Specifications

PIP ELSAP20 June 2007
Low-Voltage Automatic Transfer Switch
1. Introduction
1.1 Purpose
This Practice provides the requirements for low-voltage automatic transfer switches.
1.2 Scope
This Practice describes the requirements for the design, fabrication, inspection,
testing, and shipping of factory-assembled low-voltage automatic transfer switches.
This Practice does not cover fast transfer systems.
2. References
Applicable parts of the following Practices, industry codes and standards, and references shall
be considered an integral part of this Practice. The edition in effect on the date of contract
award shall be used, except as otherwise noted. Short titles are used herein where appropriate.
2.1 Process Industry Practices (PIP)
– PIP ELSAP20D - Data Sheet for Low-Voltage Automatic Transfer Switch
2.2 Industry Codes and Standards
• Underwriters Laboratories (UL)
– UL 508 - Industrial Control Equipment
– UL 1008 - Standard for Transfer Switch Equipment
• National Electrical Manufacturers Association (NEMA)
– NEMA ICS 10 - AC Automatic Transfer Switches
• National Fire Protection Association (NFPA)
– NFPA 70 - National Electrical Code (NEC)
3. Definitions
closed transition: Transfer switch equipment providing momentary paralleling of both power
sources during transfer in either direction. This permits the transfer of electrical loads without
a power interruption. The closed transition is possible only if the sources are properly
interfaced and synchronized.
delayed transition: Provides a timed disconnection of the load from the power sources during
transfer, primarily to allow decay of motor residual voltage.
open transition
: Transfer of power between a normal and alternate power supply if the
normal power supply is interrupted or falls outside set parameters. Transfer takes enough
time so that motors previously operating on the normal supply lose residual voltage and can
be restarted on the alternate supply.
Page 2 of 10 Process Industry Practices