Assam HS Skill Electrical Mechanics and Electrician flashcards

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

Ohm's law

Current through a conductor is directly proportional to voltage and inversely proportional to resistance, expressed as V = I × R, valid for linear resistive circuits at constant temperature.

RMS value

The effective value of an alternating quantity that produces the same heating as an equal DC value; for a sine wave it equals peak divided by root two, about 0.707 times peak.

Power factor

The cosine of the phase angle between voltage and current in an AC circuit; it equals true power in watts divided by apparent power in volt-amperes.

Kirchhoff's voltage law

Around any closed loop of a circuit, the algebraic sum of source EMFs equals the algebraic sum of resistive and reactive voltage drops, so the net loop voltage is zero.

Three-phase power formula

Total active power in a balanced three-phase load equals root three times line voltage times line current times power factor, giving watts when voltage and current are RMS line values.

Cable colour code

In Indian wiring practice the phase is red or brown, neutral is black or blue, and protective earth is green or green-yellow, enabling safe identification at every accessory.

Western Union joint

A soldered joint for solid conductors made by twisting the ends, binding with a third wire, soldering and insulating; it is compact and used inside junction boxes.

Married joint

A high-strength soldered joint for stranded conductors where strands are fanned, interlocked, bound with a binding wire and soldered, then insulated with tape or sleeving.

Insulation resistance test

A DC test using a 500 V megger between live conductors and earth; a healthy low-voltage installation reads at least one megohm, while lower values indicate moisture or damage.

Conduit fill rule

The total cross-sectional area of conductors in a conduit should not exceed about forty percent of the conduit internal area, allowing heat dissipation and easy drawing of wires.

Transformer EMF equation

The RMS induced EMF in a winding equals 4.44 times frequency times turns times maximum flux, where 4.44 is the form factor of a sine wave multiplied by two pi.

Slip of induction motor

The difference between synchronous speed and actual rotor speed expressed as a fraction of synchronous speed; typical full-load slip for standard motors is two to five percent.

Synchronous speed

The speed of the rotating magnetic field, calculated as 120 times supply frequency divided by the number of poles; a four-pole motor on 50 Hz runs at 1500 rpm.

Star-delta starter

A starter that runs the motor in star during starting, reducing phase voltage to 57.7 percent and starting torque and line current to one-third of direct-on-line values.

DC series motor caution

A series motor must never be started without load because flux falls to a small value and speed rises dangerously, risking mechanical destruction of the armature.

Back EMF

The voltage induced in a rotating armature that opposes the supply voltage; it equals supply voltage minus armature current times armature resistance, and it limits motor current.

PMMC instrument

A permanent-magnet moving-coil meter whose deflection is proportional to current; it responds only to DC and has a linear scale, so rectifiers are used for AC measurement.

Moving iron instrument

An instrument where a soft iron vane is attracted or repelled by a coil's field; it works on AC and DC, reads RMS values, and has a non-linear scale.

Energy meter constant

The number of revolutions the disc makes per kilowatt-hour consumed; energy in kWh equals revolutions counted divided by this constant, used to check meter accuracy.

Megger

A high-resistance insulation tester that applies a DC test voltage, commonly 500 V or 1000 V, and indicates insulation resistance in megohms on a hand-cranked or electronic scale.

Current transformer

An instrument transformer whose primary carries the load current and whose secondary supplies a standard five-ampere signal to meters, also isolating them from high voltage.

Earthing resistance limit

For domestic installations earth resistance should be low, commonly one to five ohms depending on supply conditions, and is measured by the fall-of-potential method.

MCB trip types

Type B trips at three to five times rated current, type C at five to ten times, and type D at ten to twenty times, chosen according to load inrush current.

RCCB

A residual current device that compares live and neutral currents and trips when the imbalance exceeds its sensitivity, commonly thirty milliamperes for personal protection.

Pipe earthing

An earth electrode made of a galvanised iron pipe about 38 millimetres diameter driven vertically into moist soil, surrounded by charcoal and salt, with a watering funnel.

Class C fire

A fire involving energised electrical equipment; carbon dioxide or dry powder extinguishers are used, and water or foam must never be applied until the supply is isolated.

Single-line diagram

A drawing that represents a power system with one line per circuit, showing sources, transformers, busbars and protective devices with their ratings, omitting individual wiring.

Bill of quantities

A tabulated list of every item of work with description, unit of measurement, quantity and rate, forming the basis of an electrical installation estimate and tender.

Diversity factor

The ratio of the maximum demand of an installation to the sum of individual connected loads, used to size main switches and cables realistically rather than by total load.

SCR

A silicon controlled rectifier conducts from anode to cathode only after a gate pulse and remains on until anode current drops below the holding current value.

TRIAC and DIAC

A TRIAC conducts in both directions for AC phase control, while a DIAC is a bidirectional trigger device used to fire the TRIAC symmetrically in dimmer and fan regulator circuits.

VFD

A variable frequency drive converts AC to DC and back to adjustable-frequency AC, controlling induction motor speed because speed is proportional to frequency divided by poles.