Why holidays matter
In immersion and buried service, a single pinhole concentrates the entire corrosive demand of the environment on a point of bare metal — the small-anode configuration that perforates fast. A lining that is 99.99 % continuous is not "almost right"; it is a lining with a scheduled failure site. Holiday testing exists because visual inspection cannot see these defects.
Low voltage or high voltage
Low-voltage wet-sponge testing suits thin films: a damp sponge and a modest voltage detect breaches through conductive contact. High-voltage spark testing suits thick linings and pipeline coatings: a pulsed voltage — set according to the coating thickness and the test standard your specification cites — jumps a spark at breaches. Voltage selection is not a detail: too low misses defects, too high can create the very pinholes it is meant to find in a marginal film.
When to require it
For tank linings, immersion service and buried or wrapped piping, a holiday test at handover — and after any repair — should be a standing requirement, with the test standard, voltage basis and repair-and-retest loop written into the specification.