Answer:
The idea is that if the critical branch fails during the presence of utility power, the area will not be blacked out. If more than one critical ATS exists, then a critical care area can be fed from both branches and normal power is not required.
Explanation:
i) Life safety is required to feed exit signs; emergency lighting in hallways, stairwells, points
of egress; evacuation speakers; fire alarms and medical gas alarming systems, automatic
doors used in evacuation, elevator cab lights and lighting/outlets at generator and ATS
locations... basically what is required to contain and evacuate occupants from a burning
building.
ii) Critical feeds patient room lighting, patient bed outlets, surgical theaters/operating
rooms, blood and bone banks, pharmacy dispensing machines, task illumination... basically
anything essential to patient care and life support.
iii) Equipment feeds HVAC equipment, elevators, pumps, medical gas equipment... anything
that is needed in the continuation of basic medical care but not immediately vital.
UPS or a tertiary system is not mandated by code and only found in some newer hospitals seen when
feeding IT equipment, MRIs, etc.
The US code also requires that critical care areas have a certain percentage of receptacles on
normal power (typically 1/3 the outlets at a patient bed location, OR, ICU, etc) and that some level
of lighting is connected to the normal branch. The idea is that if the critical branch fails during the
presence of utility power, the area will not be blacked out. If more than one critical ATS exists, then a
critical care area can be fed from both branches and normal power is not required.