A coating must be applied as a continuous film to perform its intended function. Early failure will occur if there is a discontinuity—a holiday, as it is commonly called—such as a pinhole, void, crack, thin spot, foreign inclusion, or contaminant in the coating film. Many of these defects are not readily visible, but they can be located using holiday detection equipment. Holiday detection is typically performed on coating systems designed for critical service such as immersion or chemical storage. It is also conducted on coating systems applied to steel that is in contact with soil and/or constantly wet. Examples include buried pipelines, the undersides of tanks, and sheet pilings Holiday detectors are instruments that use electricity to locate film discontinuities. Most coatings are poor electrical conductors, and so they act as insulators. On the other hand, a metallic substrate such as steel is conductive to electricity. A holiday detector consists of a power source, a ground wire, a probing electrode, and an indicator. (Note: the terms “ground” and “grounding” in this article are synonymous with “earth” and “earthing.”) Current would flow if the leads of the ground wire and the probing electrode were attached to the power source and their tips were touched. If the ground wire was connected to a coated piece of steel and the probing electrode was placed on the coating surface, the coating would act as an insulator, and no current would flow. However, if a holiday was present in the coating, there would be a pathway for the current to flow. Holiday detectors have an indicator such as a sound or a light to tell when current is flowing. So, when the indicator shows current flow, it means a holiday is present.

Low Voltage Holiday Detection

Low-Voltage Holiday Detectors Low-voltage holiday detectors are used on coatings that are thinner than 500 microns (20 mils). They are powered by a self-contained battery with a voltage that ranges from 5 to 90 volts direct current, depending on the manufacturer. Low-voltage holiday detectors have either a buzzer or a light to show when current is flowing.

High Voltage Holiday Detection

High-Voltage Holiday Detectors High-voltage holiday detection is used when a coating is thicker than 500 microns (20 mils). The basic components of high-voltage detectors still include a power supply, ground wire, probing electrode, and indicator. However, the power supply for these units will provide thousands or tens of thousands of volts, and the probing electrode will be made of copper wires or carbon-embedded rubber.

Holiday Testing on Concrete Substrates

Coatings applied to concrete substrates can also be tested for discontinuities using either low-voltage or high-voltage holiday detectors, depending on the coating thickness. Concrete can be conductive depending on its moisture content, the type and density of concrete, and the location of the reinforcing steel (rebars or wire mesh).