Handbook of Electrical Installation Practice - BeKnowledge

146 Handbook of Electrical Installation Practice
Fig. 6.13 Discharge performance (CA is the rated capacity in amperes).
This narrower band complies with the equipment regulation and as an additional
benefit removes the problems associated with possible switching spikes corrupting
stored and transmitted digital data.
ALTERNATING CURRENT SYSTEMS
A wide range of equipment in industrial and commercial premises requires a secure
a.c. power supply, and with the increasing use of computers and other sophisticated
electronic equipment, the demands placed on the quality and continuity of the a.c.
supply have grown correspondingly.
Even in the industrialised nations, where the a.c. mains supply is very reliable, it
is estimated that a computer will on average suffer from approximately 130 power
surges, sags or minor interruptions per month. In addition, between 80 and 90% of
users will be subjected to voltage drops greater than 10% of the nominal. In developing
countries, mains outages of three or four hours a day are not uncommon.
Uninterruptible power supplies (UPSs) allow a critical or sensitive electrical load
to operate reliably in spite of mains disturbances or outages. Battery-based UPSs
use static inverters or rotary converters to convert the d.c. supply from a battery
into a.c. at the voltage and frequency required by the load. Diesel rotary UPSs are
also available and use inertia energy storage to maintain supplies until the engine
starts.
A range of UPS configurations are available to suit the sensitivity, criticality and
demands of the critical load.