dacs04 Mar 23, 2012:

to the Asker In your previous question about 3РУ - three РУs (22 March), there were РУ with other numbers mentioned (like 10РУ). There was an explanation in the context provided by you that РУ means "указательное реле" - a type of relay, which not simply trips in certain cases, but also somehow indicates the reason for being tripped. The best translation I know is "indicating relay".

Of course, in this fragment, "РУ" can have a different meaning.