Maharashtra HSC Electrical Maintenance (Bifocal) key terms

The work needed to move one unit of charge between two points in a circuit, measured in volts. It is the driving force that pushes current through a resistance.

Electromotive force: the energy per unit charge supplied by a source such as a cell or generator, expressed in volts. It exists even when no current flows, unlike terminal voltage under load.

For a conductor at constant temperature, current is directly proportional to applied voltage and inversely proportional to resistance, written V = IR. It is the basis of most fault-finding calculations.

A material property giving the resistance of a unit length and unit cross-section, usually in ohm-metres. Copper has low resistivity, so it is preferred for conductors; nichrome has high resistivity for heating elements.

The ratio of true power in watts to apparent power in volt-amperes in an AC circuit, equal to the cosine of the phase angle. Low power factor increases current and cable losses for the same useful load.

The product of RMS voltage and RMS current in an AC circuit, measured in volt-amperes. It combines true power and reactive power and determines the rating of transformers and cables.

The power exchanged back and forth between a source and a reactive element such as an inductor or capacitor, measured in volt-amperes reactive. It does no useful work but is needed to magnetise motors and transformers.

The induced EMF in a coil equals the rate of change of magnetic flux linkage through it. It explains transformer action, generator output and the back EMF of a running motor.

An induced current always flows in a direction that opposes the change producing it. This explains why a motor draws a large inrush current at start and why transformers oppose load changes.

The voltage generated by a rotating armature that opposes the supply voltage. It limits armature current, so a motor with no load runs fast and draws little current until loading slows it.

The tendency of alternating current to concentrate near the surface of a conductor as frequency rises. It raises effective resistance at high frequencies, which is why stranded and hollow conductors are used.

A final circuit supplying socket outlets, lights or fixed appliances from a distribution board, protected by its own MCB or fuse. Indian practice separates lighting and power circuits with different cable sizes.

An enclosure holding incoming supply, busbars, and outgoing protective devices that split power into final circuits. It is the point where circuit identification, isolation and testing are normally carried out.

A wiring method in which insulated conductors are drawn through metal or PVC pipes fixed to walls, ceilings or buried in concrete. It gives mechanical protection and allows conductors to be replaced.

Rigid or flexible polyvinyl chloride tubing used to carry and protect wiring. It is corrosion-resistant and non-conducting, but needs separate earthing of metal boxes and accessories.

A wiring layout where the supply loops from one ceiling rose or accessory to the next, with each point taken from the loop rather than from a central joint box. It simplifies fault location and extension.

The accessory fixed at a ceiling outlet that connects the pendant luminaire to the fixed wiring. It contains terminals for loop, neutral and switched live conductors and must be correctly identified.

A switch with one common terminal and two strapper terminals, used in pairs to control one lamp from two positions such as at the top and bottom of a staircase.

A four-terminal switch inserted between two two-way switches so a lamp can be controlled from three or more positions. It reverses the strapper conductors passing through it.

The maximum continuous current a cable can carry without exceeding its rated conductor temperature. It depends on conductor size, insulation, ambient temperature and grouping or enclosure conditions.

A multiplier below one applied to a cable's tabulated current rating to account for high ambient temperature, grouping of circuits, or installation in thermal insulation. Correct application prevents insulation damage.

The reduction in voltage along a cable caused by its resistance and reactance under load. Excessive drop causes dim lamps and stalled motors, so cable size is chosen to keep it within permitted limits.

Selecting conductor cross-section so that current rating, voltage drop, short-circuit withstand and earth-fault loop impedance all meet requirements. It is a calculation, not a guess based on load alone.

The ratio of primary to secondary turns, which equals the ratio of primary to secondary voltage in an ideal transformer. It sets the secondary voltage for a given supply.

The hysteresis and eddy-current power lost in the magnetic core, present whenever the transformer is energised and nearly constant with load. It is measured by the open-circuit or no-load test.

The I-squared-R heating loss in transformer or motor windings, which varies with the square of load current. It is measured by the short-circuit test and causes winding temperature rise.

The difference between synchronous speed and actual rotor speed of an induction motor, usually expressed as a percentage of synchronous speed. Slip must exist for the rotor to develop torque.

The speed of the rotating magnetic field in an AC machine, equal to 120 times frequency divided by the number of poles. A four-pole motor on 50 Hz has a field speed of 1500 rpm.

An induction motor whose rotor is a set of short-circuited bars in a laminated core, with no external rotor connections. It is rugged and cheap but has high starting current.

A starter that connects the motor stator in star during run-up to reduce starting current to one third, then switches to delta for normal running. It suits motors designed to run in delta on the supply.

Direct-on-line starter: a contactor with overload protection that applies full supply voltage to the motor at start. It is simple and gives high starting torque but draws a large inrush current.

A protective device, usually thermal or electronic, that trips the contactor when motor current exceeds a set value for a set time. It protects windings from sustained overheating, not from short circuits.

Miniature circuit breaker: a resettable automatic switch that trips on overload by thermal action and on short circuit by magnetic action. It replaces fuses in modern distribution boards.

Earth leakage circuit breaker: a device that trips when current leaks from live conductors to earth through a fault. It protects people from electric shock and detects insulation failure.

Deliberate connection of exposed metalwork and the supply neutral point to the general mass of earth through an electrode. It keeps touch voltage low and gives fault current a return path to operate protection.

The resistance between an earth electrode and the surrounding soil, measured by a fall-of-potential test. It must be low enough for protective devices to operate quickly on an earth fault.

A measurement made with a megger between conductors and earth to check that insulation is sound. Low readings indicate moisture, damage or contamination and the circuit must not be energised.

An insulation resistance tester that applies a high DC voltage, typically 500 V or 1000 V, and displays resistance in megohms. The circuit must be isolated and discharged before testing.

An instrument that measures current by surrounding a conductor with a split magnetic core, without breaking the circuit. It reads AC directly and needs a suitable jaw for DC measurement.

A combined instrument for measuring voltage, current and resistance, with selection of AC or DC ranges. Correct range and function selection prevents damage and wrong readings.

The safety procedure of isolating a supply, locking the isolator in the off position and attaching a warning tag before work begins. It prevents accidental re-energisation of equipment being maintained.

A formal written authorisation naming the equipment, the isolation points, the work to be done and the person in charge. It is issued and cancelled by authorised staff before and after maintenance.

Variable frequency drive: an electronic converter that changes the frequency and voltage fed to an induction motor to control its speed. It also limits starting current and saves energy on pumps and fans.

A solid-state device that raises motor terminal voltage gradually during starting to limit inrush current. Unlike a VFD it does not control running speed.

Programmable logic controller: an industrial computer that reads inputs, runs a stored program and drives outputs to control machines and processes. It replaces relay logic in automated plants.

A control arrangement that prevents a machine or operation from running unless another condition is satisfied, such as a guard being closed. It protects personnel and equipment from unsafe sequences.

Scheduled inspection, cleaning, tightening, lubrication and testing carried out before failure occurs. It reduces breakdowns, extends equipment life and is planned around production schedules.

Repair work carried out only after equipment has failed. It is cheap to plan but causes unplanned downtime and secondary damage, so it is used for non-critical items.

The use of an infrared camera to find hot spots in joints, busbars, cables and motors while equipment is running. It detects loose connections and overloads before they cause failure.

Replacing or topping up grease in motor bearings at the interval and quantity specified by the maker. Over-greasing raises temperature and damages seals, so quantity matters as much as frequency.

A check that a conductor or circuit is unbroken, usually made with a low-voltage ohmmeter on an isolated circuit. It confirms earth paths, switch contacts and fuse integrity.