Units and Safety
What the domain tests
This domain checks whether you can express electrical quantities in correct SI units, handle prefixes and significant figures, and apply safe working practice. Safety is not a separate topic: wrong units can lead directly to wrong fuse ratings and unsafe circuits.
Units and prefixes
- Base electrical units: ampere (A), volt (V), ohm (Ω), watt (W), joule (J), second (s), coulomb (C), hertz (Hz).
- Prefixes: k = 10³, M = 10⁶, G = 10⁹, m = 10⁻³, µ = 10⁻⁶, n = 10⁻⁹, p = 10⁻¹².
- Keep units consistent: 4.7 kΩ = 4700 Ω; 2200 µF = 2.2 × 10⁻³ F.
- Earthing gives fault current a low-resistance path. A fuse or MCB protects cable against overload; a 30 mA RCD detects earth leakage and protects people. Double insulation avoids reliance on an earth conductor.
- Safe isolation means isolate, lock off, prove the tester, test dead, and prove the tester again.
Worked example
A 2.3 kW heater operates at 240 V. Current I = P/V = 2300/240 = 9.58 A. A 13 A plug fuse is a common standard size and can carry this current, but the cable and plug must also be rated for it. A fixed circuit might use a 10 A MCB instead. The fuse protects the cable, not the heater element.
Traps that cost marks
Do not treat µF as mF or kΩ as Ω. A fuse does not replace an RCD, and an RCD does not protect against overload. Never work live when isolation is possible; proving dead is a required step, not a formality.