Glossary: F

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Fast Charge

Increasing the Operating Voltage after a battery discharge, following an AC failure, to give a rapid battery recharge. Typically at a constant current charge rate higher than .3C. (1/3 the batteries rated ampere-hour capacity)

Fault Tolerance

The ability of a system to continue operating in the event of a fault.


In wind generators, this refers to an adjustment of the blades so that they catch less wind. This can prevent damage to the machine in high winds.


A method of removing noise from the output of a UPS preventing “dirty power” from reaching connected equipment.

Finish Charge

The final stage of battery charging, when the battery is charged at a slow rate over a long period of time.


Noticeable deformation of trees from prevailing winds. Flagging is an indication of an effective wind site. Lack of flagging is not necessarily an indication of a poor wind site.

Flat Plate Collector

A solar thermal collector that converts the sun’s radiation into heat on a flat surface. Does not use reflecting surfaces or lens arrangements to concentrate the sun’s energy.

Float Charge

A trickle charge to keep a battery fully charged at a safe voltage level with minimal gassing.

Float Service

A battery operation in which the battery is normally connected to an external current source; for instance, a battery charger that supplies the battery load under normal conditions, while also providing enough energy input to the battery to make up for its internal losses, thus keeping the battery always at full charge and ready for service.

Float Voltage

The set output voltage of the DC power system (not including temperature compensation or other adjustments).


In hydro-electric terms, flow refers to the quantity of water supplied to a water source or exiting a nozzle per unit of time. Commonly measured in gallons per minute.

Fluorescent Light

An electric lamp coated on its inner surface with phosphor and containing mercury vapor. When bombarded with electrons, the vapor emits ultraviolet light that causes the phosphor to emit visible light.

Francis Turbine

A type of reaction hydro-turbine used in low to medium heads. It consists of fixed vanes on a shaft. Water flows down through the vanes and out sideways.

Freestanding Tower

A wind generator tower with no guy wires. This can be either a lattice tower or a monopole. Freestanding towers are the most expensive type of tower, requiring large excavations and large amounts of concrete.


The number of cycles (oscillation positive and negative) completed in one second. Defined as Hertz. In North America, utility power completes 60 cycles per second, (60 Hertz).

Friction Loss

Lost energy due to friction. In hydro systems, pipe sized too small can lead to serious friction losses. In any belt drive system, there will be some losses due to friction.

Frictional Head Loss

Pipe Loss: The amount of energy or pressure lost due to friction between a flowing liquid and the inside surface of a pipe.

Full Load

The greatest load that a circuit is designed to carry under specific conditions; any additional load is considered an overload.

Full Sun

The full sun condition is the amount of power density received at the surface of the earth at noon on a clear day—about 1 KW per m^2, or 1 Sun. Lower levels of sunlight are often expressed as 0.5 sun or 0.1 sun.


Reducing a wind generator’s swept area to protect it from high winds. Common furling methods are to tilt the rotor (blades) up or sideways out of the wind, or to feather (twist) the blades to degrade the airfoil.


An electrical device that is designed to break a circuit by melting an internal conductor when the current in the circuit exceeds the maximum considered safe for the conductors or devices in the circuit.