Maharashtra HSC Electrical Technology (Vocational) flashcards

36 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

For a conductor at constant temperature, voltage equals current multiplied by resistance. It links the three basic quantities and allows any one to be found from the other two.

Kirchhoff's current law

At any node the total current entering equals the total current leaving, because charge cannot accumulate at a point. It is the basis of nodal circuit analysis.

Power factor

The cosine of the phase angle between voltage and current. It equals real power divided by apparent power and is 1 for a purely resistive load.

RMS value

The effective value of an alternating quantity that produces the same heating as an equal DC value. For a sine wave it is peak divided by root two.

Resistivity

A material property giving resistance of a unit cube. Resistance equals resistivity times length divided by cross-sectional area, so long thin conductors have more resistance.

Conduit wiring

A wiring system in which insulated conductors are drawn through metal or PVC pipes. It gives mechanical protection and allows conductors to be replaced without disturbing the building.

Fuse rating rule

The fuse must carry normal load current continuously yet melt before the cable is damaged. Its rating lies above the load current and below the cable current capacity.

Transformer EMF equation

Induced EMF equals 4.44 times frequency times turns times peak flux. It shows that voltage per turn is fixed by frequency and core flux.

Slip of induction motor

The difference between synchronous speed and actual rotor speed, expressed as a fraction of synchronous speed. It is typically two to five percent at full load.

Synchronous speed

Rotor speed of a machine locked to supply frequency, equal to 120 times frequency divided by number of poles. A four-pole motor on 50 Hz runs at 1500 rpm.

Back EMF

The voltage generated in a running motor armature that opposes the supply. It limits armature current, which equals supply voltage minus back EMF divided by armature resistance.

Shunt for ammeter

A low resistance connected in parallel with a galvanometer so most current bypasses the movement. It extends the current range while keeping full-scale deflection.

Megohmmeter

An instrument applying a high DC test voltage to measure insulation resistance in megohms. A low reading indicates moisture, damage or leakage in the insulation.

Energy meter constant

The number of disc revolutions per kilowatt-hour of energy consumed. It is marked on the meter and used with a stopwatch to check registration accuracy.

Current transformer

A transformer whose primary carries the line current and whose secondary supplies a standard low current to instruments. It also isolates measuring equipment from high voltage.

Transmission voltage advantage

Raising transmission voltage lowers current for the same power, reducing line loss and conductor size. Insulation cost rises, so an economic voltage is chosen.

Radial distribution

A system where each consumer is fed from a single path from the substation. It is simple and cheap but a fault on the feeder interrupts all downstream consumers.

Ring main system

A distribution arrangement where feeders form a closed loop so each consumer has two paths. Reliability improves and voltage drop is reduced compared with a radial system.

Distribution transformer

A transformer stepping 11 kV down to 415 V three-phase four-wire for consumers. It usually has a star-connected secondary with the neutral earthed.

Switch in phase conductor

Switches must be connected in the live conductor so that when open the lamp holder is dead. A switch in neutral leaves the fitting live even when switched off.

Insulation resistance test

A 500 V DC test between conductors and earth using a megohmmeter. A new domestic installation should read at least one megohm before being energised.

Thermal overload relay

A protective device that trips when sustained overcurrent heats a bimetal element. It is set to the motor full-load current and protects against overload, not short circuit.

Single phasing

Loss of one supply phase to a running three-phase motor. The motor hums, draws heavy current in the remaining phases and overheats unless protection trips quickly.

Earthing

Connecting exposed metalwork to the ground through a low-resistance path. It keeps touch voltage low and lets protective devices clear an earth fault rapidly.

RCCB

A residual current circuit breaker that trips when the current in live and neutral conductors differs, indicating leakage to earth. A 30 mA unit protects people.

Class C fire

A fire involving live electrical equipment. It is extinguished with carbon dioxide or dry powder, never with water or foam while the equipment is energised.

IS 732

The Indian code of practice for electrical wiring installations. It specifies circuit protection, earthing, conductor sizing and inspection requirements for buildings.

IS 3043

The Indian code of practice for earthing. It describes earthing methods, electrode materials, and the resistance values needed for effective fault clearance.

Breaking capacity

The maximum short-circuit current a protective device can interrupt safely. It must equal or exceed the prospective fault current at the point of installation.

Single-line diagram

A drawing using one line to represent a multi-conductor circuit. It shows the supply, main switch, distribution board and sub-circuits for estimation and documentation.

Wastage allowance

A percentage added to measured material quantities to cover cutting, breakage and offcuts. Cable is often allowed five percent, with smaller allowances for accessories.

Zener diode

A diode operated in reverse breakdown to hold a nearly constant voltage. With a series resistor it provides a stable reference or simple voltage regulator.

Relay

An electromagnetic switch whose coil energises to move contacts. It lets a small control signal switch a larger load and provides isolation between control and load circuits.

Bridge rectifier

Four diodes arranged so both half cycles of AC are converted to DC. It needs no centre-tapped transformer and gives a ripple frequency of twice the supply.

Crimping

Pressing a metal lug onto a conductor with a matched tool to make a permanent joint. Correct lug size and die selection give a low-resistance, mechanically strong connection.

Britannia joint

A soldered joint used for overhead conductors, made by binding the cleaned conductor ends with wire before soldering. It provides mechanical strength and good conductivity.