Page 58 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
56 Chapter Two

The changeover devices may require three or four poles depending
upon the type of supply and the details of the agreement between the
Embedded Generator and the network operator. The matter is dis-
cussed in the section titled “Neutral Connections for Single Sets Not
Intended to Run in Parallel with the Normal Supply.”

Use of Changeover Contactors
Changeover contactors are suitable for use with small generating sets
having a current rating not more than a few hundred amperes. Figure 2.1
indicates a typical interconnecting arrangement.
If the contacts of a contactor suffer misuse or severe arcing, welding
can result; for this reason contactors switching the normal and standby
supplies must be electrically and mechanically interlocked. The use of
mechanically interlocked changeover contactors requires that both the
supplies (normal and standby) are taken to the same component which
therefore becomes critical. Any failure of the contactor results in nei-
ther supply being available and a total shutdown is required for repair
(or indeed for maintenance).

Use of Electrically Interlocked Circuit
Breakers
The use of circuit breakers is applicable to generating sets of any rat-
ing; the contact arrangement and the operating mechanism are more
robust than that of contactors and mechanical interlocking is not
essential. The normal and standby supplies are taken to different com-
ponents and the failure of one circuit breaker does not necessarily ren-
der the other inoperative.
Molded-case circuit breakers may be mechanically and electrically
interlocked but this arrangement results in the two circuit breakers
being mounted close to each other and, unless live working is allowed,
a shut down will be required for repair or maintenance. For mechani-
cally interlocked circuit breakers the arrangement is similar to that
used for contactors and Fig. 2.1 applies.
Air circuit breakers constructed within a metal frame and intended
for switchboard mounting cannot usually be mechanically interlocked,
and reliance must be on electrical interlocking. Where there is no
mechanical interlocking the two circuit breakers are independent and
do not have to be mounted together. Figures 2.2 and 2.3 indicate typi-
cal interconnecting arrangements.
In Figs. 2.2 and 2.3 the normal supply is taken to the busbar section
that feeds the nonessential load, which results in an operating advan-
tage; while the essential load is being supplied from the standby sup-
ply, on restoration of the normal supply it is now permissible and

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.