UL Standard for safety Solid-State Transfer Switches
|Publication Date:||15 November 2012|
These requirements cover solid state automatic transfer switches intended for use in ordinary locations to provide for lighting and power only in optional stand-by systems in accordance with Article 702 of the National Electrical Code, ANSI/NFPA 70.
Solid-state transfer switches are not for use as service entrance equipment unless marked as such.
These requirements cover transfer switch equipment rated at 6000 A or less and 600 V or less.
These requirements cover transfer switches together with their associated control devices including voltage sensing relays, frequency sensing relays, time delay relays, and the like.
An automatic transfer switch as covered by these requirements is a device that automatically transfers a common load from a normal supply to an alternate supply in the event of failure of the normal supply, and automatically returns the load to the normal supply when the normal supply is restored.
Exception: An automatic transfer switch is allowed to be provided with a logic control circuit that inhibits automatic operation of the device from either a normal to an alternate supply, or from an alternate to a normal supply when the switch reverts to automatic operation upon loss of power to the load.
A non-automatic transfer switch as covered by these requirements is a device, operated manually by a physical action, or electrically by a remote control, for transferring a common load between a normal and alternate supply.
A transfer switch may incorporate overcurrent protection for the main power circuits.
These requirements cover completely enclosed transfer switches and also open types intended for mounting in other equipment such as switchboards.
Transfer switches are rated in amperes and are generally considered to be acceptable for total system transfer, which includes control of motors, electric-discharge lamps, electric-heating loads, and tungsten-filament lamp loads as referred to in 1.10.
A transfer switch intended for total system transfer as indicated in 1.9 is considered to be acceptable for the control of tungsten-filament lamp loads not exceeding 30 percent of the switch ampere rating unless the switch has been investigated for a higher percentage of lamp load and marked accordingly.
A transfer switch may be limited to use with one or more specific types of load if investigated accordingly and marked as indicated in 47.11.