Principle of Operation
The D.C. Holiday Detector works by detecting the current flow in a circuit. The circuit is from the high voltage output of the Holiday Detector to the handle and the to the material under test. The material is applied to a conductive substrate either steel or concrete (Concrete is conductive due to the water contained in it) which is connected back to the Holiday Detector by its earth lead. If there are no faults in the membrane then no current will flow. When the electrode passes over a fault the high voltage jumps the gap between the substrate and the electrode and a current flows, the Holiday Detector detects this and activates its alarm. The membrane stops the flow of current and therefore must be non-conductive.
Test Voltage
The level of test voltage used depends on the type of fault you are trying to find and the electrical properties of the membrane material. As a general rule the NACE standard RP-02-74 has been found to work well-in most cases. The formula can be expressed two ways:
Test Voltage formula using thousands of an inch:
or Test Voltage formula using microns:
Dielectric Strength
This is the maximum working voltage a material can withstand without breaking down; it is normally expressed in volts per millimetre. The material manufacture should be able to supply this information but if not, an approximate value can be found using the Holiday Detector and following these steps:
1. Obtain a sample of material of uniform thickness (about 1mm) applied to a sheet of metal.
2. Connect the Holiday Detector to the sample, earth lead to the metal, high voltage probe via a pointed electrode to the surface of the material.
3. Starting with the output voltage set to minimum, slowly increase it until the material beaks down and the alarm on the Holiday Detector sounds.
4. Lift the electrode off the surface and note the output voltage when the material broke down.
5. Repeat the above a number of times on a new area of the sample at least 20mm from where any break downs have occurred before, noting the break down voltage each time.
6. Take an average of the voltages and then 75% of that is approximately the Dielectric Strength of the material.
If it is not possible to break down the material then reduce the thickness by half and try again. Results obtained vary from 8kv to 40kv per mm.
For a material to be tested using high voltage its dielectric strength must be greater than 4kv per mm preferably 8kv per mm. The greater the dielectric strength, the less chance there is of making a fault.
How to test for different Types of Fault
These are the easiest faults to find as they require the minimum test voltage. If the material has a low dielectric strength of between 4 and 8kv, then the test voltage should be chosen to be between the material’s dielectric strength and the minimum necessary to find the fault.
To find the minimum voltage required to find a fault:
1. Obtain a sample of the material of the maximum thickness expected, applied to a sheet of metal.
2. Carefully make a number of holes in the material at least 20mm apart and from the edge.
3. Connect the Holiday Detector to the sample, place the electrode over one hole only and slowly increase the voltage until the alarm sounds.
4. Remove the electrode from the surface and note the voltage.
5. Repeat using a new hole each time, take the highest voltage obtained as the Minimum Voltage.
6. A new hole has to be used each time, as once the spark has made contact with the material it leaves a mark that makes it easier for the voltage to find it the next time.
Example:
* Sample Thickness = 1mm (applied evenly to a sheet of metal)
* Minimum Voltage = 3.2kv (Found using above test)
* Dielectric Strength = 5kv (Found from test in dielectric strength section or the material manufacturer)
* Test Voltage = 4.1kv (Between the Dielectric Strength and the Minimum Voltage)
It must be remembered that if the thickness of the material decreases too much the test voltage may exceed the material’s Dielectric Strength and make a fault in an otherwise good coating. Or, on the other hand, if the thickness increases too much the test voltage will be too low to find the fault.
Faults that go nearly through the membrane
These are Thin areas, Blow holes, Air Bubbles, Inclusions and Burrs. If it is required to find this type of fault, it can be achieved if the fault is large enough i.e. a Blow Hole going 90% of the way through the material or a Burr 75% the thickness of the coating. The Dielectric Strength (see Dielectric Strength above) needs to be known.
Example:
* Material Thickness = 2mm
* Dielectric Strength of material = 15kv (Found from test in dielectric strength section or the material manufacturer)
* Dielectric Strength of Air = 3kv / mm approx
* Normal Test Voltage = 11.1kv
It is required to find the above mentioned type of faults if they exceed 50% of the coating thickness.
Test Voltage to find Blow Holes and Air Bubbles.
* Air Gap between electrode and substrate =50% of Material Thickness = 1mm
* Voltage needed to jump Air Gap = Dielectric Strength of Air 3kv x 1 = 3kv
* Thickness of Material = 50% of Material Thickness = 1mm
* Dielectric Strength of 1mm of Material = 15kv
* Test Voltage = 15kv + 3kv = 18kv
Test Voltage to find Burrs, some Inclusions and Thin Areas
* 50% of Material Thickness = 1mm
* Dielectric Strength of Material = 15kv
* Test Voltage = 15kv
* Note:- The inclusion must be conductive to be located.
* As the first Test Voltage was 18kv this would be used and should find Burrs, Inclusion and Thin Areas below 1.2mm
* Minimum Material Thickness = Test Voltage divided by the Dielectric Strength 18kv / 15kv = 1.2mm
Membranes of uneven thickness
The larger the variation in thickness the higher the Dielectric Strength needs to be:
Example 1:
* Maximum Material Thickness = 3mm
* Minimum Material Thickness = 1mm
* Dielectric Strength of Material = 15kv /mm
* Test Voltage for 3mm using NACE formula = 13.7kv
* Thickness of Material to withstand 13.7kv = .91mm
No problems should be encountered.
Example 2:
* Maximum Material Thickness = 3mm
* Minimum Material Thickness = 1mm
* Dielectric Strength of Material = 10kv /mm
* Test Voltage for 3mm using NACE formula = 13.7kv
* Thickness of Material to withstand 13.7kv = 1.37mm
Problem with Test Voltage blowing holes in the thin areas of the coating. Reduce the Test Voltage to 9.5kv and check on a test sample of 3mm thick that faults are located.
