STRANGE ROAD WORK

Maintenance strategies essentially determine the length of ser-vice after which a subsystem is replaced or overhauled. If that length of time is made very short, the result is a system that is most unlikely to fail, but has unwelcome side effects. A stretch of road that is resurfaced every day never has any potholes; it also always has a sign "Road closed for repairs."

On the other hand, if the time interval is made too long, most or all subsystems will have failed before the interval is up. Since they will have to be replaced on failure anyway, that policy amounts to repairing a system only when and where it breaks down.

That, by the way, is not necessarily a bad policy. "Prevention is better than cure" is one of those folk wisdoms that are not always true. For example, you probably practice the policy in your home, and very rightly so, by replacing a light bulb only after it has failed. If you really thought prevention is better than cure, you would put in new ones in every socket of your home at calculated intervals, so as to minimize the danger of having to replace one unexpectedly.

On the other hand, you probably don't replace the oil in your car only after the engine has burnt up. Clearly when failure causes penalties far in excess of the cost and inconvenience of preventive maintenance, the latter is preferable over repair at breakdown only.

Power plants therefore always practice preventive maintenance, and while they are being overhauled, their capacity lies idle in what the industry calls "planned outages." Nevertheless, there are also "forced outages," or unplanned failures caused by anything from a tree falling across a transmission line to a major accident in a power plant. A utility needs a safety margin of spare capacity for both types of outage; but as long as it is able to meet the demand, does it matter whether it comes very close to a Dukakassia-Cuomoland brownout or avoids it with oodles of power to spare?

Yes, it does. Let me explain.