What is one limitation of low voltage holiday detectors?

One limitation of low voltage holiday detectors is that they tend to be effective for detecting holidays (or discontinuities) in coatings that are relatively thin, typically limited to coatings that are around 20 mils or less in thickness. This is primarily because their detection ability diminishes as coating thickness increases; thicker coatings can insulate the conductive path needed for the detector to identify the absence of coating.

Essentially, if a coating exceeds this thickness, the likelihood of detecting a holiday decreases significantly as the electrical current may not be able to penetrate effectively through the thicker material. Therefore, while low voltage holiday detectors are useful tools for assessing the integrity of coatings in certain applications, their effectiveness is constrained by the thickness of those coatings.