Fuse and Switching Devices Defintions

In the previous topics, in course EC-1Understanding NFPA 70 (National electrical code),  we explain most of the important definitions included in Article 100 , these definitions are very important to understand the terms used through other articles, I will put here a list of all previous topics regarding Article-100 ‘s Definitions as follows:

  1. Accessibility Terms Definitions
  2. Identified, Listed and Approved Terms Definitions
  3. Bonding Term Definitions
  4. Branch Circuit Definition
  5. Grounding Terms Definitions
  6. Circuit Breaker Definitions
  7. Electric Switch Definitions

Today, I will explain the final definitions for article 100; these definitions are applicable only to parts of the article specifically covering installations and equipment operating at over 600 volts, nominal.

Electronically Actuated Fuse:
An overcurrent protective device that generally consists of a control module that provides current sensing, electronically derived time–current characteristics, energy to initiate tripping, and an interrupting module that interrupts current when an overcurrent occurs. Electronically actuated fuses may or may not operate in a current-limiting fashion, depending on the type of control selected.


Although they are called fuses because they interrupt current by melting a fusible element, electronically actuated fuses respond to a signal from an electronic control rather than from the heat generated by actual current passing through a fusible element. Electronically actuated fuses have controls similar to those of electronic circuit breakers.

An overcurrent protective device with a circuit-opening fusible part that is heated and severed by the passage of overcurrent through it.


FPN: A fuse comprises all the parts that form a unit capable of performing the prescribed functions. It may or may not be the complete device necessary to connect it into an electrical circuit.

Controlled Vented Power Fuse:
A fuse with provision for controlling discharge circuit interruption such that no solid material may be exhausted into the surrounding atmosphere.

Vented Fuses

FPN: The fuse is designed so that discharged gases will not ignite or damage insulation in the path of the discharge or propagate a flashover to or between grounded members or conduction members in the path of the discharge where the distance between the vent and such insulation or conduction members conforms to manufacturer’s recommendations.

Expulsion Fuse Unit (Expulsion Fuse):
A vented fuse unit in which the expulsion effect of gases produced by the arc and lining of the fuseholder, either alone or aided by a spring, extinguishes the arc.

Non-vented Power Fuse:
A fuse without intentional provision for the escape of arc gases, liquids, or solid particles to the atmosphere during circuit interruption.

Power Fuse Unit:
A vented, non-vented, or controlled vented fuse unit in which the arc is extinguished by being drawn through solid material, granular material, or liquid, either alone or aided by a spring.

Vented Power Fuse:
A fuse with provision for the escape of arc gases, liquids, or solid particles to the surrounding atmosphere during circuit interruption.

Multiple Fuse:
An assembly of two or more single-pole fuses.

Switching Device:
A device designed to close, open, or both, one or more electric circuits.

Circuit Breaker:
A switching device capable of making, carrying, and interrupting currents under normal circuit conditions, and also of making, carrying for a specified time, and interrupting currents under specified abnormal circuit conditions, such as those of short circuit.

An assembly of a fuse support with either a fuseholder, fuse carrier, or disconnecting blade. The fuseholder or fuse carrier may include a conducting element (fuse link) or may act as the disconnecting blade by the inclusion of a non-fusible member.

Fuse Cutout

Disconnecting (or Isolating) Switch (Disconnector, Isolator):
A mechanical switching device used for isolating a circuit or equipment from a source of power.

Disconnecting Switch

Disconnecting Means:
A device, group of devices, or other means whereby the conductors of a circuit can be disconnected from their source of supply.

Interrupter Switch:
A switch capable of making, carrying, and interrupting specified currents.

Interrupter Switch

Oil Cutout (Oil-Filled Cutout):
A cutout in which all or part of the fuse support and its fuse link or disconnecting blade is mounted in oil with complete immersion of the contacts and the fusible portion of the conducting element (fuse link) so that arc interruption by severing of the fuse link or by opening of the contacts will occur under oil. 

Oil Cutout

Oil Switch:
A switch having contacts that operate under oil (or askarel or other suitable liquid).

Regulator Bypass Switch:
A specific device or combination of devices designed to bypass a regulator.

In the next topic, I will begin to explain Article -110 “Requirements for Electrical Installations”. Please, keep following.

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