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UL 977 – Fused Power-Circuit Devices

Please note: All interim revisions for this edition available at time of your purchase will be included.

Fused Power-Circuit Devices

UL 977

1 Scope

1.1 These requirements cover fused power-circuit devices to be employed in accordance with the National Electrical Code.

1.2 Fused power-circuit devices, as covered by these requirements, are considered to be either bolted pressure contact switches or high-pressure butt-type contact switches.

1.3 Bolted-pressure contact switches are devices in which the blade-jaw connections have an additional pressure or clamping action provided at both ends of the switch blades when the blades are in the fully closed position.

1.4 High-pressure butt-type contact switches are devices having butt-type contacts and a spring-charged mechanism.

1.5 These requirements do not apply to low-voltage AC power circuit protectors as covered by the Standard for Low Voltage AC Power Circuit Protectors Used in Enclosures, ANSI/IEEE C37.29, or to low-voltage AC integrally fused power circuit breakers as covered by the Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures, ANSI/IEEE C37.13.

1.6 Fused power-circuit devices may be manually or electrically operated and may incorporate electrical tripping means.

1.7 An electrically tripped device is one whose closing is performed manually or electrically but contact opening is performed by a release energized by a separate source of voltage.

1.8 A device with an electrical operator is one whose contact closing and opening is performed by electrical means.

1.9 Fused power-circuit devices have continuous current ratings of more than 600 A and voltage ratings of 600 V or less AC, DC, or both. Devices rated AC are intended for use on circuits having maximum available short-circuit currents of 100,000, 150,000, or 200,000 A (rms symmetrical). Devices rated DC are intended for use on circuits having maximum available short-circuit currents of 20,000, 50,000, 100,000, 150,000, or 200,000 A DC.

1.10 Fused power-circuit devices have provisions for mounting (or are intended to be used with upstream) Class L fuses or Class T fuses rated more than 600 A.1.10 revised effective October 31, 2013

1.11 These requirements cover complete, enclosed fused power-circuit devices in which the switch is integral with the enclosure; open-type fused power-circuit devices intended for mounting in other equipment, such as switchboards or in a separately shipped enclosure; and enclosures intended for mounting open-type fused power-circuit devices.

1.12 These requirements also cover electrically tripped and/or operated devices that have been investigated to determine their acceptability for ground-fault protection when combined with ground-fault sensing and relaying equipment as follows:

a) Switches for use with Class I ground-fault sensing and relaying equipment including those devices that are capable of interrupting 12 times their rated current or that have integral means intended to prevent disconnecting at levels of fault current exceeding the contact interrupting capability of the switch.

b) Switches for use with Class II ground-fault sensing and relaying equipment are capable of interrupting at 10 times their rated current and are for use in ground-fault protection systems where means intended to prevent disconnecting levels at fault current exceeding the contact interrupting capabilities of the switch are incorporated within the ground-fault sensing and relaying equipment.

1.13 A product that contains features, characteristics, components, materials, or systems new or different from those covered by the requirements in this standard, and that involves a risk of fire, electric shock, or injury to persons shall be evaluated using the appropriate additional component and end-product requirements to determine that the level of safety as originally anticipated by the intent of this standard is maintained. A product whose features, characteristics, components, materials, or systems conflict with specific requirements or provisions of this standard shall not be judged to comply with this standard. Where appropriate, revision of requirements shall be proposed and adopted in conformance with the methods employed for development, revision, and implementation of this standard.