Transfer switching modes differ in the sequence of operation to support specific applications. The following is a brief summary of their differences.

OPEN TRANSITION
In their standard configurations, ATSs transfer load from one power source to another by opening and closing contacts that serve conductors connected to utility and backup power sources. In this Open Transition mode, switching mechanisms operate in a “break-before-make” sequence. The switching contact positions and the resulting voltages to load are shown in the Figures below.

DELAYED TRANSITION
Delayed Transition transfer switches transfer loads between power sources using a timed load disconnect position that is held for a user-specified duration. This enables users to insert a delay that allows inertial load equipment, such as large motors, to slow or stop before the transfer occurs. This type of transition is commonly used to avoid high in-rush currents that could be generated by spinning motors and other equipment such as large power transformers, uninterruptible power supplies, and some older variable frequency drives. The switching contact positions and the resulting voltages to load are shown in the Figure below.

CLOSED TRANSITION
Closed Transition transfer switches feature Normal and Emergency contacts mounted on separate solenoid operators that allow them to switch asynchronously. By closing on the alternate source just before opening on the online source, closed transition switches operate in a “make-before-break” sequence that momentarily parallels power sources under monitored source parameters, avoiding power interruptions and their potential transient effects. Importantly, the entire sequence occurs in less than 100 mS to avoid impacting utility operations. Closed transition switching avoids power interruptions during planned transfers between sources, such as those associated with routine testing. For closed transition transfers, the switching contact positions and the resulting voltages to load are shown in the Figure below.

Supporting Information:

Transition Mode Basics

Transition Modes for Automatic Transfer Switches Part 1 and Part 2

Published for:ASCO Power Technologies USA