Holiday Detector Voltage Selection on a Thick Epoxy Lining
A shop is lining the interior of a carbon steel produced-water vessel with a 100% solids epoxy. The specified dry film thickness is 500-600 micrometres. The inspector must run a high-voltage holiday test on the lining per the project specification, which cites NACE SP0188 for the method but does not state a test voltage. The applicator's data sheet lists a dielectric strength of 15 kV/mm for the cured film. The inspector's holiday detector offers selectable output voltages of 500 V, 1500 V, 2500 V, and 3500 V. The vessel has been at ambient shop temperature (about 22 C) for 48 hours and the coating is fully cured per the data sheet. The inspector is asked to document the chosen voltage and justify it before testing begins.
1. How should the inspector select the test voltage, and what value should be documented?
Read the worked answer
The inspector should not simply pick the highest setting. The governing principle in SP0188 is that the test voltage must be high enough to detect a holiday through the full film but low enough that it does not damage sound coating or create a false indication.
Work the numbers:
- Maximum DFT is 600 micrometres = 0.6 mm.
- The data sheet dielectric strength is 15 kV/mm, so the breakdown voltage of the thickest sound film is roughly 15 x 0.6 = 9 kV.
- A common working rule is to test at a voltage well below the film's breakdown but above what a pinhole would withstand. A reasonable target is roughly half the breakdown voltage for the maximum DFT, i.e. on the order of 4-4.5 kV.
- Of the available settings, 3500 V is the closest below that margin and is the appropriate choice; 500 V and 1500 V are too low to reliably detect a holiday in a 600 micrometre film, and there is no setting above 3500 V to worry about over-stressing.
The inspector should document: the maximum specified DFT, the data sheet dielectric strength, the calculated breakdown voltage, the selected voltage (3500 V), and the rationale. If the specification or data sheet gives an explicit voltage formula, that governs and the calculation is a cross-check.
2. During testing at the chosen voltage, the inspector gets a spark at a location that later proves to be sound coating (no holiday). What are the plausible causes and what should the inspector do?
Read the worked answer
A false indication at a sound area usually points to one of these causes:
- Film too thin locally. If the DFT at that spot is below the minimum, the applied voltage can arc through. The inspector should gauge the DFT at the location.
- Conductive contamination or moisture. Shop dust, salt, or condensation on the surface can carry the arc. The area should be cleaned and dried and retested.
- Voltage too high for the actual local thickness. If the local film is at the low end (e.g., 500 micrometres) and the voltage is near the breakdown margin, arcing is more likely. Re-evaluating the voltage against the minimum DFT is appropriate.
- Damaged or dirty probe/electrode, or poor grounding. Check the earth connection and electrode condition.
The inspector should not simply mark the spot as a holiday and move on, nor should the inspector lower the voltage arbitrarily to make the spark disappear. The correct action is to investigate the cause, gauge thickness, clean and dry, verify grounding and equipment, and retest. Any confirmed holiday is repaired per the specification and retested. All of this is documented.