Domestic Electrical Installation Safety & Circuit Protection

A set of vocabulary flashcards summarising essential safety terms, protective devices and regulatory concepts from the lecture on electricity supply systems and domestic installation safety.

Circuit protection

Methods and devices required by BS 7671 to automatically disconnect a circuit when overcurrent occurs, preventing shock, overload and cable damage.

Safe isolation

The procedure of securely disconnecting and proving a circuit is dead before any work is carried out on electrical installations.

BS 7671

The UK Wiring Regulations that set legal standards for design, installation and protection of electrical circuits.

Overcurrent

Any current greater than that for which a circuit is designed, including overloads and short-circuits.

Maximum disconnection time

The longest time allowed by BS 7671 for a protective device to cut power after a fault; 0.4 s for TN and 0.2 s for TT systems up to 63 A.

TN system

Earthing system where the supply cable contains the earth conductor; disconnection time must not exceed 0.4 s (≤63 A) or 5 s (>63 A).

TT system

Earthing system that uses a local earth electrode; disconnection time must not exceed 0.2 s (≤63 A) or 1 s (>63 A).

Fuse

A sacrificial circuit-protection device that melts to open the circuit when excessive current flows.

Circuit breaker

Resettable protective switch that automatically opens a circuit under overcurrent or short-circuit conditions.

Residual Current Device (RCD)

Protective device that continuously compares line and neutral currents and trips when an imbalance (leak to earth) is detected.

Electric shock

Physical injury caused when current passes through the human body, possible via live parts or faulty exposed parts.

Basic protection (direct contact)

Protection against shock under fault-free conditions by preventing contact with live parts intended to carry current.

Fault protection (indirect contact)

Protection against shock if exposed or extraneous metal parts become live due to a single fault.

Equipotential zone

Area where all exposed and extraneous conductive parts are bonded to the main earthing terminal, keeping them at the same potential.

Earth fault loop

The conductive path taken by fault current from the fault point back to the supply, enabling protective devices to operate.

Automatic Disconnection of the Supply (ADS)

System where a fault causes rapid operation of the protective device, automatically cutting power within the required time limits.

Safe isolating procedures

Steps that include identifying the supply, switching off, locking off, and verifying isolation before work begins.

Overload

Excess current in a normally conducting path, often due to too many appliances drawing power.

Short circuit

Fault where current bypasses the intended load through a path of very low resistance, leading to a surge of current.