Holiday Detection

Holiday detection is a test used to determine discontinuities in a coating.

Holiday detection, also known as spark testing, is a non-destructive testing (NDT) method used to detect and locate defects such as pinholes, voids, or thin spots in protective coatings on conductive substrates. This technique is commonly employed in industries such as pipeline construction, tank fabrication, and infrastructure maintenance, where the integrity of protective coatings is crucial for preventing corrosion.

Here are the key features and aspects of holiday detection.


Holiday detection is based on the principle that an electrically conductive coating, when properly applied, creates a barrier on a conductive substrate. If there is a discontinuity or defect in the coating (a “holiday”), an electric current can pass through the defect, allowing it to be detected.


Holiday detection equipment typically includes a high-voltage, low-current power source, a probe or brush electrode, and a voltage detection unit. The power source generates a high-voltage, low-current electrical charge, and the probe is used to apply the charge to the coated surface.

Dry Film Thickness Considerations:

The effectiveness of holiday detection can be influenced by the thickness of the coating. Thicker coatings may require higher voltage settings to penetrate the film and detect defects effectively.


Holiday detection is commonly used in the inspection of coatings on pipelines, storage tanks, bridges, and other structures where protective coatings are applied to prevent corrosion. It is an essential step in quality control during the coating application process. Suitable for Coated Surfaces

Wet Sponge and High Voltage Method:

One common method of holiday detection involves using a wet sponge as the electrode. The sponge is soaked in an electrolyte solution, such as water or a specialized conductive gel. The wet sponge is then applied to the coated surface, and the high voltage is applied through the sponge. If there is a defect in the coating, the current will flow through the defect, creating a visible spark or audible signal.

Spark Testing:

Spark testing refers to the visual or audible detection of sparks when the voltage is applied to the coated surface. The size and intensity of the spark can indicate the severity of the defect. Automated spark testers may also provide numerical readouts of the defect size.

Quality Control

Holiday detection is a crucial aspect of quality control in coating applications. It helps ensure that the protective coating is continuous and free of defects, minimizing the risk of corrosion and ensuring the long-term durability of the coated structure

Coating Types:

Holiday detection is applicable to various types of coatings, including epoxy, polyurethane, rubber linings, and other electrically insulating coatings applied to conductive substrates. It’s important to note that holiday detection should be conducted in accordance with industry standards and guidelines to ensure accurate and reliable results. The method chosen may depend on factors such as the type of coating, substrate material, and specific inspection requirements.