IESL Part II Electrical key terms

The ratio of real power in watts to apparent power in volt-amperes in an alternating-current circuit, ranging from zero to one and indicating how effectively current does useful work.

The product of root-mean-square voltage and root-mean-square current in an AC circuit, measured in volt-amperes, representing the total power that appears to flow before phase difference is considered.

The portion of apparent power that oscillates between source and reactive elements such as inductors and capacitors without being consumed, measured in volt-amperes reactive and linked to magnetic or electric field energy.

The addition of capacitors or other compensating devices to an inductive load so that the phase angle between voltage and current is reduced, lowering apparent power drawn and avoiding utility penalty charges.

A method of expressing voltages, currents, impedances and powers as fractions of chosen base values, simplifying transformer networks and fault calculations by removing turns-ratio conversions between voltage levels.

A technique that resolves an unbalanced three-phase set of phasors into three balanced sets called positive, negative and zero sequence, enabling separate analysis of unbalanced faults and machine behaviour.

The total impedance of the path taken by fault current from the source through the live conductor, the fault, the protective conductor and back to the source, determining whether overcurrent protection will disconnect quickly.

A protective switch that continuously compares live and neutral currents and trips when their difference exceeds a set threshold, detecting leakage to earth that ordinary overcurrent devices may not clear.

A conductor that connects exposed conductive parts of equipment to the earthing system so that fault current returns safely and protective devices operate before dangerous touch voltages persist.

A buried conductor, rod or plate that makes direct electrical contact with the soil and provides a low-resistance path to earth for fault current and lightning discharge.

The voltage difference appearing between two points on the ground surface one metre apart during a fault current discharge, which can drive dangerous current through a person's legs standing nearby.

The voltage difference between an earthed metallic structure and a point on the ground a person can touch simultaneously, which determines the shock hazard from a faulted installation.

The ratio of the sum of individual maximum demands to the maximum demand of the whole installation, used to size supply equipment realistically because not all loads peak simultaneously.

The average load over a period divided by the peak load in that period, indicating how efficiently installed capacity is used and influencing tariffs and generation planning.

The ratio of the maximum demand of a system to its total connected load, used in installation design to estimate realistic maximum current drawn from all connected equipment.

The tendency of alternating current to concentrate near the surface of a conductor as frequency rises, increasing effective resistance and reducing the useful cross-section for high-frequency or large conductor applications.

The redistribution of current in a conductor caused by the magnetic field of a nearby parallel conductor, raising effective resistance in closely spaced cables and busbars at power frequencies.

A partial electrical breakdown of air around high-voltage conductors where the local electric field exceeds the breakdown strength, producing audible hiss, ozone and radio interference that waste energy.

A protective device that presents high impedance at normal voltage but conducts heavily during overvoltage transients, diverting lightning or switching surges to earth and then restoring normal operation.

The peak impulse voltage a piece of insulation can withstand without breakdown under a standard lightning impulse waveform, used to coordinate insulation strength with protective device levels.

A gas-actuated protective device fitted in the pipe between a transformer tank and its conservator, detecting gas accumulation from incipient faults and tripping on severe oil surge.

A scheme that compares currents entering and leaving a protected zone and trips when the difference exceeds a setting, giving fast selective clearing of internal faults in transformers, generators and busbars.

A mechanism that alters the number of turns in a transformer winding to adjust its voltage ratio, either offline or on load, compensating for supply voltage variation and controlling network voltage.

An instrument transformer that reproduces a large primary current as a proportional, phase-related smaller secondary current for metering and protection relays while isolating them from high voltage.

The total impedance of the connected secondary circuit, including relays, meters and wiring, that a current or voltage transformer must drive while staying within its accuracy limits.

A stability test for feedback systems that judges closed-loop stability from the open-loop frequency response by counting encirclements of the minus-one point in the complex plane.

A pair of graphs showing the magnitude and phase of a system's frequency response against logarithmic frequency, used to read gain and phase margins and assess relative stability.

The amount by which the open-loop phase exceeds minus one hundred eighty degrees at the frequency where open-loop gain crosses unity, indicating how much additional delay the loop can tolerate.

A feedback controller that sums proportional, integral and derivative actions on the error signal, where integral action removes steady-state offset and derivative action adds damping and speed of response.

A plot of the paths traced by closed-loop poles in the complex plane as a system parameter, usually gain, varies from zero to infinity, revealing stability and transient response trends.

The discrete-time counterpart of the Laplace transform, converting a sampled sequence into a function of a complex variable so that difference equations can be analysed algebraically.

The principle that a signal band-limited to a highest frequency must be sampled at more than twice that frequency to be reconstructed exactly from its samples.

The distortion that occurs when a signal is sampled below twice its highest frequency, causing high-frequency components to appear falsely as lower frequencies in the reconstructed signal.

A low-pass filter placed before an analogue-to-digital converter that removes frequency content above half the sampling rate so that aliasing cannot occur.

A dimensionless measure of how strongly a carrier is varied by the modulating signal; in amplitude modulation it is the ratio of modulating amplitude to carrier amplitude, and values above one cause overmodulation.

The peak difference between the instantaneous frequency of a frequency-modulated carrier and its unmodulated centre frequency, governing bandwidth and signal-to-noise performance.

The error introduced when a continuous amplitude is rounded to the nearest discrete level in digitisation, heard as a low-level hiss whose power falls as bit depth increases.

The ratio of voltage to current for a wave travelling along a uniform transmission line, determined by the line's inductance and capacitance per unit length and independent of line length.

The ratio of maximum to minimum voltage or current magnitude along a transmission line carrying a standing wave, indicating the degree of mismatch between line and load.

The complex ratio of the reflected wave amplitude to the incident wave amplitude at a discontinuity, whose magnitude and phase determine how much power returns toward the source.