What is overcurrent?
Overcurrent is an electrical current larger than the current a cable or appliance is allowed to carry. On a circuit protected at 16 amps, any current above that 16 A is overcurrent. Do not confuse it with overvoltage: there the voltage is too high, not the current.
Overcurrent is dangerous because a conductor heats up as the current through it rises. Once more heat is generated than the cable can dissipate, the insulation softens — leading to a short circuit or a fire.
Two causes
- Overload. Simply too much load connected to one circuit. The current rises relatively slowly above the permitted value. A 16 A circuit at 230 V delivers roughly 3,600 watts; a kettle, a hairdryer and a vacuum cleaner together are already at that limit. On a 120 V system the same reasoning applies with a 15 or 20 A circuit at around 1,800 to 2,400 watts.
- Short circuit. A direct connection between live and neutral, or between live and earth. The current jumps to many times the rated value within a fraction of a second.
What protects against it
- The circuit breaker (or MCB) protects against both. It has a thermal element that responds slowly to overload and a magnetic element that trips almost instantly on a short circuit. The marking gives a letter and a number, for example B16 or C16: the number is the rated current, the letter the tripping characteristic of the magnetic part.
- The fuse performs the same job in older installations.
- A residual current device does not protect against overcurrent. An RCD (or GFCI) responds to leakage current to earth and protects people against electric shock. They are two different devices with two different jobs.
| Circuit breaker (MCB) | Protects against overload (thermal element, responds slowly) and short circuit (magnetic element, trips almost instantly). |
|---|---|
| Fuse | Performs the same job as the circuit breaker in older installations. |
| Residual current device (RCD or GFCI) | Responds to leakage current to earth and protects people against electric shock; does not protect against overcurrent. |
In practice
- The protective device belongs to the cable, not to the appliance. Fitting a higher-rated breaker on an existing thin cable effectively removes the protection. It is one of the most dangerous mistakes in a domestic installation.
- If a circuit trips repeatedly, find the cause. Resetting without removing the cause means overloading the cable again and again.
- Split the heavy loads. Washing machine, dryer, oven and induction hob each deserve their own circuit.
- Work to the national wiring regulations and leave changes to the consumer unit to a qualified electrician.
Related terms
- Residual current device
- Overvoltage
- Overpressure
- Connected load
- Earthing
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