PHP Help: Very easy points!! This may be obvious, but not to me, help an idiot please.

I am trying to get all records where the difference between records is more than 30 minutes. I've tried:
SELECT enquiry_id, time_locked, locked_by
FROM candidate_enquiry
WHERE SUBTIME( time_locked,  '11:51:00' ) >= 30

AND

SELECT enquiry_id, time_locked, locked_by
FROM candidate_enquiry
WHERE TIME_FORMAT( SUBTIME( time_locked,  '11:51:00' ) ,  '%i' ) >= 30

None of these seem to bring out any records. What am I not doing
nicky2kWeb DeveloperAsked:
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