Q

R

RCD - Abbreviation for residual current device

Reactance - The result of the presence of capacitance and inductance

Reflection coefficient - Measure of the impedance change at a point in a cable. It is the ration of the signal reflected back from a point to the signal going into that point. High reflection coefficients can, on a data transmission systems, result in reflections which can be misinterpreted as data or data collisions, disrupting data transmission.

Repeaters - A form of booster or amplifier used at regular intervals along a transmission channel to regenerate message signals and to overcome impairments such as impedance.

Residual current device (RCD) - Device designed to disconnect a circuit if the earth leakage current exceeds a prescribed limit, typically 30 milliamps. Also know as an earth leakage circuit breaker.

Residual (transient) voltage - Alternative term for let-through voltage.

Resistance - the tendency of a material to resist the passage of an electric current and to convert electrical energy into heat energy. Resistance is measured in ohms.

Resistive coupling - This is the most common cause of transient overvoltages and it will affect both underground and overhead lines. Resistively coupled transients occur when a lightning strike raises the electrical potential of one or more of a group of electrically interconnected buildings (or structures).

Resistivity - A measure of the amount of resistance due to a material and independent of the dimensions of a sample of the material. Resistivity is symbolized by the Greek letter ρ.

Resonance - The maintenance of oscillation with minimum driving signal

Response time - The time it takes a surge protection device to switch from its "off" condition to an "on" or diverting mode. This occurs when an incoming surge voltage exceeds the clamping threshold level of the MOV or other suppression component. Organizations such as IEEE, NEMA, and Underwriter's Laboratories consider response time to be a non-issue since MOVs, SADs (and the surge protectors that use them) have response times that are 100 to 1000 times faster than any transient that they are likely to encounter.

RF - Abbreviation for radio frequency

RFI - Abbreviation for radio frequency interference

Ring main - The way in which wall sockets are supplied in the UK. The power supply is arranged in the form of a ring and is connected to a single point at the power distribution board.

RMS - Abbreviation for root mean square. It is used to find the DC heat equivalent of an AC voltage or current. For a sine wave the RMS value is equal to its peak value divided by square root of 2.

RS 232 - An internationally agreed standard for serial data transmission, specifying high and low voltage levels, timing and control.