# 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.

- Source : https://industryhub.site/en/standards/iec-60947
- Body : IEC
- Family : Electrical Equipment
- Status : current
- First published : 1988
- Latest revision : Various (per part, 2020–2024)
- Edition : Multiple editions per part
- Access : Paid (IEC webstore — price per part)
- Language : English

## Document structure

- **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 60947-2** — 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.

## 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

## References & further reading

- [IEC 60947 series — IEC webstore](https://webstore.iec.ch/searchform&q=60947)