Specifications

Power-Zone

Technical Overview

11/2005

Automatic Transfer Systems

Automatic transfer systems minimize power interruption by transferring the load from the normal

source to an alternate source when the normal source is temporarily unavailable. The system uses

multiple connections to power sources, usually utility sources, and a programmable logic controller

(PLC) to achieve this transfer. These systems also feature redundant supplies of control power.

Examples of automatic transfer systems with main-main circuit breakers and main-tie-main circuit

breakers are shown in Figure 42: below and Figure 43: on page 68.

Main-Main Circuit Breakers

Each main circuit breaker connects to a utility source. When the normal source becomes unavailable,

the system transfers to the alternate. If the system comes equipped with a preferred source selector

option, the system reverts to the preferred source automatically once it is available. Without the

selector, automatic retransfer does not occur.

Optional listing to UL 1008 for both closed and open transition transfer schemes is available. When an

open transition UL 1008 listed transfer scheme is specified, a mechanical interlocking cable is

provided between the two main circuit breakers (main-main) to assure that at least one breaker is

always open. This way, one circuit breaker is open prior to closing the other main.

Figure 42: Main-Main Circuit Breaker Configuration

Utility source #1 Utility source #2

Left main

circuit breaker

(normal source)

PLC

Common load

Right main

circuit breaker

(alternate source)