Glossary

Plain definitions of the electrical terms and abbreviations used on this website, in product pages, guides and datasheets.

A

Anodising: An electrochemical process that grows a controlled oxide layer on aluminium or other metals, with the part as the anode. Voltage rises as the layer grows, and hard anodising needs higher DC voltages than most plating processes.

B

Buchholz relay: A gas- and oil-operated protective relay fitted in the pipe between the main tank and the conservator of an oil-immersed transformer. It detects gas from an internal fault or a sudden oil surge and gives an alarm or trips the supply.
Buck-boost transformer: A transformer connected in series with the load that adds to (boosts) or subtracts from (bucks) the incoming voltage. In a servo voltage stabilizer, its primary is fed from the variable transformer, so the correction can be adjusted continuously.
Bypass switch: A switching arrangement that connects the load directly to the incoming supply, around the stabilizer or other equipment, so that the equipment can be isolated for maintenance or a fault. While on bypass, the load receives the unregulated supply.

C

Carbon brush: The sliding contact that runs along the exposed winding of a variable autotransformer in a servo voltage stabilizer. It wears with use and is inspected and replaced during routine maintenance; some designs use a roller contact instead.
Case temperature point (Tc): A marked point on the case of a driver or power supply at which the manufacturer specifies a maximum temperature and, often, an expected life. Measuring it in the finished product is the most direct check of thermal design.
Commissioning: The checks and tests carried out after installation to confirm that equipment is correctly connected, set and working as specified before it is put into service. The results are recorded in a commissioning report.
Conservator: A small oil tank mounted above the main tank of an oil-immersed transformer. It allows the oil to expand and contract with temperature while keeping the main tank full, and is usually fitted with a breather to keep out moisture.
Constant current (CC): An output mode in which a power supply or LED driver holds its output current at a set value while the output voltage follows the load. It is the usual mode for driving LED modules and for plating rectifiers.
Constant voltage (CV): An output mode in which a power supply holds its output voltage at a set value, such as 12 V, 24 V or 48 V DC, while the current follows the load. LED strips and parallel loads connected to a CV supply control their own current.

D

Datasheet: A controlled technical document that states a product's ratings, characteristics and conditions in a standard format, with a revision identity. On this site, datasheets are named by product class and rating band.
Dead band: The small band around the set point within which a servo controller does not drive the motor, typically 1–3% of the set point. Too narrow a band makes the motor hunt back and forth; too wide lets the output drift further before correction.
Derating: Operating equipment below its full rating to suit conditions harsher than the rating assumes, such as high ambient temperature, altitude or restricted cooling. A derating curve shows the allowed output against temperature.
Design-in: The process of selecting, integrating and validating a component, such as a power supply, in an OEM product, from requirements and samples through validation to production release.
Digital Addressable Lighting Interface (DALI): A digital protocol for lighting control in which each driver on a two-wire bus has its own address. It allows individual dimming, grouping and status reporting, and is common in commercial building lighting.
Distribution transformer: A transformer that steps a medium-voltage supply, commonly 11 kV or 33 kV, down to the low voltage used by buildings and plant, typically 433 V at no load for a 415 V system. Ratings run up to a few thousand kVA.
Dry-type transformer: A transformer with no insulating liquid; the windings are insulated with solid materials such as cast resin and cooled by air. With no oil to leak or burn, it suits indoor installation close to the load, in clean and ventilated rooms.

E

Electrolysis: The use of direct current to drive a chemical reaction in an electrolytic cell, for example to produce gases or refine metals. Cells often need high, steady currents at low voltage, supplied by an industrial rectifier.
Electromagnetic compatibility (EMC): The ability of equipment to work in its electromagnetic environment without being disturbed by it and without disturbing other equipment. EMC covers both emissions and immunity and is tested against standards that depend on the product and market.
Electroplating: A process that deposits a metal coating on a conductive part by passing direct current through an electrolyte from an anode to the part. The coating rate follows the current, so plating rectifiers usually run in constant-current mode.

F

Factory acceptance test (FAT): Tests carried out at the manufacturer's works, often witnessed by the customer, to confirm that equipment meets the agreed specification before dispatch. The scope and acceptance criteria are agreed in advance.

H

Harmonics: Currents or voltages at whole multiples of the supply frequency, such as 150 Hz and 250 Hz on a 50 Hz supply. They are produced by non-linear loads including rectifiers, drives and electronic power supplies, and add heating in cables, transformers and motors.
Hold-up time: The time a power supply keeps its output within specification after the input supply is lost, typically a few to tens of milliseconds. It decides whether connected equipment rides through a brief interruption or dip.

I

Independent phase control: A three-phase stabilizer arrangement in which each phase is measured and corrected separately. It suits supplies feeding unequal single-phase loads, where phase voltages drift apart; balanced control corrects all three phases together and suits balanced loads.
Ingress protection (IP): A rating code, such as IP20 or IP67, defined in IEC 60529, that states how well an enclosure keeps out solid objects (first digit) and water (second digit). Higher numbers mean greater protection.
Input voltage window: The range of incoming voltage over which a stabilizer can deliver its rated output at the stated accuracy, for example 340–480 V AC for a three-phase unit. Outside the window, output accuracy or rated output is no longer maintained.
Inrush current: The brief current peak drawn when a power supply or driver is switched on, mainly to charge its input capacitors. It can be many times the running current and decides how many units can share one circuit breaker.
Insulation monitoring: Continuous measurement of the insulation resistance between an unearthed electrical system and earth, with an alarm when it falls below a set value. On a floating DC system it reveals a first earth fault before a second one causes a short circuit.
Interlock: An electrical or mechanical arrangement that prevents an unsafe sequence of operation, for example stopping a rectifier from running while a cover is open, cooling is absent or tank ventilation is off.

K

Kilovolt-ampere (kVA): The unit of apparent power: voltage multiplied by current, divided by 1,000, with a factor of √3 for three-phase circuits. Stabilizers and transformers are rated in kVA because their heating depends on current, whatever the power factor of the load.
Kilowatt (kW): The unit of active power, the part of the supplied power that does useful work as heat, light or motion. Active power in kW equals apparent power in kVA multiplied by the power factor.

L

LED driver: A power supply designed to operate LEDs, converting the AC mains to a controlled DC output. Most drivers for LED modules are constant-current types; constant-voltage types feed strips and parallel loads. Many support dimming.
Load loss: The heat produced as load current flows through a transformer's winding resistance, also called copper loss. It varies with the square of the load, so at half load it is a quarter of its full-load value.

N

Nameplate: The permanent plate fixed to equipment that records its identity and ratings: manufacturer, serial number, rating, voltages, currents, frequency and, for a transformer, vector group, impedance, tappings and cooling. Quote it in any service request.
No-load loss: The power a transformer consumes in its core whenever it is energised, also called core or iron loss. It is almost constant whatever the load, so it is paid for every energised hour of the year.

O

Off-circuit tap changer (OCTC): A switch that changes a transformer's turns ratio in steps, such as ±5% in 2.5% steps, to suit the actual supply voltage. The transformer must be de-energised before the tap position is changed.
Oil natural, air natural (ONAN): A transformer cooling code: the internal coolant is oil circulating by natural convection, and the external coolant is air circulating naturally around the tank and radiators. No pumps or fans are used. ONAF indicates fans added on the air side.
Oil-immersed transformer: A transformer whose core and windings are immersed in insulating oil inside a steel tank. The oil insulates the windings and carries heat to the tank and radiators. It is compact for its rating and is widely used outdoors and in substations.
On-load tap changer (OLTC): A mechanism that changes a transformer's turns ratio while it remains energised and carrying load, allowing the output voltage to be adjusted as the supply varies. It is more complex than an off-circuit tap changer and needs its own maintenance.
Original equipment manufacturer (OEM): A company that builds a product incorporating components from other suppliers, such as a luminaire maker using a bought-in LED driver, and sells it under its own name.
Output accuracy: The band within which a stabilizer or power supply holds its output, stated as a percentage of the set value, for example ±1%. The figure is meaningful only with the input range, load and temperature conditions that it assumes.
Overvoltage: A supply voltage that stays above its nominal value for an extended period, often at night when local demand is low. It increases magnetising current and losses in motors and transformers and stresses electronic input stages.

P

Percentage impedance (%Z): The voltage needed to circulate full-load current through a transformer with its secondary short-circuited, expressed as a percentage of rated voltage. It determines the available fault current and the voltage drop under load; distribution transformers typically fall around 4.5–6.25%.
Power factor (PF): The ratio of active power (kW) to apparent power (kVA), between 0 and 1. Motors and some electronic loads have a power factor below 1, so they draw more current than their kW rating suggests. A value of 0.8 is a common working assumption until measured.
Power quality: How closely the supply matches the ideal of a steady, sinusoidal voltage at the nominal value and frequency. It covers sustained voltage variation, dips and swells, unbalance, harmonics, flicker, transients and interruptions, each of which needs its own measure.
Product change notification (PCN): A formal notice from a supplier that a product will change in components, process, manufacturing location or specification, issued in advance so that customers can assess and, if needed, re-validate.
Pulse number: The number of DC pulses per supply cycle produced by a rectifier circuit. A six-pulse bridge gives 300 Hz ripple on a 50 Hz supply; a twelve-pulse arrangement gives 600 Hz with lower ripple and lower supply harmonics.

R

Rectifier: Equipment that converts alternating current (AC) to direct current (DC). An industrial rectifier combines a transformer, a bridge of diodes or thyristors, and control and protection, delivering controlled DC to processes such as plating and electrolysis.
Ripple: The small AC component that remains on a DC output after rectification, expressed as a percentage of the DC value. Its size and frequency depend on the rectifier's pulse number, the control setting and any filtering.

S

Scope of supply: The written statement of what a supplier will provide: equipment, configuration, accessories, documentation, testing and any installation or commissioning, together with exclusions and the customer's responsibilities.
Semiconductor fuse: A high-speed fuse that clears a short circuit fast enough to protect diodes and thyristors. It is selected by comparing its let-through energy, expressed as I²t, with the withstand of the device it protects.
Servo controller: The control board of a servo voltage stabilizer. It measures the output voltage, compares it with the set value and drives the servo motor to raise or lower the voltage, usually with adjustable set point, sensitivity and time delay.
Servo voltage stabilizer: A voltage stabilizer in which a servo motor adjusts a variable transformer to correct the output voltage. Correction is continuous and mechanical, so output can be held within a narrow band while the incoming supply drifts, at a limited correction rate of typically 6–25 V per second.
Static voltage stabilizer: A voltage stabilizer with no motor-driven parts in the correction path. Electronic switches such as thyristors or IGBTs select transformer taps or control a buck-boost stage, so correction is faster than in a servo design, though often in steps.
Switched-mode power supply (SMPS): A power supply that converts power by switching semiconductors at high frequency and using small transformers and filters. It is lighter and more efficient than a linear supply of the same rating, and is the basis of most modern power supplies and LED drivers.

T

Tap-changing stabilizer: A voltage stabilizer that keeps its output within a stated band by switching between taps on a transformer using relays or thyristors. It corrects in steps, so accuracy depends on the number of taps and is typically ±2–5%.
Temperature rise: The increase in oil or winding temperature above the surrounding ambient at rated load. Transformer and equipment ratings are tied to temperature-rise limits set in the applicable standard, so a hotter site or a restricted room needs review.
Thyristor (SCR): A semiconductor switch, also called a silicon-controlled rectifier, that conducts once triggered and stops at the next current zero. In a controlled rectifier, delaying the trigger point in each cycle adjusts the DC output.
Total harmonic distortion (THD): A single figure for the harmonic content of a voltage or current, expressed as a percentage of the fundamental. Current THD describes how distorted a load's current is; voltage THD describes the effect on the supply.
Traceability: The ability to link a supplied unit, by its serial number or batch, to its configuration, components, production records and test results. It supports investigation of field issues and controlled changes.
Transient: A very short voltage spike, lasting microseconds to milliseconds, caused by switching operations or lightning. Transients are outside the scope of voltage regulation; they are handled by surge protection devices coordinated with the installation.

U

Undervoltage: A supply voltage that stays below its nominal value for an extended period, minutes or hours rather than cycles. It is common at the far end of long feeders at times of high demand and makes motors draw more current and run hotter.

V

Variable autotransformer: A transformer with a single winding and a movable contact that slides along it, giving an adjustable output voltage. In a servo voltage stabilizer, a servo motor moves the contact to change the correction applied by the buck-boost transformer.
Vector group: A code describing how a three-phase transformer's windings are connected and the phase shift between primary and secondary. In Dyn11, the primary is delta, the secondary is star with neutral brought out, and the secondary lags the primary by 30°.
Voltage controller: Equipment that sets an adjustable output voltage, rather than holding a fixed one, using a manual or motorised variable transformer. Typical duties include heater and furnace control, motor and lamp testing, and process equipment that needs a chosen voltage.
Voltage regulation: The ability of equipment to hold its output voltage near a set value as the input voltage or the load changes. It is usually stated as a percentage band around the set value, together with the input range and load conditions under which it applies.
Voltage sag: A short reduction in supply voltage, also called a dip, lasting from half a cycle to a few seconds. Sags are often caused by large motors starting or by faults elsewhere on the network, and can make contactors drop out and drives trip.
Voltage swell: A short increase in supply voltage lasting from half a cycle to a few seconds, for example when a large load is switched off or during certain network faults. It is distinct from sustained overvoltage and from very short transients.
Voltage unbalance: A condition in which the three phase voltages of a three-phase supply differ in magnitude or phase angle, usually because single-phase loads are unevenly shared. A small voltage unbalance causes a much larger current unbalance and extra heating in three-phase motors.

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