Uninterruptible AC output power supplies should include the following
additional requirements that are unique to this type of equipment:
Hold-Up Time - During input power interruptions, the UPS derives its power
from an energy storage device. Hold-Up time is the parameter that
specifies the UPS capacity to sustain load with no input power present.
Hold-Up time is directly related to the capacity of the storage device.
Therefore, by varying the storage capacity, UPS systems may be configured to
support power interruptions ranging between milliseconds and hundreds of
Most energy storage devices are extremely environment-sensitive. For
example, the capacity of some batteries falls below 10% in temperatures under
-20'C, and effects of aging and frequent power cycling are almost as
pronounced. Therefore, specifications for Hold-Up time should include:
- Hold-Up Time as a function of load,
- operating temperature
- expected frequency of power interruptions, and
- life expectancy or battery replacement
Recharge Time - An UPS returns to normal operation when the input power is
restored. In anticipation of the next power outage, the depleted energy
must be replenished. The period required to return the storage element to
90% of its full capacity is defined as the Recharge Time. Recharge times
normally vary between 30 minutes and 36 hours as a function of Hold-Up capacity
and UPS architecture. Shorter recharge ratings are usually more
costly. Conversely, longer recharge times impose the risk of mission
failure if a subsequent power interruption occurs before the storage devices are
Transfer Time - This requirement applies primarily to stand-by UPS
configurations, i.e., the backup power supply remains idle until a failure of
the primary AC Source is detected and addressed. Transfer time is the
delay between the primary source failure and the backup engagement.
Commercially, these times typically vary from 1 to 100 milliseconds.
Transfer time requirements below 5 - 10 milliseconds become a severe cost