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Resistance and impedanceWhat slows current down, and why does it matter?
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- Resistance
- Opposition to current flow. Measured in ohms, Ω. Symbol R.
- Impedance
- Opposition to current in an AC circuit only (never DC). Symbol Z, also in ohms. It accounts for loads, such as the inductor in a fluorescent fitting, that shift current and voltage out of step. You'll meet Ze and Zs in fault loop impedance testing, and R when testing continuity.
Think of rocks in a stream: the water can't flow straight and fast, so it slows down. Every conductor has resistance, and so do cables, loads and your body. Increase resistance and current goes down, as long as the voltage stays the same.
| Best conductors | Why it matters |
|---|---|
| 1. Silver | Lowest resistance, but far too expensive to use in cable |
| 2. Copper | Second best and cheap, so it's what cables use. Aluminium is also used, for example in concentric supply cables. |
| 3. Gold | Doesn't tarnish, so it's used to plate contacts |
- Low resistance is good in neutrals and earth conductors (CPCs). If there's a fault, enough current has to flow to trip the protective device quickly.
- Your body has resistance too, and it's much lower when wet, for example straight out of the shower. That's why wet skin is so dangerous.
- A protective device (breaker or fuse) is there to protect the cable from carrying too much current. More current means more heat.
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