IEC 60947-1 General rules
Scope, definitions, service conditions, constructional and performance requirements applicable to all LV switchgear. The reference part for interpreting all subsequent parts.
IEC 60947
Low-Voltage Switchgear and Controlgear
IEC 60947 is the multi-part standard governing low-voltage switchgear and controlgear — circuit breakers, contactors, motor starters, fuses, relays and disconnectors. It defines performance, testing and safety requirements for equipment operating at voltages up to 1000 V AC and 1500 V DC.
Document structure
IEC 60947-2 Circuit-breakers
Requirements for MCBs, MCCBs, ACBs including breaking capacity, selectivity, zone-selective interlocking, and arc flash energy. Defines categories A and B (non-selectivity vs. selectivity).
IEC 60947-3 Switches, disconnectors, switch-disconnectors and fuse-combination units
Isolation and switching requirements. Defines utilisation categories (AC-21, AC-22, AC-23 for resistive, mixed and motor loads).
IEC 60947-4-1 Contactors and motor starters — Electromechanical contactors and motor starters
The core reference for contactors (AC-1, AC-2, AC-3, AC-4 utilisation categories) and direct-on-line (DOL) starters. Includes thermal overload relay requirements.
IEC 60947-4-2 AC semiconductor motor controllers and starters
Soft-starters — requirements for thyristor-based reduced-voltage starting equipment.
IEC 60947-5-1 Control circuit devices and switching elements — Electromechanical control circuit devices
Push-buttons, selector switches, indicator lights, emergency stops. Defines IEC colours for push-buttons (red=stop, green=start).
IEC 60947-5-2 Proximity switches
Inductive, capacitive, magnetic and ultrasonic proximity sensors used as control circuit devices.
IEC 60947-6-1 Multiple function equipment — Transfer switching equipment
Automatic Transfer Switches (ATS) for mains/generator changeover in power continuity applications.
IEC 60947-7 Ancillary equipment — Terminal blocks
Requirements for terminal blocks, bus bars and similar equipment used in LV assemblies.
Key concepts
- Utilisation category
- Defines the switching duty of a device based on the nature of the load. AC-3 (squirrel-cage motors, switching off during running) is the most common contactor duty. AC-4 (plugging, inching) requires significantly larger contactors for the same motor rating.
- Breaking capacity (Icu / Ics)
(Icu / Ics) - Icu = ultimate breaking capacity (single breaking event). Ics = service breaking capacity (device remains functional after). For MCC and panel design, Ics is the relevant figure — typically 25%, 50% or 75% of Icu.
- Selectivity (discrimination)
- Ability of the upstream protective device to not operate when the downstream device clears a fault. Partial selectivity up to a limit current; total selectivity above. Critical for maintaining supply continuity in process plants.
- Rated impulse withstand voltage (Uimp)
- Peak impulse voltage the device can withstand without damage. Defines the overvoltage category of the installation (Cat II for appliances, Cat III for distribution boards, Cat IV for utility interface).
- AC-3 / AC-4
- Contactor utilisation categories. AC-3 = normal motor starting and switching off during running (standard industrial use). AC-4 = plugging and inching — requires derating or larger contactors due to higher breaking energy.
Notes & guidance
Overview
IEC 60947 is the standard family for low-voltage switchgear — the equipment that switches, protects and controls electrical circuits at voltages up to 1000 V AC. It covers the core components of any motor control centre (MCC), distribution board or control panel.
Part Selection Guide
| Equipment | Relevant part |
|---|---|
| Air circuit breakers (ACB) | IEC 60947-2 |
| Moulded case circuit breakers (MCCB) | IEC 60947-2 |
| Miniature circuit breakers (MCB) | IEC 60898 (household) or 60947-2 |
| Contactors | IEC 60947-4-1 |
| DOL motor starters | IEC 60947-4-1 |
| Soft starters | IEC 60947-4-2 |
| Push-buttons, E-stops | IEC 60947-5-1 |
| Terminal blocks | IEC 60947-7 |
| Transfer switches (ATS) | IEC 60947-6-1 |
Utilisation Categories — Contactors (AC)
| Category | Load type | Typical application |
|---|---|---|
| AC-1 | Non-inductive or slightly inductive | Resistive heating |
| AC-2 | Slip-ring motor | Starting, plugging |
| AC-3 | Squirrel-cage motor | Normal starting and running |
| AC-4 | Squirrel-cage motor | Plugging, inching, reversing |
AC-3 is by far the most common. AC-4 duty requires contactors 2–4 frame sizes larger for the same motor.
Key Design Considerations
- Coordination with IEC 60947-2: always verify Ics (service breaking capacity) ≥ prospective short-circuit current at panel busbars
- Selectivity: use manufacturer selectivity tables — it cannot be calculated from catalog ratings alone
- ATEX zones: LV switchgear in hazardous areas must additionally comply with IEC 60079-0 and relevant protection concept (Ex d, Ex e, Ex p)
Applicable industries
- process
- manufacturing
- energy
- oil-and-gas
- water-treatment
- mining