What Power Interruptions Cost, and Which Departments to Back Up

How to read this sheet

Time a restart before using this sheet, because everything on it turns on that one figure and it is never in the maintenance system. Stand in the ring department during a trip and count what it takes to get back to saleable output: the ends down, the doffs interrupted, the material run out and downgraded, the shift that never recovers its rhythm. The number is a cost per event, not per hour, and it is the reason an outage log measured in hours understates the year by a factor rather than by a margin. The comparison against fewer, longer events is the argument to take to a utility or to a board. Feeders and supply contracts are usually negotiated on availability, expressed as hours, and a mill will accept more frequent interruptions in exchange for shorter ones believing it has gained. On these numbers that trade is a large loss, and knowing so before signing is worth more than the generator. Size the backup to the departments that earn it. Restart cost is what varies, not load, so the ranking by net saving per kilowatt is rarely the ranking by size - and a plant that backs up everything spends most of its capital on the departments that would simply have started again. What is not modelled. Utilities and humidification are excluded and should not be entered as a department with no contribution of their own: they gate everything downstream, so a mill that backs up spinning without backing up its humidification has not backed up spinning. Add their load to the department they serve. Nothing here prices the quality consequence of a stop beyond the restart figure entered, and on continuous processes - a stenter, a continuous range, a bath in progress - that consequence can exceed everything else on the sheet. Voltage dips too shallow to register as an interruption are outside it and on a weak feeder they are frequently the larger problem. And a generator that has to be started is not a generator that prevents the ends coming down; only stored energy does that, which is a different and dearer machine than the one this sheet prices.