When dealing with critical systems such as servers and associated equipment we work on N+1, where N is the number of backup power sources and Uninterruptable Power Supplies necessary to maintain power during any eventuality. +1 is the addition of an extra module to backup the first alternative power source and UPS. The =1 model ensures that there will be no time that all systems can possibly fail at the same time. It also creates an environment where maintenance and servicing can be conducted in a systematic way safe in the knowledge that a backup system still remains for every unit undergoing maintenance.
Tier 1 systems just comprise a single power path, cooling and ventilation system with no redundant components.
Tier 2 systems comprise a single main active power supply, cooling and ventilation systems with redundant power and UPS.
Tier 3 systems have multiple active power supplies, cooling and ventilation systems with multiple redundant components.
Finally a Tier 4 system will have multiple active power supplies, cooling and ventilation with redundant components that have additional fault tolerance.
To learn more about power systems, the use of UPS and other backup power systems, take a look at our Network and Systems training course at www.nstuk.com/course-schedule.html