Inspection And Test Methods For Coating And Lining

Electrical high-voltage holiday detection is a simple method for testing coated or lined equipment using Holiday / Spark Test Detectors which are instruments that use electricity to locate film discontinuities.


Why Is Holiday Testing Important?

A holiday is an imperfection or discontinuity in the coating or lining film such as a pinhole, void, crack, inclusion or contaminant. A high proportion of these defects are not visible to the naked eye but can be found and located by the use holiday / spark test detection equipment. Holiday detection is typically carried out on items that are designed for critical service such as immersion, chemical storage or transportation as a coating or lining failure could lead to catastrophic event if not detected.

How Does High Voltage Holiday Testing Work?

The vast majority of coatings and linings are poor electrical conductors and as such act as insulators. However steel items are conductive to electricity.

A holiday /spark test detector consists of a power source, a ground wire, an electrode (typically a drum brush type), and an indicator. Current flows if the leads of the ground wire and the electrode are attached to the power source and their tips are touched. When the ground wire is connected to an uncoated part of a coated piece of steel and the electrode is placed on the coated surface, the coating acts as an insulator and no current flows. However, if a holiday is present in the coating, there is a pathway for the current to flow. Holiday detectors have indicators both audible and visual to reveal when current is flowing and a defect located.

Calculation and selection of the correct voltage is critical when using high voltage 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.


✓ 100% inspection of the coated or lined items surfaces
✓ Usually invisible to the naked eyes defects can be easily detected
✓ Both newly applied or aged coatings and linings can be tested
✓ Test Equipment is easily portable


× Voltage control is user dependant – high voltages may damage a coated or lined surface, whereas too low a voltage may not identify defects
× High Voltage Holiday (Dry Spark) Detection is not compatible with damp, dirty, or soiled surfaces
× Not compatible with DSEAR (potentially explosive atmosphere) environments due to spark generation

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