Short circuit
A short circuit is an unintended connection of very low between points that should have a difference, such as line and neutral. It bypasses the intended load and can allow a large to flow, limited by the supply and circuit impedance. It differs from an caused by too many loads. PSCC describes the current that could flow in such a fault, rather than a current flowing in normal use.
Where Short circuit fits in the learning path
The Installation section covers these units:
- Wiring Systems & Enclosures
- Cable Selection & Circuit Design
- Earthing, Bonding & Consumer Units
Curriculum mapping: C&G 2365 Units 203/204/305.
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- Prospective Fault CurrentTesting & Inspection
- Consumer UnitsInstallation
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The highest current that would flow in a short circuit between live conductors — line to neutral, or line to line on three-phase supplies. Together with the prospective earth fault current it makes up the prospective fault current, and the larger of the two is what switchgear breaking capacity is checked against.
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PSCC — Prospective Short-Circuit Current
The highest current that would flow in a short circuit between live conductors — line to neutral, or line to line on three-phase supplies. Together with the prospective earth fault current it makes up the prospective fault current, and the larger of the two is what switchgear breaking capacity is checked against.
Read the PSCC entryPFC — Prospective Fault Current
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.
Read the PFC entryOvercurrent
Any current above the value a circuit is designed to carry. It comes in two flavours with different time scales: overload, where a healthy circuit is simply asked for too much for too long, and fault current, where a short circuit drives an enormous current that must be broken in milliseconds. Protective devices are chosen to handle both.
Read the Overcurrent entryMCB — Miniature Circuit Breaker
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.
Read the MCB entryOverload
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.
Read the Overload entryImpedance
Impedance is the combined opposition that resistance and reactance present to alternating current, measured in ohms (Ω). Reactance comes from inductance and capacitance, so impedance is not always the same as a resistance reading. Earth fault loop impedance, Zs, describes the whole fault-current path; R1+R2 describes the resistance of two circuit conductors.
Read the Impedance entryA definition is a start. Using it in a real question is where it sticks.