Maharashtra HSC Multi-Skill Electrical (NSQF) 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 the voltage across it and inversely proportional to its resistance: I = V/R, valid for a constant-resistance element at constant temperature.

Three-phase power formula

For a balanced three-phase load, total active power equals √3 multiplied by line voltage, line current and power factor: P = √3 × V_L × I_L × cos φ.

Insulation resistance test

A DC test voltage, typically 500 V for low-voltage circuits, is applied between conductors and earth; a reading of at least one megohm indicates acceptable insulation.

RCCB sensitivity

A residual current circuit breaker rated at 30 mA sensitivity is used for personnel protection; it trips when earth leakage reaches that value within the specified time.

Megger use precaution

An insulation resistance tester must only be used on a de-energised, isolated circuit, and electronic equipment must be disconnected to avoid damage from the test voltage.

Clamp meter limitation

A clamp meter measures current from the magnetic field around one conductor; if line and neutral both pass through the jaw, the fields cancel and the reading is zero.

Energy unit calculation

One unit of electricity is one kilowatt-hour; energy in kWh equals power in kW multiplied by time in hours, so a 2 kW load for 3 hours uses 6 units.

Fall-of-potential test

Earth resistance is measured by driving auxiliary electrodes into the soil at specified spacings and passing current through them, then reading the potential difference.

Two-way switch wiring

Two strapper wires connect the two switches, with the live feed to the common of one switch and the switched line from the other to the lamp.

Domestic sub-circuit rating

A lighting sub-circuit uses a 5 A protective device with 1.0 to 1.5 square millimetre copper conductor; a power sub-circuit uses 15 to 16 A with 2.5 square millimetre.

Voltage drop limit

Cable size is chosen so that voltage drop from the origin to the load stays within the permitted percentage of supply voltage, commonly three to five percent.

IP44 rating

The first digit 4 means protection against solid objects larger than one millimetre; the second digit 4 means protection against splashing water from any direction.

Transformer EMF equation

The induced EMF in a winding equals 4.44 times frequency times turns times maximum flux, which explains why the volts-per-turn ratio is fixed by core area.

Synchronous speed

The speed of the rotating magnetic field in an induction motor is 120 times frequency divided by the number of poles, giving 1500 rpm at 50 Hz for four poles.

Slip

Slip is 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.

Back EMF

In a DC motor the back EMF equals supply voltage minus the armature current multiplied by armature resistance, and it limits the current drawn by the motor.

Holding contact

A normally open auxiliary contact of the contactor, wired in parallel with the start push button, keeps the coil energised after the button is released.

Thermal overload relay

A device with a bimetallic element heated by motor current; it trips after sustained overload and is set to the motor full-load current from the nameplate.

MCB type C

A miniature circuit breaker with a C characteristic trips magnetically at five to ten times its rated current, suiting loads with moderate inrush such as small motors.

Star-delta starting

The motor starts with windings in star, reducing phase voltage and starting torque to one third, then switches to delta when the timer completes.

Busbar short-circuit rating

The rated short-time withstand current, quoted in kiloamperes for a stated duration such as one second, must exceed the prospective fault current at that point.

Power factor correction

Capacitor bank rating in kVAR equals active power multiplied by the difference between the tangents of the initial and target power factor angles.

Selectivity

Discrimination between protective devices means the device nearest the fault operates first, so that healthy parts of the installation remain supplied.

Conduit fill rule

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

Earth fault location

Insulation resistance is measured section by section with the circuit isolated and loads disconnected; the section showing a low reading contains the earth fault.

Lockout tagout

The supply is isolated, proved dead with a tester, earthed where required, and locked with a personal tag before work begins on a panel or machine.

Open circuit symptom

A break in a circuit shows full supply voltage across the break when measured with a voltmeter, while no current flows and the load does not operate.

Single-line diagram

A drawing that represents the distribution system with one line per circuit, showing the incoming supply, main switchgear, sub-boards and outgoing feeders.

Bill of quantities

A priced schedule listing each item with description, unit, quantity and rate; the total cost is the sum of quantity multiplied by rate for every item.

Break-even quantity

The number of units at which total revenue equals total cost, calculated as fixed cost divided by the difference between selling price and variable cost per unit.

STAR interview method

A structured answer describing the Situation, the Task required, the Action taken and the Result achieved, used to present practical experience clearly.

Completion test sequence

After visual inspection, the installer checks earth continuity, insulation resistance, polarity, residual current device trip time and finally the functional operation of every circuit.