The Importance Of Holiday/Spark Testing

A coating or lining 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, area of low dry film thickness, inclusion or contaminant in the coating or lining film. Many of these defects are not seen by the naked eye, but they can be located using holiday/spark test detection equipment. Holiday detection is typically performed on coating systems designed for critical service such as immersion or chemical storage.


Holiday / Spark Test Detectors are instruments that use electricity to locate film discontinuities. Most coatings are poor electrical conductors and as such act as insulators. However, a metallic substrate such as steel is conductive to electricity. A holiday/spark test detector consists of a power source, an earthing wire, a probing electrode, and an indicator. 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 earthing 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 indicators both audible and visual to reveal when current is flowing.

Here at KUE Group we utilise two methods of Holiday/Spark Testing – the first being the Wet Sponge Method; the second being the more widely used High Voltage DC Method.

Wet Sponge Method

A low voltage, typically 9v is applied to a moist sponge, when the sponge is moved over a coating flaw such as a pinhole, the liquid penetrates to the substrate and completes the electrical circuit, triggering an audible alarm and the alarm LED to flash. KUE Group use the Wet Sponge Method where specified or the dry film thickness is typically below 200 microns.

High Voltage DC Method

A high DC voltage, minimum of 1kv (1000v) is passed through the selected probe, typically a drum brush type or circular brush and passed over the coated or lined surface, should a flaw such as a pinhole, void, crack, area of low dry film thickness, inclusion or contaminant be located then the electrical circuit would be complete, again triggering an audible alarm, the alarm LED to flash and, if at a high voltage, a visible spark.

A high-voltage holiday detector causes the air between the drum brush/circular brush and the substrate to conduct electricity or break the air gap in the holiday.

Calculation and selection of the correct voltage is critical when using high voltage DC Holiday/Spark Testers as too high a voltage may actually cause a defect rather than locate it. It is imperative that the dry film thickness of the coating or lining to be tested is determined together with the coating or linings insulation properties. A general rule of thumb within the industry is acknowledged as 4 volts per micron or material being tested – for example, a coating or lining applied to a dry film thickness of 1mm (1000 microns) would be tested at 4kv [1000 microns x 4 volts]. The speed of the test is also critical and should be carried out at a rate of approximately 0.3m/second.


Holiday/Spark Test Detection is vital in finding and locating coating and lining film discontinuities that are not visible to the naked eye and form an essential part of the coating and lining inspection procedure particularly when the item being treated is handling, processing or storing hazardous materials.

To find out more about the importance of holiday/spark testing please contact one of our experienced team by calling 01274 721188 or by emailing