What is a short circuit?

Glossary · Wiring, circuits and faults

What causes a short circuit

In a healthy circuit, current flows out along the live conductor, through the load, and back along the neutral. The load limits how much current flows. In a short circuit, that load is bypassed. A live conductor touches a neutral conductor, or a faulty appliance gives current a direct route, with almost no resistance in the way. The current rises sharply as a result.

Common causes are damaged or aged insulation, a nail or screw through a cable, a faulty appliance and a loose or damaged connection. Electrical Safety First says not to ignore burn marks, arcing (buzzing or crackling) sounds, fuses blowing, circuit-breakers tripping or anything hot to the touch, and to unplug the item and contact the retailer, manufacturer or a qualified repair technician.^{[4]Electrical Safety First, Electrical Safety Advice for Earthing and Bonding}

BS 7671 treats this as a fault current, separate from an overload. NICEIC says an overload is an overcurrent in an electrically sound circuit, and that all circuits, whether liable to overload or not, must be protected against fault current, with a few exceptions.^{[2]NICEIC, Overcurrent protection - overload} See overload for the difference.

How the protection responds

The consumer unit exists to stop this. The IET says fuses provide protection against overcurrent and short-circuit faults, and that the modern equivalent is the circuit-breaker, with one device per final circuit.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes} NICEIC says circuit breakers break the circuit when tripped by short-circuit issues or overloads, and switch off faster than traditional fuses, which helps minimise damage to wiring and appliances.^{[1]NICEIC, Consumer units and fuse boxes: installation and replacement} Our guide to the circuit breaker explains the device and the guide to fuses covers the older type.

The protective device is matched to the cable. NICEIC explains how a 30 A or 32 A device on a ring protects the 2.5 mm² conductors of an unfused spur against short-circuit current.^{[5]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?} Choosing and matching those ratings is design work for a registered electrician.

What you can check yourself

If a breaker has tripped, the IET says the cause is likely to be a fault with an appliance rather than the installation, and describes a fault-finding routine: unplug every appliance on the circuit fully, reset the breaker, and if it holds, plug appliances back one at a time to find the faulty one. A faulty item should not be used again unless it has been repaired.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes} If the breaker will not reset with everything unplugged, the IET says to consult a registered electrician.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes}

The IET also warns that attempting to reset a breaker or RCD on a fault repeatedly could damage the device, so keep attempts to a minimum.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes} If you notice a burning smell, excessive heat, smoke or charring at the consumer unit, the IET says to isolate the installation immediately and consult a registered electrical contractor.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes} Our guide to resetting a tripped consumer unit has the routine in order.

When to call an engineer

Call a registered electrician for any short circuit that you cannot trace to a single faulty appliance, any trip that repeats, and any sign of heat, smoke or scorching. If there is a burning smell from a socket, see our guide to burning smell from a socket.

Example

Peter in Burley plugs in an old lamp and the whole lighting circuit trips with a bang. He unplugs the lamp, resets the breaker once and the circuit holds. The lamp’s flex has a split in the insulation and he does not use it again. He keeps the lamp out of use until it is repaired and tells his electrician when he next calls, so the circuit can be checked.

What people get wrong

Many people think a short circuit means the breaker has failed because it keeps tripping. The breaker is doing its job. It is responding to a fault, and the fault is still there until it is found and fixed.

The second mistake is resetting again and again, or fitting a larger breaker to stop the tripping. The IET warns repeated resetting on a fault can damage the device.^{[3]IET, Consumer units - safe fuse-box upgrades for modern homes}

The bottom line

A short circuit is a fault that gives current a very low-resistance path, producing a large, sudden current. Fuses and circuit breakers disconnect it, which is why a trip is a warning, not a nuisance. Unplug, reset once and find the faulty appliance if you can. If it trips again or you see heat, smoke or scorching, leave it off and call a registered electrician.