Technical Knowledge: Electrician Trade
Electrician Trade covers circuit laws, AC and DC machines, wiring practice, protective devices and safety. Numerical items on Ohm's law and series-parallel networks are common and quick to score.
Core concepts
- Ohm's law: V = IR; power P = VI = I2R = V2/R.
- Series: same current, voltages add, resistances add.
- Parallel: same voltage, currents add, 1/R = 1/R1 + 1/R2.
- AC fundamentals: RMS value = peak/root 2; frequency = 1/period.
- Transformer: EMF per turn is constant; a step-down transformer reduces voltage and increases current.
- Earthing protects against leakage current; fuse and MCB isolate faults.
Worked example
A 6 ohm and a 3 ohm resistor are connected in parallel across 12 V. Equivalent resistance = (6 x 3)/(6 + 3) = 2 ohm. Total current = 12/2 = 6 A. Branch currents are 12/6 = 2 A and 12/3 = 4 A, and they sum to 6 A, confirming the result. Power drawn = 12 x 6 = 72 W.
Traps
In parallel circuits the voltage is common, not the current. Energy billed is in kWh: kWh = watts x hours / 1000. Power factor below unity means more current for the same real power. Never connect an ammeter in parallel or a voltmeter in series.
Key points
- V = IR and P = VI = I2R = V2/R.
- Parallel resistance: product over sum for two resistors.
- RMS value = peak value divided by root 2.
- Energy in kWh = watts x hours / 1000.
- Ammeter in series, voltmeter in parallel, always.