Voltage Selection and Coating Thickness

Checkline Europe's Technical Support 10 Jul 2026

The single most consequential setting on a holiday detector is the test voltage, and it is not a number carried over from the last job. It follows from the coating: how thick the film is, what it is made of, and which standard governs the work. This page explains how that figure is derived and why it has to be re-established for each coating system.

Set it too low and the detector sweeps clean over real defects; set it too high and it drills fresh holidays into sound film. Everything downstream — a defensible result, an undamaged coating, a report an auditor will accept — rests on getting the thickness right first and applying the specified rule to it.

1. Why the setting cannot be guessed

The detector is trying to bridge a defect in the coating without breaking down intact film. That means the correct voltage sits within a practical window that changes with coating thickness, coating material and the standard being followed. Too low and defects may be missed. Too high and the inspection can create fresh damage.

Because of that balance, voltage selection should always be based on measured dry film thickness and the formula or table required by the project specification.

2. How thickness affects the decision

The starting point is a trustworthy thickness reading. If the dry film thickness is wrong, the voltage logic that follows will also be wrong. Where coating thickness varies across the asset, the thinnest relevant areas usually deserve the most attention because they are the first places that become vulnerable to induced damage.

Multi-layer systems add another layer of caution. Even when the total build is known, differences between layers, edges and local geometry can change how safely the coating handles the selected voltage.

3. Where inspectors usually go wrong

Thickness data is weak: the setup starts from unreliable dry film thickness readings.
The wrong rule is used: the operator follows a familiar formula rather than the project requirement.
The coating system is oversimplified: local thin areas, geometry changes or multilayer effects are ignored.
The setting is not verified: the instrument is assumed to be outputting the chosen voltage without a proper check.

4. Why this matters in real jobs

On pipelines, tank linings and structural steel, the voltage decision directly affects whether the continuity test produces meaningful evidence. A careful setup can prevent both missed defects and unnecessary coating damage, which is why voltage selection sits so close to the center of competent holiday detection practice.

6. Next Step

If coating build and voltage-setting logic are now clear, the next step is choosing the detector around that testing requirement.

Choose a Holiday Detector if you are ready to compare detector options after confirming the required holiday-testing method and coating range.

7. Frequently Asked Questions

1. Why is the minimum dry film thickness often used for voltage calculation?

Because the thinnest relevant area is the most vulnerable to induced damage if the voltage is too high.

2. Can the same voltage be used for all coatings of the same thickness?

No. Coating chemistry, build structure and specification rules can change the correct decision even at similar thickness levels.

3. What if the coating thickness varies a lot across the asset?

The inspector may need a more cautious setup strategy or zone-based logic rather than treating the whole asset as uniform.

4. Why should the instrument output be verified?

Because the intended setting is only useful if the detector is actually delivering it in service conditions.

5. Do different standards give the same voltage for the same thickness?

Not necessarily. NACE SP0188, ASTM D5162 and a coating manufacturer's data sheet can each specify a different voltage rule, so the governing specification — not the most familiar formula — decides which one applies to the job.

8. Glossary

Test Voltage	The voltage applied during continuity testing, chosen to suit coating thickness and the governing rule set.
Dielectric Strength	The voltage a coating can withstand before electrical breakdown occurs.
Dry Film Thickness	The measured thickness of the cured coating.
Induced Damage	Damage created by testing rather than by a pre-existing coating defect.
Voltage Formula	A rule or equation used to convert coating thickness into a holiday-test voltage.
ASTM G62	A standard commonly used for holiday testing on suitable coatings over conductive substrates.
ISO 29601	An international standard covering holiday detection methods and related setup guidance.