Overcurrent

An automatic switch that turns a circuit off when too much current flows, whether a slow overload or a sudden short circuit. It has replaced the rewirable fuse in modern boards. Each one is chosen by its current rating, such as 6 A for lights or 32 A for sockets, and by its type: B for most home circuits, C or D where equipment draws a big surge as it starts.

The highest current that could flow at a given point if a fault of negligible impedance occurred there — whether between live conductors or from live to earth. Every protective device must be able to break the prospective fault current at its position without failing, which is why PFC is measured or calculated at the origin of every installation.

The current a cable can carry continuously in its installed conditions without its insulation exceeding the temperature it was designed for. Tabulated values assume reference conditions; the real, installed capacity is what remains after correction factors for temperature, grouping and insulation have taken their share.

An overload occurs when an otherwise sound circuit carries more current than its intended rating because of the loads connected to it. Several high-power appliances on one extension lead can exceed its rating even without a short circuit. Overload is a kind of overcurrent, and can cause overheating. An RCD alone does not provide overload protection; that is a different protective function.

A short circuit is an unintended connection of very low impedance between points that should have a voltage difference, such as line and neutral. It bypasses the intended load and can allow a large current to flow, limited by the supply and circuit impedance. It differs from an overload caused by too many loads. PSCC describes the current that could flow in such a fault, rather than a current flowing in normal use.