Holiday testing is a non-destructive test method applied on protective coatings to detect unacceptable discontinuities that affect the integrity of linings such as pinholes and voids. The test involves checking an electric circuit to see if there is insufficient coating to resist the electrical charge and current flows to complete the circuit. If electrical flow is detected then it indicates the presence of a pinhole or a holiday (discontinuity). Holiday testing is also known as continuity testing.
A holiday detector is an electrical device used to conduct the holiday test described above. There are various techniques and equipment available for holiday detection. It can be performed by applying low voltage or high voltage across the chosen path depending on the coating thickness. Low-voltage holiday testing is used when the coating system is less than 500 microns (20 mils) thick. This is typically done using the wet sponge method. High-voltage holiday testing is used when the coating system is thicker and employs device producing a high voltage discharge. High-voltage holiday testing requires special care not to damage the coating or cause injury to the operator.
Key applications for holiday testing include finding discontinuities in thin coating films , piping and structures, storage tank interiors, and rubberized waterproofing on concrete. Industries that use holiday testing include oil/gas, transportation, construction, tank manufacturers, and coatings/inks/adhesives suppliers.
Each inustry has different guidelines on best practices and standards (e.g. ASTM) that need to be met on lining/coating preparation, inspection and repair. Here are some interesting links we have found that may be helpful to you:
ASTM Holiday Testing Standards:
Electro-Technic Products offers a high-voltage holiday detector (model BD-50E) for customers looking to test thicker linings (1/4 inch and up) and a High/Low Voltage detector (model BD-60) for customers who need flexibility and features such as audible and visual alarms when holidays are detected.