Lockout (also tagout if all that is done is affix a tag to a valve, switch, etc. informing people not to operate it) is an operation performed prior to maintenance on systems where there would be danger if someone were to turn apparatus on during the operation. There is bound to be some administrative papertrail attached to it. Whether this involves "certificates", "cards", etc. I don't know.

The best I know for "déconsignation" is "removal of lockout" (the AFNOR nuclear dict. gives "removal of administrative lockout"), but it would be nice if s.o. could find sth. snappier.

"Card" - and hence "green card" - will make it sound like a soccer match or US immigration.


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Note added at 34 mins (2006-12-05 01:42:54 GMT)
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Lockout card: A card indicating the procedure to be followed to lock out equipment. Each time it is used and equipment is locked out, the person responsible for the lockout signs the card and indicates the date of the lockout.
http://www.uottawa.ca/services/immeub/eng/lockout.htm
[site gives lots of info on lockout procedures]