Page 99 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
Additional Information Relating to the Standby Supply Installation 97

The planning of the complete installation
The standard to which the equipment has been installed
The standard of maintenance
The operating procedures that are used including such matters as:
Ensuring that fuel is available in the daily service tanks
Ensuring that plant rooms are clean and not used as storage areas
Personnel responding correctly to alarms initiated by the equipment
Personnel following the operating and safety rules of the location

For high reliability each of the above activities must be undertaken
to a high standard. An installation designed and planned to a high
standard will be compromised if the operation or maintenance proce-
dures are not of a similar standard.

Redundancy
Redundancy is introduced in Chap. 2, where it is given as one reason
for the use of multiple sets. Redundancy may be defined as the provi-
sion of additional equipment such that, in the event of equipment fail-
ure, the remaining healthy equipment is capable of continuing to
supply power to the load; manual intervention may be required.
A common example of redundancy is the ring main which is almost
universally used for distribution at 11 kV. This allows, by manual inter-
vention, the disconnection of a faulty section of cable and the restoration
of supply to all consumers. If a second section of cable develops a fault,
some consumers will be deprived of supply until a repair can be effected.
An example of redundancy relevant to standby supplies is the provision
of additional generating sets so that, in the event of a failure of one set,
the redundant set can replace the failed set, either automatically or
with manual intervention.
Redundancy for generating sets is expressed in two forms of notation,
as a percentage or as the N 1 notation. If the installation includes two
sets each capable of supplying the entire load, the redundancy is said to
be 100 percent or 1 1; if there are three sets each capable of supply-
ing one-half of the load the redundancy is said to be 50 percent or 2 1,
and so on. It is possible, although unusual, to have a redundancy above
100 percent. Thus, three sets each capable of supplying the entire load
would have a redundancy of 200 percent or 1 2.
The most common failure experienced by users of generating sets is
a failure to start. Once having started, they usually continue running
until stopped by their control systems or they fail due to some external
circumstance such as a lack of fuel. For this reason, and because
standby sets run only occasionally, reliability is usually assessed as the

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