The 36 pads are divided into 6 circuits, and each circuit is evenly distributed on the glass. Loss of one circuit would be no problem. Loss of two circuits will increase the glass and operator stress into the uncomfortable range. Loss of three circuits does not precipitate doom with certainty but four does. The pumps run continuously and with power loss auxiliary reserve tanks are automatically engaged.
Power can be off for at least a day and probably a week before things get critical.
This should be enough time to lower the mirror or find a portable generator. Lots of glass has been handled with these devices but we are still a little paranoid. We have special shaped pads that match our radius of curvature and a thick waffle pad to be sure. The stress from the edge reaction of a disk would be worrisome to any curved thin plate, but if the vacuum is locally reacted against there is little problem. We thoroughly tested the damage to glass, optical coatings, applying optical coatings afterwards and lifting with protective coatings between the glass and pads.