Page 63 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 61

cally 5 min, and only for the purposes of maintaining continuity of
supply while changing over from one source to the other.

For standby power installations interest is limited to the alternative
connection and to occasional paralleling. The document describes the
technical requirements for earthing the standby supply, interconnect-
ing the neutrals of the two supplies and the changeover devices, and
the electrical protection required during parallel running. These mat-
ters are discussed in the sections titled “Earthing the Neutral of the
Standby Supply,” “Neutral Connections for Single Sets Not Intended to
Run in Parallel with the Normal Supply,” and “Paralleling the Standby
and Normal Supplies.”
There is an associated guidance document Engineering Technical
Report 113 (ETR 113) which discusses and describes these matters in
detail. Many of the diagrams reproduced in this chapter have been
derived from diagrams appearing in ETR 113.

Earthing the Neutral of the Standby Supply
It is important that the standby supply has a reliable earthing system;
it is not safe to rely on any earthing arrangements that are not under
the control of the site operating personnel. If the earthing system
for the normal supply is local to the generating set and is under the
control of the site operating personnel it may be permissible to use it
as the earth for the standby supply. Figure 2.6 illustrates typical

Generator Manual changeover or
circuit mechanically and electrically
Generator breaker interlocked devices
R

Y
Normal
supply
B
N

Essential Non-essential Adjacent earth
load load electrode
Figure 2.6 Typical connections for alternative supplies when the normal supply
is from an adjacent substation with its own local earth electrode.

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