Circuit Breakers & Switchgear Terminology Glossary: Every Term Explained Specifying switchgear correctly starts with speaking the same language as the datasheet. This glossary runs through the core terminology you'll encounter across circuit breakers, contactors, relays and distribution components — including the din rail power relay, a term that comes up often but is rarely defined clearly. Bookmark it as a quick reference the next time a datasheet, RFQ or drawing uses a term you want to double-check before ordering. Need help specifying switchgear for your site? Talk to Our Expert Team Core Terms A-Z Term Definition Typical Standard MCB (Miniature Circuit Breaker) Automatically-resettable overcurrent protection for final circuits, sized up to around 125A BS EN 60898-1 / IEC 60947-2 MCCB (Moulded Case Circuit Breaker) Higher-current breaker with adjustable trip settings, used for distribution boards and larger loads IEC 60947-2 ACB (Air Circuit Breaker) Large-frame breaker used at main incomer and busbar level, often with electronic trip units IEC 60947-2 RCD / RCCB / RCBO Devices that detect current leaking to earth and disconnect the circuit to protect people from shock BS EN 61008 (RCCB) / BS EN 61009 (RCBO) Contactor An electrically-operated switch for repeatedly making and breaking a power circuit, typically motor loads IEC 60947-4-1 / UL 489 Thermal Overload Relay A device that trips a motor circuit on sustained overload current, protecting the motor from overheating IEC 60947-4-1 Switch Disconnector A manually-operated device for isolating a circuit for maintenance, not intended for fault interruption IEC 60947-3 Auxiliary Contact Block An add-on module providing extra signalling contacts on a contactor or breaker Tested with parent device (IEC 60947 series) Busbar Distribution System Rigid conductor system distributing power along a run, with tap-off points for individual circuits IEC 61439 Din Rail Power Relay A general-purpose electromechanical or solid-state relay mounted on standard DIN rail, used to switch control or small power loads independently of a motor contactor UL 508 / IEC 60947-5-1 Motor Protection Circuit Breaker (MPCB) A breaker combining short-circuit and adjustable overload protection sized specifically for motor starting current IEC 60947-2 ACB (Air Circuit Breaker) Large-frame breaker typically used at main incomer or busbar level, often with an electronic trip unit and higher breaking capacity than an MCCB IEC 60947-2 RCCB Standalone device that detects earth leakage current and disconnects the circuit, with no overload protection function of its own BS EN 61008 RCBO Combines RCD earth-leakage detection with MCB-style overcurrent protection in a single device BS EN 61009 Isolator / Switch Disconnector Manually-operated device for isolating a circuit for maintenance; not rated to interrupt fault current IEC 60947-3 Contact Configuration (NO/NC) Describes whether a contact is Normally Open or Normally Closed in its unoperated state, used on auxiliary contacts and relays — Coil Voltage The voltage required to energise a contactor or relay coil (e.g. 24V dc, 230V ac) — must match the control circuit supply — DIN Rail Standard 35mm metal rail used to mount circuit breakers, relays and terminals inside a distribution board or enclosure EN 60715 Prospective Fault Current (PFC) The maximum current that could flow at a point in the installation under fault conditions, used to check a breaker's breaking capacity is adequate BS 7671 Discrimination / Selectivity Design principle ensuring only the breaker closest to a fault trips, leaving upstream circuits energised IEC 60947-2 Enclosure / IP Rating A code (e.g. IP54, IP65) describing an enclosure's protection against dust ingress and water ingress IEC 60529 Overload Protection Protection against sustained excess current below short-circuit level, typically provided by a thermal overload relay or MCCB trip unit IEC 60947-4-1 Short-Circuit Protection Protection against a sudden, very high fault current, provided by the magnetic or instantaneous trip element of a breaker IEC 60947-2 Cross-Reference The process of matching a discontinued or obsolete part number to a current equivalent from the same or a different manufacturer — Related Standards Terms Breaking capacity — the maximum fault current a breaker can safely interrupt without damage, usually expressed in kA. Trip curve (B/C/D) — defines how quickly an MCB trips relative to its rated current, matched to the type of load it protects. IP rating — describes the level of protection an enclosure provides against dust and water ingress. Pole count — the number of phases/conductors a device switches (1P, 2P, 3P, 4P). IEC 60947 — the international standard family covering low-voltage switchgear and controlgear, including circuit breakers (Part 2), switches and disconnectors (Part 3), and contactors and motor starters (Part 4-1). BS EN 60898 — the British/European standard for MCBs used in ho…