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GLOSSARY

Electrical equipment glossary

Plain-language definitions of the terms used when buying motors, switchgear, transformers, batteries, inverters and solar equipment in India.

A B C D E F G H I K L M N O P R S T V X

A

ACB

Air circuit breaker. A large low-voltage breaker used on main incomers, bus couplers and heavy feeders, typically from about 800 A up to 6300 A. Most are draw-out types with adjustable electronic protection, which allows maintenance without isolating the whole board. It is chosen for its rated current, short-time withstand rating and breaking capacity at the busbar fault level.

AMF panel

Auto mains failure panel. It monitors the incoming grid supply, starts the diesel generator when the mains fails or goes out of limits, transfers the load once the generator is stable, and returns to the grid after the mains has recovered, stopping the generator after a cooling run. It is used for hospitals, buildings and plants that cannot wait for manual changeover.

APFC

Automatic power factor correction. A panel in which a controller measures the load's power factor and switches banks of capacitors in and out to hold it near a target, commonly 0.95 to 0.99. It reduces the reactive current drawn from the supply, cutting losses and avoiding low power-factor penalties. Sites with drives or harmonic-producing loads usually need detuned reactors in series with the capacitors.

B

BMS

Battery management system. Electronics that monitor each cell in a lithium battery for voltage, temperature and current, balance the cells, and disconnect the battery to protect it from overcharge, deep discharge, overcurrent and over-temperature. Quality of the BMS strongly affects the safety and life of a lithium battery.

Brake motor

A motor with a spring-applied electromagnetic brake built into the end shield. The brake releases when power is applied and engages when power is cut, so the load is held safely and stopped quickly. It is common on hoists, cranes, conveyors and machine tools. Brake torque, response time and the number of starts per hour should be checked against the application.

Breaking capacity

The highest fault current a breaker or fuse can safely interrupt, stated in kA. For breakers, Icu is the ultimate capacity and Ics the service capacity, usually a percentage of Icu. The device must have a breaking capacity at least equal to the prospective short-circuit current at its point of installation, which a system study or the transformer data can estimate.

Bus duct

An enclosed system of copper or aluminium busbars, also called busbar trunking, that carries large currents between a transformer and a main panel or along a building riser. It has lower voltage drop and a neater layout than multiple parallel cables, and tap-off units allow connections along the run. It is rated by current and short-circuit withstand and comes in ratings up to several thousand amperes.

C

C-rate

A way of expressing charge or discharge current relative to battery capacity. For a 100 Ah battery, 1C is 100 A, C/10 is 10 A and 0.5C is 50 A. Capacity is quoted at a stated rate, for example C10 for a ten-hour discharge, and falls if the battery is discharged faster.

Cable gland

A fitting that seals and anchors a cable where it enters a panel, motor terminal box or junction box, maintaining the enclosure's ingress protection and relieving strain on the termination. Glands are chosen by cable outer diameter, armour type and material. Flameproof areas require certified glands rated for the area classification.

Cast resin transformer

A dry-type transformer whose windings are encapsulated in epoxy resin, so it needs no oil. It is fire resistant and suited to buildings, hospitals, malls and indoor substations. It needs adequate ventilation, protection from moisture and dust, and temperature sensors in the windings. It generally costs more than an oil-filled unit of the same rating.

Contactor

An electrically operated switch used to make and break power circuits, usually for motors, lighting and heaters, controlled by a low-power coil. Contactors are selected by utilisation category, for example AC-3 for squirrel-cage motors and AC-1 for resistive loads, and by rated current and coil voltage. A contactor switches the load but gives no overload or short-circuit protection on its own.

Current transformer

A CT steps down a large primary current to a standard secondary, typically 1 A or 5 A, for meters and relays. It is rated by ratio, burden in VA and accuracy class: metering classes such as 0.5 or 1, protection classes such as 5P10. The secondary must never be left open while the primary is energised, as the voltage that develops is dangerous.

D

Depth of discharge

DoD is the percentage of a battery's capacity that has been used in a discharge. A battery discharged from full to 50 percent charge has a DoD of 50 percent. Cycle life falls as DoD increases, so a bank sized to run at a shallower DoD lasts longer. Manufacturers quote cycle life at stated DoD levels.

Detuned reactor

A reactor connected in series with a power capacitor so that the combination resonates below the lowest significant harmonic, commonly with a 7 percent reactor tuned near 189 Hz on a 50 Hz system. It protects the capacitor from harmonic overload and prevents resonance with the supply, and is used wherever drives, UPS or LED lighting add harmonics.

DOL starter

Direct-on-line starter. It connects the motor straight to full supply voltage through a contactor, with an overload relay for protection. It is the simplest and cheapest starter, but draws a starting current of roughly six to eight times full-load current and gives full starting torque at once. It suits small motors and loads where the supply and the machinery can tolerate the jolt.

E

Earth fault

An unintended connection between a live conductor and earth or an exposed metal part. The fault current depends on the earthing system and fault impedance, and can be small enough for ordinary overcurrent protection to miss. Dedicated earth-fault relays, RCDs or insulation monitoring are used to detect it quickly, because an earth fault can cause shock, fire or equipment damage.

Earth pit

A buried electrode, such as a plate, pipe or rod, connected to the electrical system's earthing conductor to dissipate fault and surge current into the ground. Its resistance depends on soil, moisture and electrode design, and it is measured with an earth tester. Maintenance pits, backfill and periodic watering or testing keep the resistance within the value set by the designer.

F

Flameproof Ex d

A protection method for equipment in explosive gas atmospheres. The enclosure is built to withstand an internal explosion and to cool escaping gases so that they cannot ignite the outside atmosphere. It is specified under the IEC 60079 series, and is certified for gas groups (IIA, IIB, IIC) and a temperature class. Selection and installation must follow the area classification, confirmed by a qualified engineer.

Related: Flame Proof Motors, AC Motor

Float charging

The method of keeping a standby battery fully charged by holding it at a constant voltage just above its open-circuit voltage, so it supplies only a small trickle current. It is used for UPS, telecom and switchgear batteries that are rarely discharged. The float voltage per cell is set by the manufacturer and should be corrected for temperature, since a wrong setting shortens life.

FRLS cable

Flame retardant low smoke cable. A PVC-insulated cable whose compounds are formulated to resist spreading fire along the cable and to give off less smoke than ordinary PVC. It is used in buildings and plants for improved safety. It is not the same as LSZH cable, which also avoids halogen emissions.

G

Grid-tied inverter

A solar inverter that converts DC from panels into AC synchronised with the utility grid and feeds the power into the site's loads and grid. It has no battery and, for safety, shuts down when the grid fails. It is the simplest and most economical way to use rooftop solar where the grid is reliable.

H

Harmonics

Currents or voltages at whole-number multiples of the 50 Hz supply frequency, produced by non-linear loads such as drives, rectifiers, UPS, welders and LED drivers. They overheat transformers, neutrals and capacitors, cause nuisance tripping and distort the supply. Remedies include line reactors, detuned capacitor banks, harmonic filters and well-chosen transformer ratings.

Helical gearbox

A gearbox that uses helical-cut gears, which mesh gradually and run more smoothly and quietly than straight-cut gears. It is efficient, handles high loads and is made in parallel-shaft, inline and right-angle (bevel-helical) forms. Select it by output speed or ratio, torque, mounting position and service factor, which accounts for shock loads and daily running hours.

Related: Gearbox, Helical Gear Box

HMI

Human machine interface. A screen, often touch-enabled, through which operators view process values, alarms and trends and give commands to a PLC or drive. Selection depends on screen size, protocols supported, memory and environmental rating. A well laid-out HMI helps operators spot and respond to faults quickly.

HRC fuse

High rupturing capacity fuse. A cartridge fuse that clears very high fault currents safely by melting a calibrated element surrounded by quartz sand. Type gG fuses protect general cables and loads, while aM fuses protect motor circuits and tolerate starting current. Fuses act faster than most breakers on severe faults, but must be replaced after they operate and replaced with the same rating and type.

Hybrid inverter

An inverter that manages solar panels, a battery and the grid together. It can run loads from PV, store surplus in the battery, draw from the grid when needed, and keep essential loads running during an outage. It is more flexible than a grid-tied inverter, but the battery adds cost and requires proper sizing.

I

IDMT relay

Inverse definite minimum time overcurrent relay. Its operating time shortens as fault current rises, following a standard inverse curve, with a plug setting for pickup current and a time multiplier for delay. Setting relays in sequence along a feeder so that the one nearest the fault trips first is called grading or discrimination.

IE2

An efficiency class for induction motors defined in IEC 60034-30-1, known as high efficiency. IE1 is standard, IE2 is high, IE3 is premium and IE4 is super premium. A higher class wastes less energy as heat, which matters for motors that run for many hours a year. Always compare the efficiency figure at the actual load, not only the class label.

Related: AC Motor, Motors

IE3

The premium efficiency class for induction motors under IEC 60034-30-1. An IE3 motor has lower losses than an IE2 motor of the same rating, so it runs cooler and uses less electricity, usually at a higher purchase price. The saving is greatest on motors that run continuously. Check the nameplate for the class and the efficiency at full load.

Related: AC Motor, Motors

Impedance voltage

The percentage of rated voltage that must be applied to a transformer's primary to drive rated current through its short-circuited secondary. It decides the fault current: roughly rated current divided by the per-unit impedance, so a 5 percent unit passes about twenty times rated current. It also affects voltage regulation and parallel operation.

Inrush current

The brief, large current drawn when equipment is switched on. Motors draw several times full-load current while starting, and transformers can draw several times rated current for a few cycles when energised, due to core magnetisation. Inrush must be considered when choosing breaker curves, fuses, soft starters and generator size, so that the protection does not trip on a normal start.

IP55

An ingress protection rating under IEC 60529. The first digit, 5, means dust-protected: some dust may enter, but not enough to interfere with operation. The second digit, 5, means protected against water jets from any direction. IP55 is a common rating for motors, panels and junction boxes in indoor industrial areas. Outdoor and washdown use may need IP65 or higher.

K

kVA

Kilovolt-ampere, the unit of apparent power. For a three-phase system it equals the square root of three times line voltage times line current, divided by 1000. Transformers, generators and UPS are rated in kVA because their size is limited by current, regardless of power factor. Real power in kW is kVA multiplied by power factor.

Related: Transformers, Online UPS

kVAr

Kilovolt-ampere reactive, the unit of reactive power. It is the power that moves back and forth between source and load in inductive or capacitive equipment without doing useful work. Capacitor banks are rated in kVAr, and the size needed to improve power factor depends on the load in kW and the starting and target power factors.

L

LFP battery

Lithium iron phosphate battery. A lithium-ion chemistry with a nominal 3.2 V per cell, valued for thermal stability, long cycle life and the ability to be discharged deeply. Used for solar storage, telecom and UPS, and typically has a higher upfront cost than lead-acid. It must be paired with a battery management system and a compatible charger.

Line-interactive UPS

A UPS that passes mains power to the load through voltage correction, boosting or reducing it with a tap-switching transformer, and switches to battery power within a few milliseconds when the mains fails. It is cheaper than an online UPS and suits offices and networks where supply is generally steady and loads tolerate a brief transfer.

M

MCB

Miniature circuit breaker. A small thermal-magnetic breaker used in final distribution boards to protect circuits against overload and short circuit. Ratings run from a few amperes to about 125 A, with B, C and D tripping curves chosen for the load: C suits mixed and light motor loads, D suits high inrush. Check the breaking capacity in kA against the fault level at the board.

MCC

Motor control centre. A set of draw-out or fixed cubicles housing starters, breakers or fuses, overload protection and control wiring for a group of motors, fed from a common busbar. It centralises motor control and makes maintenance simpler. Intelligent MCCs add communication so that status and current can be monitored from a control system.

Related: MCC Panel, Switchgear Panels

MCCB

Moulded case circuit breaker. A breaker in an insulated moulded housing, used on feeders and incomers from roughly 16 A up to about 1600 A. Many models offer adjustable overload and short-circuit trip settings and a higher breaking capacity than an MCB. Select it by rated current, breaking capacity (Icu and Ics) and whether you need earth-fault or motor protection.

MPCB

Motor protection circuit breaker. A compact breaker designed for motor feeders, combining adjustable overload protection and fixed magnetic short-circuit protection. It is set to the motor's full-load current and, with a contactor, forms a complete starter without separate fuses or overload relay. Many models also offer phase-failure sensitivity, which helps prevent single-phasing damage.

MPPT

Maximum power point tracking. A control method used in solar charge controllers and inverters that continually adjusts the operating voltage of the PV array to extract the most power as sunlight and temperature change. It usually harvests more energy than simpler PWM charging, especially when the array voltage is higher than the battery voltage.

N

NGR

Neutral grounding resistor. A resistor connected between the neutral of a transformer or generator and earth to limit earth-fault current to a defined value. This reduces damage and arc-flash energy at the fault while still allowing protection to detect it. It is common on medium-voltage systems and is rated for the fault current and the time it must carry it.

Ni-Cd battery

Nickel-cadmium battery. An alkaline battery with a nominal 1.2 V per cell, known for tolerating high and low temperatures, deep discharge and long service life. It is used for switchgear DC supplies, railways and critical backup, where reliability counts. It costs more than lead-acid, and cadmium requires proper disposal.

O

ONAN

Oil natural, air natural. A cooling method for oil-immersed transformers in which oil circulates by natural convection and heat is lost to the surrounding air through radiators, without fans or pumps. It is the most common cooling for distribution transformers. Larger units add fans (ONAF) or pumps to raise the rating.

Online UPS

A double-conversion uninterruptible power supply that continuously converts incoming AC to DC and back to clean AC, so the load runs from the inverter at all times and sees no transfer time when the mains fails. It isolates the load from sags, surges and noise, and is used for servers, medical and process equipment. It is rated in kVA and kW.

Related: Online UPS, Inverters & UPS

OPzS

A 2 V flooded lead-acid cell with tubular positive plates and a transparent container, which lets the electrolyte level be seen. It is built for long standby and cycling life and is used in power plants, substations, telecom and larger solar installations. It needs periodic topping-up and a ventilated room, and is often selected where a long design life is wanted.

Related: Industrial & Standby Batteries

P

PCC

Power control centre. The main low-voltage distribution panel fed from the transformer, containing the incomer, bus coupler and outgoing feeders to MCCs, sub-distribution boards and large loads. It typically uses ACBs and MCCBs and a heavy busbar system rated for the site's short-circuit level. Its design is usually set by the transformer rating and the fault level.

Related: PCC Panel, Switchgear Panels

PCU

Power conditioning unit. A solar inverter that combines a battery charger, MPPT or PWM solar charge controller and an inverter in one box, so that it can run loads from solar, batteries and the grid. PCUs are sold in off-grid and hybrid forms and are rated in kVA, with the battery bank voltage specified.

Plante cell

A lead-acid cell with positive plates formed from pure lead rather than pasted active material. It is a long-life, flooded 2 V cell used in substations, power stations and telecom exchanges, where reliability matters more than size. It needs a ventilated battery room, regular checking and a properly set float charger.

Related: Industrial & Standby Batteries

PLC

Programmable logic controller. An industrial computer built for machine and process control, which reads inputs from switches and sensors, runs a stored program and drives outputs such as contactors, valves and drives. Selection depends on the number and type of I/O points, communication ports and scan speed. Remote I/O modules extend it to distant equipment.

Potential transformer

A PT, also called a voltage transformer, steps high system voltage down to a standard secondary, commonly 110 V, for meters, relays and synchronising equipment. It is rated by ratio, burden and accuracy class, and must be protected on the secondary by a fuse or small breaker, because a shorted secondary can damage the unit.

Power factor

The ratio of real power in kW to apparent power in kVA, equal to the cosine of the angle between voltage and current for a sinusoidal load. Induction motors and transformers draw reactive current, which lowers power factor and raises current for the same useful power. Utilities often penalise a low power factor, and it is corrected with capacitors.

R

RCBO

Residual current breaker with overcurrent protection. It combines the earth-leakage function of an RCCB with the overload and short-circuit protection of an MCB in one device. It is useful where each circuit needs its own leakage protection, so that a fault on one circuit does not trip the others. It is more compact than fitting a separate MCB and RCCB for every circuit.

RCCB

Residual current circuit breaker. It compares the current flowing out and back through a circuit and trips if the difference, which means leakage to earth, exceeds its rating. A 30 mA unit is used for protection against electric shock, while 100 mA and 300 mA units are used for fire protection and equipment. It does not protect against overload, so it needs an MCB or fuse upstream.

S

Service factor (gearbox)

A multiplier applied to the required power or torque when selecting a gearbox, to allow for shock loads, starts per hour and daily running time. A smooth load such as a lightly loaded conveyor has a low factor; a crusher or heavy mixer has a high one. Selecting without it is a common cause of premature gearbox failure.

Slip-ring motor

A wound-rotor induction motor whose rotor windings are brought out to slip rings and brushes. External resistance in the rotor circuit gives high starting torque with low starting current and allows some speed control. It suits cranes, mills, crushers and other heavy or high-inertia starts. Brushes and rings need regular maintenance, and many applications now use a drive with a cage motor instead.

SMPS

Switch-mode power supply. A compact, efficient supply that converts AC mains into a regulated DC output, commonly 24 V for control circuits, sensors and PLCs. DIN rail types mount in panels. Select by output current with spare capacity, input voltage range, and features such as short-circuit protection and redundancy.

Soft starter

A solid-state starter that ramps up motor voltage gradually using thyristors, limiting starting current and mechanical shock, and can also provide a controlled stop. It does not vary running speed. A bypass contactor is often fitted to remove losses once the motor is at full speed. It is common on pumps, fans, compressors and conveyors where the supply or the machinery is sensitive to a hard start.

SPD

Surge protection device. It limits transient overvoltages from lightning or switching by diverting surge current to earth. Devices are coordinated in types: Type 1 at the service entrance, Type 2 at distribution boards, Type 3 near sensitive loads. It needs a short, low-impedance earth connection to work, and a suitable back-up fuse or breaker. SPDs wear out and should be inspected periodically.

Star-delta starter

A reduced-voltage starter for motors rated for delta connection at the supply voltage. The motor starts with its windings in star, so each winding sees about 58 percent of line voltage and starting current and torque fall to roughly one-third, then switches to delta once up to speed. It suits light-start loads such as fans and pumps, but not heavily loaded starts.

T

Tap changer

A device that changes the number of turns in a transformer winding to adjust output voltage, usually in steps of a few percent. An off-circuit tap changer must be changed with the transformer de-energised, while an on-load tap changer (OLTC) can be operated under load. OLTCs suit larger transformers and sites where supply voltage varies widely.

TEFC

Totally enclosed fan cooled. A motor enclosure in which the internal parts are sealed from the outside air and an external fan on the shaft blows air over ribbed cooling fins on the frame. It keeps dust and moisture out of the windings and suits most industrial locations. It is not by itself suitable for explosive atmospheres, which need a certified flameproof motor.

THD

Total harmonic distortion. It is the RMS value of all harmonic content as a percentage of the fundamental, measured for voltage (THDv) or current (THDi). A power analyser or multifunction meter can display it. Higher THDi means a load injects more harmonics, which affects transformer heating, capacitor life and supply quality.

Thermal overload relay

A device that protects a motor from sustained overload. A bimetallic element heats with the motor current and trips the contactor after a delay that grows shorter as current rises. It is set close to the motor's full-load current, and its trip class, such as 10 or 20, indicates how long a motor can take to start. It does not protect against a short circuit.

Tubular battery

A flooded lead-acid battery with tubular positive plates, designed for deep discharge and long life. It is the common choice for home inverters and solar storage in India. It needs periodic topping-up with distilled water and should be kept in a ventilated place. Life depends strongly on depth of discharge, charging voltage and temperature.

V

VCB

Vacuum circuit breaker. A medium-voltage breaker, commonly used at 3.3 kV to 33 kV, that interrupts current inside sealed vacuum bottles. It needs little maintenance, produces no oil or gas, and suits frequent operation. It is the standard breaker in most MV panels feeding transformers, large motors and substations, and works with protection relays that decide when to trip.

Vector group

A code on a three-phase transformer nameplate showing how the high-voltage and low-voltage windings are connected and the phase shift between them. For example, Dyn11 means a delta primary, a star secondary with neutral, and a 30 degree shift. Transformers must have matching vector groups to run in parallel, and the group affects earth-fault behaviour and harmonics.

VFD

Variable frequency drive, also called an AC drive. It converts incoming AC to DC and then to AC at adjustable frequency and voltage, giving smooth speed control and soft starting for induction motors. Because power drawn by fans and centrifugal pumps falls steeply with speed, reducing speed even modestly can save considerable energy. Drives create harmonics and need correct cabling and earthing.

VRLA battery

Valve-regulated lead-acid battery. A sealed battery, in AGM or gel form, in which gases recombine inside the cell so no water topping-up is needed. It is compact and maintenance-light, used in UPS, telecom and switchgear DC systems. It is sensitive to high temperature and overcharging, which shorten life, so ambient temperature and charger settings need care.

X

XLPE

Cross-linked polyethylene, a cable insulation with a higher temperature rating than PVC. XLPE allows a continuous conductor temperature of 90 degrees Celsius against 70 for PVC, and a short-circuit limit of 250 degrees against 160, so a given cable carries more current. It is widely used in LT and HT power cables, particularly at 11 kV and above.

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