Maharashtra HSC Multi-Skill Electrical (NSQF) cheat sheet

Ohm's law: V = IR; power P = VI = I²R = V²/R. In a purely resistive circuit these hold for AC rms values as well.

Series circuit: same current through all elements, R_total = R1 + R2 + …; voltage divides in proportion to resistance.

Parallel circuit: same voltage across all branches, 1/R_total = 1/R1 + 1/R2 + …; current divides inversely with resistance.

Three-phase power: P = √3 × V_L × I_L × cos φ for a balanced load; single-phase P = V × I × cos φ.

Insulation resistance is measured with a megger at 500 V DC for LV circuits; a common acceptance rule is ≥ 1 MΩ, with higher values expected on healthy new wiring.

First aid for electric shock: isolate the supply, do not touch the casualty until de-energised, then assess breathing and start CPR if trained; report and record the incident.

Earth wire is green (old) or green-with-yellow (IEC); line is red/brown, neutral is black/blue. Never rely on colour alone - verify with a tester.

Leakage current to earth operates an ELCB/RCCB; 30 mA sensitivity is standard for personnel protection in domestic circuits.

Insulation resistance tester (megger): apply DC test voltage, read after 60 s for polarisation index; never test live circuits or electronic equipment.

Multimeter on voltage range has high input impedance; on current range it is near short circuit - always break the circuit and connect in series for current.

Clamp meter reads current by magnetic field around a single conductor; reading is invalid if both line and neutral pass through the jaw.

Continuity test uses a low-resistance range with the circuit dead; a healthy joint reads close to 0 Ω, open circuit reads overload.

Wattmeter/household energy meter: kWh = kW × hours; a 1 kW load for 2 h consumes 2 units (2 kWh).

Earth resistance is measured with a fall-of-potential (3- or 4-pole) tester; electrodes are spaced according to the instrument manual, typically 5–10 m apart.

Torque screwdrivers, crimping tools and cable strippers must match conductor size; a loose terminal raises contact resistance and causes heating.

Phase sequence tester confirms R-Y-B order before paralleling motors or connecting a three-phase meter.

Lighting sub-circuit: 5 A fuse/MCB, conductor 1.0–1.5 mm² copper; power sub-circuit: 15–16 A, conductor 2.5 mm² copper minimum.

Total connected load per sub-circuit must not exceed the protective device rating; diversity factor reduces the calculated maximum demand.

Two-way switch wiring uses two strapper wires between the switches plus the live feed and the switched line to the lamp; either switch toggles the lamp.

Loop-in (joint-box-free) method: supply loops from ceiling rose to ceiling rose; switch drop goes to the switch and back as switched line.

Earth continuity must be maintained to every socket, metal enclosure and fixture; metal conduit and trunking must be bonded.

Bathroom and outdoor sockets require additional protection (RCD) and IP-rated accessories; IP44 resists splashing water.

Cable sizing depends on current-carrying capacity, voltage drop (typically ≤ 3–5 % of supply) and derating for grouping and ambient temperature.

Before energising a new installation: insulation resistance, polarity, earth continuity and RCD trip-time tests must be recorded.

Transformer EMF equation: E = 4.44 × f × N × Φ_max; ratio V1/V2 = N1/N2 = I2/I1 for an ideal transformer.

Synchronous speed: Ns = 120f/P (rpm); induction motor slip s = (Ns − Nr)/Ns, typically 2–5 % at full load.

Single-phase induction motor is not self-starting; capacitor-start, capacitor-run or shaded-pole arrangements provide the starting torque.

DC motor back EMF: Eb = V − IaRa; torque T ∝ ΦIa, so field weakening raises speed.

Star–delta starting reduces phase voltage to 1/√3 of line, giving one-third starting torque compared with direct-on-line.

Transformer losses: core (iron) loss is constant at fixed voltage and frequency; copper loss varies as I² and is found by the short-circuit test.

Motor nameplate gives rated voltage, frequency, kW/HP, current, power factor, insulation class and IP rating; connections must match the nameplate star/delta data.

Commutator and brush gear on DC machines need correct brush pressure and spring tension; sparking at brushes indicates wrong pressure, brush grade or overload.

DOL starter circuit: main contactor, overload relay, start push button (NO) in parallel with a holding auxiliary contact, stop push button (NC) in series.

Star–delta starter changes the motor from star (starting) to delta (running) after a timer; interlocking prevents both contactors closing together.

Thermal overload relay is set to the motor full-load current and protects against sustained overload, not short circuit; an MCB/fuse provides short-circuit protection.

MCB types: B trips at 3–5 × In, C at 5–10 × In, D at 10–20 × In; choose according to inrush current.

Contactor coil voltage must match the control supply (e.g. 230 V AC or 24 V DC); an interposing relay is used when it does not.

No-volt protection: the contactor drops out on supply failure and does not restart automatically unless the start button is pressed.

Control wiring is numbered and terminated on terminal blocks; a wiring diagram shows the physical layout, a schematic shows the logical circuit.

Emergency stop must be hard-wired in the control circuit, not implemented only in software or a PLC output.

LT distribution chain: transformer → main switchgear → busbar → MCCB distribution board → MCB/MCCB final circuits → load.

Busbar current rating and short-circuit withstand (kA for 1 s) must exceed the prospective fault current at the point of installation.

Cable tray, trunking and conduit fill should not exceed about 40 % of the cross-section; grouping derates current-carrying capacity.

Power factor correction capacitors reduce reactive demand; kVAR required = P (tan φ1 − tan φ2).

Earthing systems: TT, TN-S and TN-C-S differ in where neutral and protective earth are combined; the installation must match the declared supply system.

Selectivity (discrimination) means the downstream device clears a fault before the upstream device; check manufacturer selectivity tables.

Cable glands and lugs must match cable size; aluminium conductors require bimetallic lugs to avoid galvanic corrosion at copper terminals.

Voltage drop in a feeder = I × R × length factor; long feeders need larger cross-section to keep drop within limits.

Five-step fault finding: gather symptoms, form hypothesis, isolate and test, repair, then verify and record.

Divide-and-conquer: test at the midpoint of a circuit to halve the search area; compare live readings with expected values from the drawing.

Open circuit shows full supply voltage across the break; a short circuit shows abnormally low resistance and trips protection.

Earth fault is located by insulation resistance testing each section separately with the circuit isolated and loads disconnected.

Preventive maintenance schedule includes thermographic scanning of terminals, torque checks, cleaning of insulation, and contact wear inspection.

Recurring overload trips indicate undersized cable, high ambient temperature, loose terminal, or a failing bearing - not always a faulty relay.

Lockout/tagout: isolate, prove dead with a tester, apply earth, tag and lock before working on any panel or machine.

Record every fault, cause and remedy in the maintenance log; history shortens future diagnosis.

Single-line diagram shows the distribution hierarchy with one line per circuit; wiring diagram shows every conductor and terminal.

Electrical layout plan places points (lights, fans, sockets, switches) and shows circuit routing, conduit runs and switchboard positions.

Estimation: quantity take-off from drawings, then rate per point/item plus material, labour, overheads and profit.

Labour cost is estimated per point or per metre of conduit; wastage allowance for cable is typically 5–10 %.

Load calculation: sum connected loads per sub-circuit, apply diversity, then select main switch and cable size.

Bill of quantities lists item, description, unit, quantity and rate; total = Σ (quantity × rate).

Standard symbols per IS/IEC drawing practice: use the legend sheet and never invent symbols without a note.

As-built drawings must be updated after installation; deviations from the design must be recorded and approved.

Communication model: sender → message → channel → receiver → feedback; noise distorts the message.

Active listening: maintain eye contact, paraphrase the message back, and confirm understanding before acting.

Resume essentials: contact details, objective, qualification, technical skills, project and practical experience; keep it to one or two pages.

Interview technique: STAR method (Situation, Task, Action, Result) for answering experience questions.

Entrepreneurship: identify a market need, prepare a business plan with cost, price, break-even and cash flow, and register the enterprise.

Break-even quantity = fixed cost ÷ (selling price − variable cost per unit).

GST registration and invoicing are statutory requirements for electrical contracting above the turnover threshold; keep records of material and labour.

Workplace ethics and safety culture: report hazards, follow the permit system, and never bypass a protective device.

Practical assessment criteria: correct tool use, safety compliance, workmanship, time, and testing/verification of the completed job.

Staircase (two-way) wiring practical: verify strapper continuity, switch both ends, and test lamp operation from both positions.

Board wiring practical: mark layout, fix accessories, run conduit with bends at correct radius, terminate with correct torque, then test insulation resistance.

Motor connection practical: identify six terminals with a multimeter, connect star or delta per nameplate, check direction of rotation before coupling the load.

Project report structure: title, objective, components list, circuit diagram, procedure, observations, result, conclusion and safety precautions.

Viva answers must be specific: state the instrument used, the expected reading and the reason for the choice.

Sequence of tests on completion: visual inspection → earth continuity → insulation resistance → polarity → RCD/ELCB trip test → functional test.

Record readings in the logbook at the time of measurement; never fill them in from memory afterwards.

Practical assessment criteria: correct tool use, safety compliance, workmanship, time, and testing/verification of the completed job.

Staircase (two-way) wiring practical: verify strapper continuity, switch both ends, and test lamp operation from both positions.

Board wiring practical: mark layout, fix accessories, run conduit with bends at correct radius, terminate with correct torque, then test insulation resistance.

Motor connection practical: identify six terminals with a multimeter, connect star or delta per nameplate, check direction of rotation before coupling the load.

Project report structure: title, objective, components list, circuit diagram, procedure, observations, result, conclusion and safety precautions.

Viva answers must be specific: state the instrument used, the expected reading and the reason for the choice.

Sequence of tests on completion: visual inspection → earth continuity → insulation resistance → polarity → RCD/ELCB trip test → functional test.

Record readings in the logbook at the time of measurement; never fill them in from memory afterwards.

Practical assessment criteria: correct tool use, safety compliance, workmanship, time, and testing/verification of the completed job.

Staircase (two-way) wiring practical: verify strapper continuity, switch both ends, and test lamp operation from both positions.

Board wiring practical: mark layout, fix accessories, run conduit with bends at correct radius, terminate with correct torque, then test insulation resistance.

Motor connection practical: identify six terminals with a multimeter, connect star or delta per nameplate, check direction of rotation before coupling the load.

Project report structure: title, objective, components list, circuit diagram, procedure, observations, result, conclusion and safety precautions.

Viva answers must be specific: state the instrument used, the expected reading and the reason for the choice.

Sequence of tests on completion: visual inspection → earth continuity → insulation resistance → polarity → RCD/ELCB trip test → functional test.

Record readings in the logbook at the time of measurement; never fill them in from memory afterwards.