Page 62 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
60 Chapter Two

supply is available within the consumer’s installation, two forms of con-
nection are recognized:

1. The alternative connection in which none of the conductors of one
supply (except those connected with earth) are ever connected with
the conductors of the other supply.
2. The parallel connection which allows the two supplies to be inter-
connected, provided that a suitable written agreement exists
between the two suppliers, and that the maintenance and operation
of the plant is undertaken in a competent and safe manner. The
agreement is to include such matters as the means of synchronizing,
earthing, maintenance records, competence of personnel, and means
of communication between operators of the interconnected supplies.”

Engineering Recomendation G.59/1
The full title of this document is “Recommendations for the connection of
embedded generating plant to the public electricity suppliers’ distribu-
tion systems.” It originated in 1985 as Engineering Recommendation
G.59, and following the privatization of the electricity supply industry,
was revised in 1991 by the Electricity Association of the United Kingdom
who now publish it as Engineering Recommendation G.59/1 (ER G.59/1).
It is intended for the use of the U.K. public electricity suppliers and their
consumers and applies to generating plants not exceeding 5 MW rating
which is connected to systems operating at 20 kV or below.
The term “embedded generating plant” is used to describe any gen-
erating plant connected to a public electricity supply, whether it is
intended for parallel operation or not. Persons who operate such a plant
are defined as embedded generators. Three types of connection are rec-
ognized:
■ The alternative connection in which the embedded generator operates
as an alternative to the public electricity supply. The arrangement
must be such that the two supplies cannot in any circumstances be
paralleled.
■ The parallel connection in which the embedded generator may run in
parallel with the public electricity supply for unlimited periods. For
this mode of operation an important consideration is the safety of the
public electricity supplier’s personnel who may find themselves
working on a distribution system which unexpectedly becomes con-
nected to the embedded generating plant.
■ Occasional paralleling in which the embedded generator may run in
parallel with the public electricity supply for a limited period, typi-

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