Tony M Nov 25, 2009:

Yes, but that's different... There, you have the true plural of 'main (something)' — like 'main circuit breaker unit', of which the plural would normally be 'm. c. b. units', but if they abbreviate the whole thing to just 'main', then obviously, the plural becomes 'mains'; but this not the same as 'the mains' (electricity), which is in any case a rather informal, non-technical layman's term, which ought to be avoided as far as possible in formal technical documents.

What you actually have here is different types of units: 'main units', 'feeder units', 'direct on line starter units' — see what I mean? When an adjective, it remains singular, but when the adjective is used as short-hand for he whole noun expression, then it takes the plural.

Cf.

"mains switches need to be well-insulated" (= switches for use on mains electricity)

"main switches need to have a high enough current rating" (= principal / master switches for a system)

samy_trad (asker) Nov 25, 2009:

Thank you for answering, in fact it also appear with an "s", see "Mains with fusible disconnects are available" or "different types of units, such as mains, feeders, direct on line starters ". It deals with Motor Control Centers.