Lightning Protection, Grounding,
& Surge Suppression Glossary of Terms

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Facility
A building or other structure, either fixed or transportable in nature, with its utilities, ground networks, and electrical supporting structures. All wiring, cabling as well as electrical and electronic equipment are also part of the facility.
Facility Ground Network
The electrically conductive network, including all structures and grounding cables bonded to the earth grounding counterpoise but excluding the instrumentation ground network and electrical enclosures, conduit, and raceway systems. In steel frame structures, the structural members may be bonded together and connected to the earth grounding counterpoise to form the basic network. In buildings using nonconductive structural methods and materials such as masonry and in outside facility areas such as gas, propellant, or oxidizer service facilities, the facility ground network consists of conductors, sized according to established criteria, bonded to an earth grounding counterpoise and extending to all areas containing equipment to be grounded.
Facility Ground System
The electrically interconnected system of conductors and conductive elements that provides multiple current paths to earth. The facility ground system includes the earth electrode subsystem, lightning protection subsystem, signal reference subsystem, fault protection subsystem, electronic multipoint ground system, electronic single point ground system, as well as the building structure, equipment racks, cabinets, conduit, junction boxes, raceways, duct work, pipes, and other normally noncurrent-carrying metal elements.
Failure Mode
The effect by which a failure is observed to occur.
Faraday Shield
An electrostatic (E field) shield made up of a conductive or partially conductive material or grid. A Faraday cage or screen room is effective for protecting inside equipment from outside radiated RF energies.
First Return Stroke
That current flow along the previously ionized path occurring when that path is complete from cloud to ground
Flash
The total lightning discharge.
Flash-bang Rule
A rough calculation to determine how close lightning is: When you see the flash, begin to count the seconds until you hear the thunder. Divide this number by 5. The number you get is the approximate distance of the lightning in miles.
Flashover
Arcing or sparking between two or more (isolated) conductors. See "thermal sparking".
Flicker
A variation of input voltage, either magnitude or frequency, sufficient in duration to allow visual observation of a change in electric light source intensity.
Follow Current (Power)
The current from the connected power source that flows through an arrester during and following the passage of discharge current.
Forked Lightning
The common form of cloud-to-ground (CG) discharge always visually present to a greater or lesser degree which exhibits downward-directed branches from the main lightning channel. In general, of the many branches of the stepped leader, only one is connected to ground defining the primary, bright return stroke path, and the other incomplete channels decay after the ascent of the first return stroke.
Form Factor (Periodic Function)
The ratio of the root square value to the average absolute value, averaged over a full period of the function.
Forward Transfer Impedance
An attribute similar to internal impedance of a power source, but at frequencies other than the nominal (e.g. 60 Hz power frequency). Knowledge of the forward transfer impedance allows the designer to assess the capability of the power source to provide load current (at the harmonic frequencies) needed to preserve a good output voltage waveform. Generally, the frequency range of interest is 60 Hz to 3 kHz for 50 Hz to 60 Hz power systems, and 20 KHz to 25 kHz for 380 Hz to 480 Hz power systems.
Foundation Earth Electrode
Reinforcement steel of foundation or additional conductor embedded in the concrete foundation of a structure and used as an earth electrode. Also called "UFER" ground.