UL-1008A:2017 pdf free download – STANDARD FOR SAFETY Transfer Switch Equipment, Over 1000 Volts.
16.1.10 When the means used to comply with 16.1.9 renders the control circuit non-functional, audible or visual signals or equivalent means shall be provided to indicate the status of the control circuit.
16.1.11 Operation of an overcurrent protection system shall not cause a switching device to interrupt an overcurrent greater than the rated interrupting current of the switching device or initiate a transfer.
16.1.12 Unless the transfer switch is rated for switching fault currents, transfer under low supply voltage conditions shall be inhibited under short-circuit conditions.
16.1.13 Undervoltage transfer initiation shall be blocked when an overcurrent is sensed.
16.2 Transfer switch operator interlocks
16.2.1 The operating mechanism of open transition switches shall be interlocked to reduce the possibility of simultaneous connection to both the normal and alternate supplies.
16.2.2 The interlock circuit wiring shall be factory connected and located entirely within the transfer switch enclosure. It shall be additionally protected from possible damage due to the operation of the transfer mechanism or during any servicing of the switch. Where transfer equipment consists of multiple sections of equipment, provisions shall be made for field connection of the interlock circuit wiring, and detailed instructions for this connection shall be provided.
16.2.3 When an interlock system includes solid-state components, the electric interlocking system shall be evaluated to the requirements of the Standard for Tests for Safety-Related Controls Employing Solid-State Devices, UL 991 (refer to Annex B, Item 18).
16.2.4 An alarm or test means shall be provided to indicate an inoperative condition of an interlocking system which includes solid-state components, when the failure analysis indicated in 16.2.3 results in showing that a single failure renders the system inoperative.
16.2.5 The interlocking mechanism shall operate in the intended manner at the conclusion of all tests, as specified n the Performance section of this document for the particular type of transfer switch construction. In addition, all wiring connections shall be examined to determine that there has been no adverse effect – for example, connections should not become loose, parts should not rotate, and the like.
16.2.6 The mechanism and interlocking means shall be constructed to reduce the possibility of transfer in either direction in the event of welding of one or more contacts in the power circuit or one contact in the control circuit.
16.3 Location and protection of transfer control circuits
16.3.1 When a transfer switch is marked for service equipment use, and the control circuit is connected on the line side of the service disconnecting means, the control circuit shall have short-circuit protection and a disconnecting means suitable for the available current of the supply.
16.3.2 Control circuits may extend from a transfer switch to an external engine-generator set, when a marking indicates that the control circuit wiring is to be in conduit. See 56.7.
16.3.3 Other control circuits that are depended upon for the proper operation of a transfer switch shall be located wholly within the transfer switch enclosure and shall not have overload protective devices connected in them, but may have short-circuit and ground-fault protection. In a transfer switch consisting of multiple vertical sections, the control circuits may be in any vertical section of a continuous lineup of sections.
16.3.4 When an electrical motor on a transfer switch drive mechanism is provided with overload protection, the motor shall be protected against locked-rotor burnout only.
16.3.5 To be in compliance with 16.3.4, the overload protector shall not open during the Mechanical Endurance Tests, 40.
16.4 Transfer switches for emergency use
16.4.1 The operating mechanism and control circuit shall be such that the load cannot remain simultaneously disconnected from both the normal and alternate supplies when either or both supplies are available with voltage and frequency sufficient to permit proper operation. When intended for use with paralleled engine generator sets, transfer to the generator source or sources may be inhibited until sufficient power for the connected load is available. See 56.4.5.