BTEB Electrician (EIM) Exam cheat sheet

Electric shock severity depends on current path, magnitude and duration: 30 mA across the heart can cause ventricular fibrillation; 50 mA is a commonly cited lethal threshold.

Body resistance falls from about 100 kΩ dry skin to roughly 1 kΩ when skin is wet, so touch voltage limits are set low: 50 V AC in dry, 25 V AC in wet or confined locations.

First aid for a shock victim: isolate supply, do not touch the casualty until proved dead, call for help, start CPR if breathing has stopped, place unconscious but breathing casualties in the recovery position.

Fire classes: A solid, B liquid, C gas, D metal, F cooking oil; use CO2 or dry powder on live electrical (Class C/E) fires, never water or foam.

Lockout/tagout sequence: identify all energy sources, isolate, lock and tag, prove dead with a tested voltage detector, discharge stored energy, then work.

A permit-to-work is required for live working, work in confined spaces, and work on high-voltage equipment; it records isolation points, limits and authorisation.

Personal protective equipment hierarchy: eliminate, substitute, engineer controls, administrate, then PPE last; PPE for electrical work includes insulating gloves, arc-rated clothing, face shield and safety footwear.

Environment: waste segregation (e-waste, lamps, batteries, solvents), spill kits for transformer oil, and no burning of PVC insulation because it releases hydrogen chloride.

Ohm's law V = I × R; power P = V × I = I²R = V²/R; for three-phase, P = √3 × V_L × I_L × cos φ.

Series: R_total = R1 + R2 + …; parallel: 1/R_total = 1/R1 + 1/R2 + …; two resistors in parallel give product over sum.

Kirchhoff's laws: current law, sum of currents at a node is zero; voltage law, algebraic sum of EMFs and drops around a loop is zero.

Energy in kWh = power in kW × time in hours; one unit of electricity = 1 kWh = 3.6 MJ.

AC: f = 1/T; ω = 2πf; X_L = 2πfL; X_C = 1/(2πfC); Z = √(R² + (X_L − X_C)²); cos φ = R/Z.

Star (Y) and delta (Δ) relations: in a balanced star, V_L = √3 V_ph and I_L = I_ph; in delta, V_L = V_ph and I_L = √3 I_ph.

RMS and peak: V_rms = V_peak/√2 ≈ 0.707 V_peak; V_peak = 1.414 V_rms; form factor 1.11, peak factor 1.414 for a sine wave.

Temperature coefficient: R2 = R1[1 + α(T2 − T1)]; conductor resistance rises with temperature, insulation resistance falls with temperature and moisture.

Copper: resistivity about 1.72 × 10⁻⁸ Ω·m, excellent conductor, used for conductors and busbars; annealed copper is soft, hard-drawn is stronger.

Aluminium: resistivity about 2.83 × 10⁻⁸ Ω·m, about 60% of copper conductivity by volume but far lighter; requires antioxidant paste and bimetallic joints to prevent electrolytic corrosion with copper.

Insulation classes and limiting temperatures: Y 90 °C, A 105 °C, E 120 °C, B 130 °C, F 155 °C, H 180 °C, C above 180 °C.

PVC insulation: good dielectric, flame-retardant, maximum conductor temperature 70 °C; XLPE: 90 °C continuous, higher current rating and better overload withstand.

Magnetic materials: soft iron and silicon steel (low hysteresis, high permeability) for cores; hard magnetic materials such as Alnico and ferrite for permanent magnets.

Contact materials: silver alloys for low contact resistance; tungsten for arc resistance in heavy contacts; brass and phosphor bronze for springs and terminals.

Solder: tin-lead 60/40 melts near 183–190 °C; lead-free SAC alloys melt near 217–227 °C and need higher iron temperatures.

Conduit and trunking: PVC for corrosive or damp areas, steel for mechanical protection and earth continuity; minimum conduit size is chosen so cable fill does not exceed about 40%.

Insulated hand tools must be rated 1000 V and marked; check insulation for cuts before use and never use them as a substitute for isolation.

Wire stripping: set the stripper to the conductor gauge so strands are not nicked; a nicked strand reduces current capacity and creates a hot spot.

Crimping: match the lug to conductor size and use the correct die; a correct crimp shows no loose strands, passes a pull test and is insulated or heat-shrunk.

Bending conduit: minimum bend radius is commonly 6–10 times the conduit diameter; use a bending spring for PVC and a hickey or bender for rigid steel.

Terminal tightening: use a torque screwdriver where specified; loose terminals cause arcing and overheating, over-tightened terminals damage strands and threads.

Soldering a joint: clean and tin the iron, heat the joint and apply solder to the joint not the iron, aim for a shiny concave fillet, avoid a cold dull joint.

Measuring tools: vernier caliper for conductor and conduit diameter, micrometer for wire gauge, megger for insulation, multimeter for volts, amps and continuity.

Safe hand-tool practice: sharp-edged tools stored sheathed, chisels and files used away from the body, no loose clothing near rotating tools, eye protection for drilling and cutting.

Domestic lighting circuits are commonly wired in loop-in (loop at the ceiling rose) or loop-in at the switch; the three-plate rose carries permanent live, switched live and neutral.

Radial circuits serve one direction from the board; ring circuits return to the board, so a broken ring must be detected by continuity tests of all three conductors.

Two-way switching uses two strappers between the switches plus common and switched live; intermediate switches are inserted in the strappers for three or more control points.

Cable colour convention (IEC): line brown, neutral blue, protective earth green-yellow; old UK colours were red line, black neutral, green earth.

Cable sizing depends on design current, protective device rating, voltage drop (typically 3% lighting, 5% other loads) and correction factors for grouping and ambient temperature.

Voltage drop mV/A/m tables: drop = (mV/A/m × current × length)/1000; long runs often force a larger cable than current-carrying capacity alone.

Conduit and trunking systems: draw wires after conduit is erected, use inspection bends and boxes, and keep bends to a minimum to allow rewiring.

Segregation: extra-low-voltage and mains cables in the same enclosure require separate compartments or a barrier; data and power cables cross at right angles to limit interference.

MCB types: B trips at 3–5 × In, C at 5–10 × In, D at 10–20 × In; choose B for resistive loads, C for inductive motor loads, D for high inrush.

Fuse: rewirable and HRC types; HRC cartridge fuses have a high breaking capacity and current-limiting action; the fuse rating must not exceed the cable current capacity.

RCD residual current device: 30 mA for supplementary protection of socket outlets, 100–300 mA for fire protection; test button checks the trip mechanism, not the wiring.

Earthing systems: TN-S separate earth, TN-C-S combined neutral and earth (PME), TT local earth electrode; TT installations require an RCD because loop impedance is high.

Earth fault loop impedance Zs must satisfy Zs ≤ U0/Ia, where Ia is the current that trips the device within the required disconnection time.

Bonding: main protective bonding connects gas, water and structural metal to the main earth terminal; supplementary bonding is used in bathrooms and special locations.

Discrimination: the upstream device must not trip before the downstream device; check time-current curves and use a ratio of at least 1.6 for MCBs in series.

IP rating: first digit solid protection 0–6, second digit water protection 0–8; bathrooms require IPX4 in zones 1 and 2, and SELV (12 V AC) for luminaires in zone 0.

DC machine EMF: E = (p × φ × Z × N)/(60 × a), where p poles, φ flux per pole, Z conductors, N rpm, a parallel paths; a = p for lap, a = 2 for wave.

DC motor torque T = 0.159 × φ × I_a × Z × p/a (N·m) or T ∝ φ I_a; shunt motor speed falls slightly with load, series motor must never start unloaded.

Three-phase induction motor synchronous speed Ns = 120f/p; slip s = (Ns − N)/Ns; rotor frequency = s × f.

Starting methods for induction motors: direct-on-line, star-delta (reduces starting current and torque to one third), autotransformer, soft starter, variable frequency drive.

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

Transformer losses: copper loss I²R varies with load squared, core loss (hysteresis plus eddy current) is nearly constant; maximum efficiency occurs when copper loss equals core loss.

Single-phase motors: capacitor-start, capacitor-run and split-phase types rely on a phase-split auxiliary winding; shaded-pole motors are low torque and low efficiency.

Machine cooling and rating: insulation class limits winding temperature rise; a motor rated for 40 °C ambient must be derated above that, and frequent starts raise winding heating.

DOL starter circuit: main contactor with overload relay, start push button in parallel with a holding (seal-in) auxiliary contact, stop button in series with the coil.

Forward-reverse control uses two contactors with electrical interlock (NC contacts in each other's coil circuit) and preferably a mechanical interlock.

Star-delta starter sequence: main and star contactors close, then after a timer the star opens and delta closes; never allow star and delta to close together (short circuit).

Overload relay: thermal type uses a bimetal strip and is set to motor full-load current; magnetic type responds to high current; it protects the motor, not the operator.

Contactor vs relay: a contactor switches high current motor loads and has arc-suppressing contacts and a strong magnetic blowout; a relay switches control circuits.

Limit switches, float switches, pressure switches and thermostats are pilot devices wired in the control circuit to start, stop or reverse the motor at a set condition.

VFD: rectifies AC to DC, then inverts to variable frequency and voltage; it reduces starting current and gives speed control, but needs shielded cable and an EMC earth.

Control panel protection: short-circuit protection by fuses or MCCB, overload by thermal relay, no-volt release by the contactor coil, and emergency stop hard-wired in series.

Continuity test: prove the instrument on a known source, isolate the circuit, measure between the ends of the conductor and record the value; expected value is under 1 Ω for a healthy protective conductor.

Insulation resistance test: 500 V DC for 230/400 V circuits, minimum acceptable 1 MΩ; disconnect electronics and surge devices, and test line-neutral, line-earth, neutral-earth.

Polarity test: verify that the line conductor is at the centre contact of the lamp holder and the switch is in the line conductor, not the neutral.

Earth fault loop impedance: measured at the farthest point; compare with the maximum Zs for the protective device from the manufacturer's table.

RCD test: use an RCD tester to confirm trip current and time; a 30 mA device should trip within 300 ms at rated residual current, and a half-rated current test should not trip.

Fault finding sequence: gather symptoms, form a hypothesis, test in a logical order (supply, control, load), confirm the fault, repair, then retest and record.

Common faults: open circuit (no continuity), short circuit (low resistance, blown fuse), earth fault (insulation failure), high resistance joint (overheating, voltage drop).

Maintenance schedule: thermal imaging of joints, torque checks, cleaning of vents and filters, megger tests of motors, and inspection of contacts for pitting and wear.

Standard symbols: a circle with a cross is a lamp, a rectangle with a diagonal is a resistor, an arrow through a contactor coil indicates a mechanical link, and dashed lines indicate an enclosure.

Single-line diagram shows the distribution path with one line per circuit; a schematic shows the electrical connections and control logic in detail.

Wiring diagram vs schematic: a wiring diagram shows physical terminal numbers and wire colours; a schematic shows functional relationships without physical layout.

Estimation: take off quantities from drawings, add for wastage (typically 5–10% for cable, 3–5% for accessories), then apply labour rates and overheads.

Load estimation: sum connected loads with diversity factor applied, then size the main switch, cable and meter to the maximum demand.

Bill of quantities lists items with description, unit, quantity and rate; a schedule of rates is used for variations and extra work.

As-built drawings record the installed arrangement and must be updated after any modification; cable schedules list circuit number, size, type, length and protective device.

Documentation set for a job: drawing register, test certificates, operation and maintenance manuals, risk assessments and the handover pack.

Diode: forward biased above about 0.7 V for silicon, 0.3 V for germanium; reverse bias blocks current until the reverse breakdown voltage.

Rectifiers: half-wave gives V_dc = 0.318 V_peak; full-wave and bridge give V_dc = 0.636 V_peak; a smoothing capacitor reduces ripple.

Transistor as a switch: base current I_B must exceed I_C/h_FE for saturation; the transistor then behaves like a closed switch between collector and emitter.

Thyristor (SCR) latches on when the gate is pulsed and the anode is positive; it turns off only when anode current falls below the holding current.

LED needs a series resistor R = (V_supply − V_LED)/I_LED; a typical LED drop is 2 V and a current of 10–20 mA.

PLC basics: inputs from sensors, outputs to contactors and valves; ladder logic uses normally open and normally closed contacts, coils, timers and counters.

Sensors: PIR detects infrared movement, LDR resistance falls with light, thermistor resistance changes with temperature, proximity sensors detect metal inductively or any object capacitively.

Solar PV: panels give DC, an inverter converts to AC, and a charge controller regulates battery charging; a blocking diode prevents night-time reverse current.

Before working: identify the circuit, isolate, lock off, prove dead with a tested voltage indicator, and place warning notices.

Cable preparation: strip only the length needed, keep the sheath inside the enclosure, and ensure the earth conductor is the last to be disconnected and the first to be reconnected.

Terminal connection: form a neat loop clockwise around the screw so tightening pulls the wire in, use ferrules on flexible conductors, and avoid mixing dissimilar metals without a bimetallic connector.

Board work: keep line and neutral separated, label circuits, maintain a wiring schedule, and leave sufficient slack for re-termination.

Viva topics: state Ohm's law, explain why the earth is needed, describe how an RCD works, name the insulation classes, and explain star-delta starting.

Safe testing: use the correct category (CAT III or IV) meter, check the meter on a known live source before and after the test, and never test alone on live equipment.

Handover: demonstrate the installation, explain the isolation points, hand over test certificates, and record any departures from the drawing.

Housekeeping: clear offcuts, coil and label spare cable, remove temporary supplies, and restore covers and enclosures before energising.