).
So what if the dielectric strength is below the calculated test voltage. Let’s look at the same example as was shown above, a coating 2mm thick but this time it has a dielectric strength of 5,000V/mm. Again the test voltage is calculated to be 11,180V but now the breakdown voltage of the material is 10,000 Volts (2 x 5,000). This is clearly less than our test voltage and attempting to use 11,180V to test this coating would result in the creation of more holes.
In this instance high voltage holiday detection may still be used to locate flaws in the coating, but some further testing is required to ensure that this method is valid.
Referring to the example above of a 2mm coating with a dielectric strength of 5000V/mm, the validation test would be as follows:
- Make a small hole in a test piece.
- With the electrode over the hole slowly increase the voltage until the spark jumps the gap. Note the voltage (which in this instance, on a 2mm coating, would be ~5,000 V).
- To determine the test voltage, use a value midway between the test voltage calculated using the NACE formula (in this case 11,180V) and the minimum voltage determined from the above test (~5,000V). This works out to be 8090 V. ((11,180 – 5,000) / 2) +5,000).
- Now make some more holes in the test piece (making sure there is more than 20mm between each hole), this time at angles, and using your test voltage (in our example 8,090V) ensure that it is possible to locate the faults.
This method of finding the test voltage is fine if all you are looking for is cracks in the coating (that is complete faults that go all the way through the coating to the substrate). Indeed, many standards only require this type of fault to be detected. However, with careful selection of the test voltage, it is possible to find a variety of different flaws. See our Guide to Using DC Holiday Detectors for more information.