Short circuit (cortocircuito)

What happens in a short circuit

Normally, current passes through an appliance (a bulb, a motor, a heating element) that limits how much flows. In a short circuit, the live wire touches the neutral or another phase directly and the current meets only the cable’s tiny resistance. The current jumps to huge values, there is a spark or a small bang and the cable heats until its insulation melts.

The magnetic part of the MCB (magnetotérmico) detects that peak and cuts within milliseconds. To do so safely it needs sufficient breaking capacity.

Common causes at home

• Cables whose insulation has deteriorated with heat or age, typical of old installations.
• Poor joints or junction boxes with loose connections.
• A nail or drill going through a buried cable.
• Water or damp in sockets, light fittings or boxes.
• Faulty appliances or appliance leads that are crushed or stripped.

Short circuit or overload

In an overload the circuit works but has too many appliances connected, and the MCB takes a while to trip. In a short circuit there is a fault and it trips instantly, sometimes with a spark or a noise.

Warning signs

A spark when plugging in, black marks on a socket or switch, a smell of burning plastic, or a breaker that trips as soon as you switch it on.

What to do and how we repair it

Do not keep switching the breaker back on: every attempt repeats the spark. Unplug the appliances on that circuit and, if it still trips, the fault is in the installation. We find the point, replace the damaged section and test the insulation. As a guide, a simple repair costs 50 to 100 euros plus VAT. See our 24-hour emergency electrician in Seville, Spain.