Fast Speed Ratio - A speed ratio of melting characteristics from 6-8 (refer to Speed Ratio).

Fault Close Rating - The ability of a switching device to close into a fault current of a specific magnitude without excessive arcing.

Fault Current - The current from the connected power system that flows in a short circuit.

Fault Indicator- A device which indicates fault current by sensing the magnetic field caused by current flowing through the conductor. Once the current is above the fault indicator's current rating, the fault indicator will trip.

Feeder - Any portion of an electrical circuit between the service box or other source of supply and the branch circuit overcurrent devices.

Ferroresonance - A phenomenon characterized by overvoltages and very irregular wave shapes which are potentially damaging to a transformer. It typically occurs when there is no ground on the system except through the transformer connected line to ground. It is always associated with the excitation of one or more saturable inductors through capacitance in series with the inductor. When one or two phases are disconnected from the source by single-pole fault clearing or switching, it is possible for the transformer windings connected to the open phases to be excited through the system capacitances to ground and between phases.

Fire-Resisting when Applied to a Building - Means constructed of masonry, reinforced concrete, or equivalent materials.

Flashover - A disruptive discharge around or over the surface of an insulator.

Flat Conductor Cable Type FCC - the following special terminology applies to the installation of Type FCC under-carpet wiring systems:

• Bottom shield - a protective layer that is between the floor and the Type FCC cable to protect the cable from physical damage.
• Insulating end - an insulator designed to electrically insulate the exposed ends of Type FCC cables. Metal tape - a metal overlay to prevent physical damage to the Type FCC system.
• Top shield - an electrically conductive covering for under-carpet components of a Type FCC system that provides a degree of protection against physical damage and electric shock and may or may not be incorporated as an integral part of a Type FCC cable assembly.
• Transition assembly - an assembly specifically approved for the purpose of connecting a Type FCC system to other types of wiring systems.
• Type FCC cable - a cable consisting of 3 or more flat separated conductors laid flat and parallel in the same plane and enclosed within an insulating assembly.
• Type FCC cable connector - a device used for joining Type FCC cables, with or without the use of a junction box.
• Type FCC system - a complete wiring system for installation only under carpet squares and includes cable and associated fittings.

Flexible Metal Conduit - A metal conduit that may be easily bent without the use of tools.

Flicker - Impression of fluctuating brightness or color, occurring when the frequency of the observed variation lies between a few hertz and the fusion frequencies of the images.

Fluctuation - A surge dip in voltage amplitude, often caused by load switching or fault clearing.

Follow Current - The current which flows through an arrester, caused by the power frequency voltage across it, during and after the passage of surge current.

Frequency - On ac circuits, the designated number of times per second that polarity alternates from positive to negative and back again, expressed in Hertz (Hz)

Fulgurite - tunnel-like structure formed when a current-limiting fuse operates; glass-like structure formed by the temperature of a high voltage arc when a current-limiting fuse operates; the fused silica sand that remains after prolonged abnormal current flow.

Fuse - A device that will heat up, melt and electrically open the circuit after a period of prolonged abnormal current flow.

Fuse Arcing Time - The amount of time required of the fuse to extinguish the arc and clear the circuit; the specific amount of time required for an interrupter to clear a circuit after its fusible element has melted.

Fuse, Buttonhead - A relatively common type of expulsion fuse utilized in open or enclosed cutouts; the name given to the upper assembly of a distribution fuse link which is then used in a reusable fuseholder for outdoor cutout applications.

Fuse Canister - Dry well full range current-limiting fuse holder - loadbreak and non-loadbreak. Fuses are mounted in an oil tight canister which extends into the tank beneath the oil. Accessible from the high voltage compartment, the fuse is air insulated.

Fuse, Cartridge - Expulsion type fuse with either current sensing or dual (current and oil temperature) sensing elements mounted internally under-oil. Used to remove an apparatus from the primary circuit in the event of a current fault. Cartridge fuses have limited interrupting ratings.

Fuse Clearing Time - The total time required for the fuse to melt and clear the circuit; melting time plus arching time.

Fuse, Clip-Mounted - A full range current-limiting fuse mounted in clips in the high voltage compartment.

Fuse, Current Sensing - An expulsion fuse having elements whose characteristics are not affected by elevated oil temperatures; a fuse having elements that are made of copper, tin or silver that will react to current flow only.

Fuse, Dual Sensing - An expulsion transformer fuse having elements that are sensitive to fault current, load current and temperature (top oil); a fuse that senses the top oil temperature of a transformer in addition to the temperature from the current flow; a fuse that can monitor the aging effects of elevated transformer hot spot temperatures regardless of cause; sometimes know as "load" sensing.

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

Fuse, Expulsion - A replaceable one-shot current interrupting device having definite time current characteristics; a device which has a metal melting element and uses the principles of heated fibers releasing de-ionizing gases, and alternating current passing through a zero reference periodically for arc extinction.

Fuse Link - The (metal) fusible element in an expulsion cutout; the composite construction of materials that house the fusible element.

Fuse Maximum Interrupting Current - That upper current level that a fuse can successfully clear with assured reliability; the maximum current at a specified maximum design voltage that a fuse if required to clear as specified by the manufacturer.

Fuse Maximum Operating voltage - The maximum voltage that a fuse can properly operate at with assured clearing.

Fusing Melting Time - The amount of time required to melt a fusible element at a specific current.

Fuse Minimum Interrupting Current - That lower limit of current that up0on melting a fuse can be assured of successfully clearing; a term generally associated with back-up current-limiting fuses indication the lower limit of current that the fuse can clear upon melting.

Fuse, Open-Link - An outdoor expulsion fuse for use in an open-link cutout; a one-shot fuse that is usually suspended between two spring arms which hold the link in tension; a "flipper" fuse.

Fuse, (silver) Sand - An abbreviated name for a type of current-limiting fuse that has silver for its element and sand as its surrounding media (refer to Current-Limiting Fuse).

Fuse Spider - The element support material used in a current-limiting fuse of various compositions and design.

Fuse, Type D - Refer to D-Link.

Fuse, Type K - A specific type of expulsion fuse available in button and open-link configurations. The "K" designation refers to specific time current characteristics, defined by ANSI, allowing customers to purchase fuses from more than one manufacturer without adversely affecting the overall operation of the system.

Fuse, Type N - Expulsion fuse links having time current characteristics unique to and defined by individual fuse manufacturers. Since the characteristics are unique, various manufacturers' products cannot be interchangeably used on a distribution system without adversely affecting its operation. In addition to type "N", other links that fall into this category are Type H, Type Q, Type QA, Type KS, Type 200, etc.

Fuse, Type T - A specific type of expulsion fuse available in button and open-link configurations. The "T" designation refers to specific time current characteristics as defined by ANSI. In reference to the type "K" links, the type "T" links are time delayed or retarded in operating characteristics.

Fuse, Weak Link - A coined name for the cartridge fuse. The name is take from the thought that the fuse would be the weakest link in a chain causing the fuse to open the circuit before equipment is damaged.