Three reliability categories per PUE/IEC
PUE Chapter 1.2 defines 3 power supply reliability categories. The choice is not at the designer's discretion — it's mandated by regulations and technological necessity.
Category I (critical)
Requirements: two independent mutually-reserved sources + third independent (for special group). ATS with transfer time ≤ 0.5 sec. Diesel generator often added.
Consumers: operating rooms, ICUs, fire pumps, smoke exhaust, data centers Tier III+, continuous-cycle industrial equipment.
Cost: highest. Two inputs + generator + ATS increase panel cost 4-6× vs Category III.
Category II
Requirements: two independent sources. Switching can be manual or automatic (ATS not mandatory). Interruption during switching — up to 1 hour for manual.
Consumers: residential buildings >5 floors, schools, shopping centers >2000 m², offices >50 people, non-continuous production lines.
Implementation: two inputs with ATS or manual transfer switch.
Category III
Requirements: one source. Interruption for repair — up to 24 hours.
Consumers: private houses, apartments in low-rise buildings, small shops, garages, temporary structures.
Common mistake: over-specifying category
Doubles panel cost, adds unnecessary cable — more elements = more potential failures.
Common mistake: under-specifying category
Project rejection, regulatory orders, risk of full outage on single-input failure.
Practical approach: 10-floor residential building
- Fire pumps: Category I
- Elevators (<17 floors): Category II
- Emergency lighting: Category I
- Apartments: Category II
Design: two inputs to main panel. Fire pumps and emergency lighting with ATS. Rest — sectioned across two inputs with tie breaker.