「機会の点検・保守作業を行うとき、必ず主電源を切ること。」という一文の英訳としては、

Shut off source power while inspected and maintained.

よりも、

shut off the source of power while inspections and maintenance operations are being performed.

の方が適切で、さらに厳密に言えば、

In anticipation of (or 「before」) periods during which inspections and maintenance operations are to be performed, the source of power should invariably be switched off.

とした方がさらに良いでしょう。