Hi Tim, not sure what 'ferrous enclosures' you have in mind presume sw.gear to trunking or conduit as that would be most used case for what is called a 'High Current Bond Test' in regs.
Also used to confirm metal-to-metal continuity of pipes, ducts, ceiling grids ect.
Reason for carrying out test at 25A is that research has shown that continuity tests at much lower currents such as from a multi-meter may well show a low resistance path, but if called upon to carry an earth fault current of many amps the joints resistance rises rapidly with time/temp. and fails to maintain 'equipotential between parts.
The 50Volts bit is chosen for safety of personnel during such testing as P.D. of more than that between different parts could arise and cause injury to someone unlucky enough to be in contact with them during the tests.
Trust I have covered what you wanted to know but if not please come back with more detail,
Ferrous enclosures have one thing different to other metal enclosures in that they can have magnet effects and running feed and return through different holes can generate heat so 521.5.2 tells us how it should be arranged like a big tube but I canâ€™t find anything about a 25 amp test.
25 amp is the current normally used by PAT testers to check earth wires are good enough.
50 volts a.c. is considered as the maximum volts the human body can touch when dry and cause no permanent damage.
May be you can expand a little and tell us what tin boxes need testing?
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