A holiday test is an inspection method used to detect discontinuities in painted/coated surfaces using specialized tools and equipment. These tools, called holiday detectors, are portable devices that are swept across the coated surface.
Holidays tests work on the concept of electrical conductivity. Metal substrates are excellent conductors of electricity, and therefore allow current to flow through them. On the other hand, many coatings are poor conductors of electricity and resist the flow of electricity. Using this principle, holiday tests use instruments to locate flaws in anticorrosive paints and coatings.
During holiday testing, a ground wire and probing electrode are attached to the same power source. The ground wire is clamped to the specimen being tested while the probe is swept across the surface of the metal substrate. If the probe comes into contact with a coating discontinuity, the exposed metal completes the electrical circuit between the electrode and the grounding wire, resulting in a flow of electricity. This electricity shows up on an indicator, alerting the equipment operator of the defect.
While holiday tests are effective, they do possess limitations. Because these tests depend on the coating being non-conductive, they are not effective on conductive metallic coatings, such as zinc-rich primers.
A holiday test is also known as a continuity test.
Free Download: All About Holiday Testing
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.
Check out this video demonstrating holiday testing a pipeline:
Each industry 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:
Industry Links:
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.
Download our White Paper on Leak Testing of Tank Linings Using High Voltage
Topics: Holiday Testing
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