Guide to Low Voltage Circuit Breaker Standards

Introduction

Low voltage circuit-breakers are fundamental components of every electrical installation, providing essential protection against overcurrents and short circuits. However, selecting the correct circuit-breaker for a specific application requires navigating a complex landscape of standards, each designed for different environments and user types.

The BEAMA Guide to Low Voltage Circuit-Breaker Standards provides a practical resource for designers, specifiers, and installers to enable them to specify low voltage circuit-breakers in accordance with BS EN 60898-1, BS EN 60898-2 and BS EN 60947-2. This guide should be read in conjunction with these standards as it provides additional explanation on each section.

The Standards Framework

International Harmonisation

The compatibility of manufactured goods across a wide geographical area can remove barriers to trade and result in efficiency of scale due to increased manufacturing volumes. In the electrical industry, appropriate standardisation means common supply networks and products, enabling:

• Use of compatible equipment
• No need to adapt or modify products
• Fewer limitations on the source of supply

The International Electrotechnical Commission (IEC) coordinates the work of national standards committees from over 80 nations to formulate world standards. Within Europe, CENELEC (Comité Européenne de Normalisation Electrotechnique) produces European standards, generally based on the work of the IEC.

United Kingdom Adoption

Adoption of European Standards within the EU is mandatory. In the UK, such standards are further endorsed with the additional "BS" prefix, for example BS EN 60898. Following the UK's exit from the EU, the UK government implemented a system that confers presumption of conformity with the safety objectives of the Electrical Equipment (Safety) Regulations via "Designated standards".

Circuit-breakers conforming with the safety objectives of the Electrical Equipment (Safety) Regulations are required to be UKCA marked. EU directives continue to apply in Northern Ireland (NI) as dictated by the Northern Ireland Protocol of the Withdrawal Agreement. Circuit-breakers placed on the NI market need to conform with the LVD and be CE marked.

Types of Circuit-Breaker

Miniature Circuit-Breakers (MCBs) – BS EN 60898 Series

MCBs to BS EN 60898 are suitable for operation by ordinary persons and have fixed protection settings, generally a two-position on/off operating handle. They are typically the final overcurrent protection measure in an electrical system, for example before sockets or lighting circuits.

Key Characteristics:

• Current ratings: 0.5 A to 125 A
• Short-circuit ratings: up to 25 kA
• Fixed tripping characteristics: Types B, C and D
• Suitable for Pollution Level 2 environments

MCBs are designed for use in household and similar installations such as offices, commercial premises, schools, hospitals and public buildings.

Moulded Case Circuit-Breakers (MCCBs) – BS EN 60947-2

MCCBs may have fixed or adjustable protection settings, normally a three-position toggle operating handle giving on-off-tripped indication plus reset function. They offer a performance level relative to the incoming supply such that they can be installed at a point close to the supply transformer.

Key Characteristics:

• Current ratings: 16 A to 1,600 A (up to 3,200 A available)
• Short-circuit ratings: up to 100 kA
• Adjustable protection settings available
• Suitable for Pollution Level 3 environments

Air Circuit-Breakers (ACBs) – BS EN 60947-2

ACBs are normally used as the main incoming protection and have a spring-operated mechanism to open and close the device, often charged by an internal motor. The protection settings include time delays and the devices have a short-time withstand value to give full discrimination under fault conditions with downstream protection devices.

Key Characteristics:

• Current ratings: 630 A to 6,300 A
• Short-circuit ratings: up to 150 kA
• Time-delayed protection settings
• Designed for full discrimination with downstream devices

Key Differences Between Standards

BS EN 60898-1 vs BS EN 60947-2, by feature:

• Application: Household and similar installations (60898-1) vs Industrial and large commercial installations (60947-2)
• User Type: Ordinary persons (60898-1) vs Instructed/skilled persons (60947-2)
• Voltage Range: Up to 440 V (60898-1) vs Up to 1,000 V AC, 1,500 V DC (60947-2)
• Current Range: 0.5 A to 125 A (60898-1) vs 0.5 A to 6,300 A (60947-2)
• Protection Settings: Fixed (60898-1) vs Fixed or adjustable (60947-2)
• Pollution Level: Level 2 (60898-1) vs Level 3 (60947-2)
• Short-Circuit Rating: Icn, marked (60898-1) vs Icu and Ics (60947-2)
• Maintenance Required: Minimal (60898-1) vs Regular (60947-2)

Understanding Short-Circuit Breaking Capacities

BS EN 60898 – Icn (Rated Short-Circuit Capacity)

Circuit-breakers to BS EN 60898-1 have the rated short-circuit capacity Icn marked on the device. The standard recognises preferred short-circuit capacities up to 25 kA. The test sequence for Icn is: O – t – CO

Where: O = opening operation under fault conditions; t = time interval before re-closing (3 minutes); CO = closing operation on to a fault

BS EN 60947-2 – Icu and Ics

BS EN 60947-2 recognises both a rated ultimate short-circuit breaking capacity (Icu) and a rated service short-circuit breaking capacity (Ics).

Icu (Ultimate Short-Circuit Breaking Capacity): This is the maximum short-circuit current the circuit-breaker can interrupt without sustaining damage. The test sequence is O – t – CO. After this, only dielectric and overcurrent release tests are required.

Ics (Service Short-Circuit Breaking Capacity): This is the maximum short-circuit current the circuit-breaker can interrupt and still remain in service. The test sequence is O – t – CO – t – CO. After this, load-switching, dielectric, terminal temperature and overcurrent release tests are applied. The circuit-breaker must meet certain test parameters to ensure it has not deteriorated in performance and can be put back into service.

Ics must be at least 25% of Icu. For high-performance devices, Ics can be 100% of Icu, meaning the circuit-breaker can safely break its maximum fault current and remain fully operational.

Installation Factors and Practical Application

Energy Let-Through (I²t)

Energy let-through is a measure of the energy released by the circuit-breaker under short-circuit conditions. BS EN 60898-1 classifies B and C characteristic MCBs into energy-limiting classes 1 or 3. Class 3 MCBs offer superior performance by reducing the amount of let-through energy, thereby protecting downstream cables and components.

Permissible I²t values for Type B and Type C MCBs are provided in the BEAMA guide, allowing designers to verify cable protection using the adiabatic equation.

Back-up Protection

Back-up protection (also referred to as cascading) consists of an upstream short-circuit protective device (SCPD) that helps a downstream circuit-breaker to break fault currents greater than its maximum breaking capacity. This creates a conditional rating. However, back-up protection can only be accurately verified by testing, and appropriate data must be obtained from the manufacturer.

Determination of Maximum Earth Loop Impedance (Zs)

For circuit-breakers to BS EN 60898-1, Zs values are tabulated in BS 7671. For circuit-breakers to BS EN 60947-2, Zs must be calculated from the basic equation: Zs ≤ (Uo × Cmin) / Ia

Where: Uo = nominal voltage to earth; Cmin = minimum voltage factor (0.95 for LV supplies); Ia = current required to achieve the required disconnection time

For circuit-breakers with adjustable or electronic trip units, the selected settings must be applied to the determination of Zs, and any applicable tolerances must be taken into account.

Circuit-Breaker Substitution

BEAMA warns against installing circuit-breakers of one manufacturer as replacements for devices of another manufacturer without the necessary verification of performance. Substituting devices not verified by the assembly manufacturer invalidates any testing/verification and warranty.

BS 7671 regulation 510.3 requires that the installer takes into account the manufacturer's instructions in regard to devices fitted. It is the installer's responsibility to either obtain authority from the assembly manufacturer or undertake appropriate verification.

Harmonic Currents

Harmonic currents can affect circuit-breaker operation in several ways:

• Circuit-breakers with bimetallic thermal overload protection respond to the true RMS value of the current waveform, providing protection in the case of overload currents including harmonics.
• Third harmonics and their multiples accumulate in the neutral conductor, requiring consideration of neutral conductor sizing and potentially the use of 4-pole circuit-breakers with protected neutral poles.

Circuit-breakers with electronic overcurrent protection conforming to Annex F of BS EN 60947-2 have immunity to unwanted operation in the presence of specific percentages of odd harmonic currents.

Conclusion

Understanding the differences between BS EN 60898-1 and BS EN 60947-2 is essential for specifying the correct circuit-breaker for any application. The BEAMA Guide to Low Voltage Circuit-Breaker Standards provides a practical bridge between these complex standards and real-world design decisions, covering history, characteristics, rated values, installation factors and technical application data.

The guide is regularly updated to reflect changes to product standards and amendments to BS 7671, the IET Wiring Regulations. The current edition incorporates Amendment 2:2022 to the 18th Edition. It remains an essential reference for designers, specifiers, and installers working with low-voltage circuit-breakers.

This article summarises key concepts from the 'BEAMA Guide to Low Voltage Circuit-Breaker Standards'.

Download the full free guide here: BEAMA Guide to Low Voltage Circuit-Breaker Standards

The guide is available as a free download from BEAMA.

References: BEAMA (2015, updated 2022). Guide to Low Voltage Circuit-Breaker Standards in accordance with BS EN 60898-1, BS EN 60898-2 and BS EN 60947-2. BEAMA Ltd, London. BS EN 60898-1:2019, Electrical accessories. Circuit-breakers for overcurrent protection for household and similar installations. Circuit-breakers for a.c. operation. BS EN 60898-2:2016, Electrical accessories. Circuit-breakers for overcurrent protection for household and similar installations. Circuit-breakers for a.c and d.c. operation. BS EN 60947-2:2017+A1:2020, Low-voltage switchgear and controlgear. Circuit-breakers. BS 7671:2018+A2:2022, Requirements for Electrical Installations. IET Wiring Regulations.

Disclaimer: This article provides a summary of the BEAMA Guide to Low Voltage Circuit-Breaker Standards. While efforts have been made to accurately represent the content, please refer to the official guide and the relevant standards for complete technical details. Final design, specification, and implementation decisions should be made by qualified professionals in accordance with all applicable regulations and standards.