These two sentences don't sound right. I don't think 'at hand' and 'on hand' go well when referring to small physical items like alarms clocks. I'd say 'I always set my alarm clock before I go to sleep.' I'd probably just say 'I've got an alarm clock.' I'd say 'The lifeboat was on hand to pick up survivors.' There's the colloquial 'handy'. More @Wikipedia
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