If the displayed voltage drops sharply when the probe is applied to the test surface or the alarm sounds continuously, then the coating may be conductive. The usual occurrences of conductive coatings are described in the following:
Existence of Metallic, Carbon or other Conducting Particles in the Coating
During normal use the particles in this type of coating are not electrically or physically linked. However, when the coating is subjected to high voltages, the material between the particles can break down. This results in the coating becoming conductive and the detector indicating the presence of a flaw. To overcome this effect the voltage should be reduced so that it is still high enough to detect flaws but low enough to avoid break down of the coating. However, in some cases, the coating will still conduct at voltages that are too low to locate a flaw. In this case, the holiday detector is not a suitable method for checking the coating and the UV torch may be considered.
Surface Moisture or Contamination
Soluble salts attract moisture from the atmosphere and this, along with other forms of surface contamination, can form a path across the surface to the high voltage that is not due to a coating flaw telegraphing. Under these conditions the detector indicates non-existent flaws. When these circumstances occur, the surface should either be dried using a suitable cloth or cleaned with a non-conducting cleaner or solvent which will not damage the coating.
Rubber linings may be slightly conductive due to their carbon content. As with other conductive coatings, reduce the sensitivity so that the detector indicates a known flaw but does not sound when the probe is placed on a flawless coating.
Coating May Not Be Fully Cured
If the coating is not fully cured it may still contain solvents which allow the path to the current to be formed, even if a flaw is not present. To overcome this problem, allow the coating to cure before undertaking the test.
If a concrete or cement substrate contains enough moisture, then it will conduct electricity and the holiday detector can be used to detect flaws in its coating. It should be noted that hammering a masonry nail or similar conducting spike into the concrete or cement makes the earth signal return contact.
The suitability of the concrete for use with a holiday detector can be checked as follows:
- Make a high voltage return contact by hammering a nail or similar into the concrete
- Attach the earth signal return cable to the nail
- Set test voltage for the thickness of coating (or in the range 3kV - 6kV if the test voltage is not known) and set the sensitivity to maximum (5μA current)
- Place the probe on uncoated concrete about 4 metres (13 feet) from the nail. If the alarm sounds, then the concrete is sufficiently conductive.
If the concrete is too dry and the alarm does not sound, then it is unlikely that the holiday detector will be a suitable inspection method.