RECLAIMING THE LAST FOUR HOURS
The UPS industry has a secret, and it’s four hours long. That’s the average length of time that the UPS and
switchgear must be completely de-energized back to the service entrance each year for maintenance.
There’s no way to escape this downtime. Remember: Deferred maintenance is NOT the same as high availability.
This does not mean that your critical load will not have access to power during those four hours. It simply means that most (but not all) system configurations will expose your critical load to unconditioned generator or mains power during that interval. This unconditioned power is politely referred to as “bypass” power.
Those four hours of unconditioned power cast a dark shadow over your system availability calculations. Therefore, for the remainder of this paper, the term “availability” will refer to the availability of power that has been conditioned by the UPS and is backed up by a reliable battery system.
The good news is that certain UPS system configurations permit concurrent maintenance – supporting the load equipment on conditioned power while de-energizing one complete power system back to the service entrance. We will describe these system configurations in later sections.