General Standard for Holiday Testing of Coatings: Procedures for Low and High-Voltage Testing

General Standard for Holiday Testing of Coatings

Holiday testing, also known as holiday detection or voltage indication testing, is a type of quality control procedure used to identify defects in protective coatings on pipelines, tanks, and other structures. These defects, known as "holidays," are areas where the coating is not present and the underlying substrate is exposed to the environment. Holiday testing helps ensure that coatings are applied consistently and effectively, and can detect holidays that may not be visible to the naked eye.


The general standard for holiday testing of coatings is defined by a number of industry organizations, including NACE International (formerly known as the National Association of Corrosion Engineers) and the American Society for Testing and Materials (ASTM). These standards provide guidance on the appropriate procedures and requirements for holiday testing of coatings, including the voltage and current levels to be used, the minimum acceptable level of coating coverage, and the acceptable level of holiday detection sensitivity.

Procedures for Low-Voltage Testing

Low-voltage holiday testing is a common method for detecting holidays in coatings with a dry film thickness of less than 50 mils (1.27 mm). This method uses a holiday detector with a voltage range of 50 to 500 volts and a current range of 1 to 10 milliamperes. The operator moves the holiday detector probe over the surface of the coating, and the presence or absence of coating is indicated by the holiday detector. If the coating is continuous and the probe is in contact with it, the electrical current will flow from the probe to the ground rod. If there is a holiday, or area where the coating is not present, the electrical current will not be able to flow and the holiday detector will indicate the presence of a defect.


Low-voltage holiday testing is generally less sensitive than high-voltage testing, but it has the advantage of being less likely to damage the coating or the substrate. It is also less prone to false positives due to stray currents, and is less affected by surface roughness or other factors that may interfere with the flow of electrical current.

Procedures for High-Voltage Testing

High-voltage holiday testing is a method for detecting holidays in coatings with a dry film thickness of 50 mils (1.27 mm) or greater. This method uses a holiday detector with a voltage range of 1000 to 30,000 volts and a current range of 1 to 10 milliamperes. The operator moves the holiday detector probe over the surface of the coating, and the presence or absence of coating is indicated by the holiday detector. If the coating is continuous and the probe is in contact with it, the electrical current will flow from the probe to the ground rod. If there is a holiday, or area where the coating is not present, the electrical current will not be able to flow and the holiday detector will indicate the presence of a defect.


High-voltage holiday testing is generally more sensitive than low-voltage testing, and is able to detect smaller holidays that may not be detectable with low-voltage testing. It is also more effective at detecting holidays in coatings with higher film thicknesses. However, high-voltage testing has some drawbacks as well. It is more prone to false positives due to stray currents, and may also damage the coating or the substrate if the voltage is too high. It is important to follow the manufacturer's recommendations for high-voltage holiday testing to ensure that the coating is not damaged and that accurate test results are obtained.


In conclusion, holiday testing is a crucial quality control procedure for ensuring the effectiveness of protective coatings on pipelines, tanks, and other structures. Both low-voltage and high-voltage testing have their own advantages and disadvantages, and the appropriate method to use depends on the specific application and requirements. It is important to follow industry standards and manufacturer's recommendations for holiday testing to ensure accurate and reliable test results.