The manufacturers of the Seaward Live Line Tester (LLT) used by staff on the 750V DC electrified network have advised that there are a small number of units in circulation that have a manufacturing defect that can result in the tester being polarity sensitive. The result is that the tester can, depending on which contact is applied to the conductor rail, give a "dead" indication when the rail is in fact still live. As designed, the tester is not polarity sensitive and can be applied either way round.
Advice provided by Seaward is that the Live Line Tester should be proved both ways round on the proving unit to confirm correct operation prior to, and after, use. Not doing so may result in a tester with a fault appearing to indicate correctly when proved, then, if reversed, indicate incorrectly when applied to a conductor rail that is still live.