Myth Busters #7 - Out with the old, in with the new?

The introduction of the 18th Edition of the IET Wiring Regulations (BS 7671:2018) saw a new regulation buried in Section 536 Co-ordination of electrical equipment for protection, isolation, switching and control.

Section 536 is about ensuring (amongst other aspects) that the performance of devices for protection against faults and overloads are coordinated so that the effectiveness of the operation of individual items of equipment, under both normal and abnormal operating conditions, does not impair the safety or proper functioning of the installation. It includes, for example, the set of regulations that provides requirements for selectivity of protective devices for residual currents, overloads and short-circuits.

This issue’s Myth Buster is not so much ‘busting a myth’, but explaining something that is commonly misunderstood, and which is a source of a steady trickle of calls to the IET Technical helpline.

The regulation in question, 536.4.203, was introduced by BEAMA (the UK trade association for manufacturers of electrical equipment including switchgear) to warn designers and installers of the possible dangers of mixing devices from different product ranges or manufacturers in the same distribution board; mainly, but not exclusively, circuit-breakers. When installing a new final circuit for example, if there is spare space in a consumer unit and a circuit-breaker that appears to fit is to hand, it is tempting to use it. Another example is replacing a circuit-breaker with an RCBO to afford better protection for the user.

Before delving into the detail of this change, as ever it pays to start at the beginning – namely the Fundamental Principles of Part 1.

Regulation 133.1.1 requires that:

Every item of equipment shall comply with the appropriate British or Harmonized Standard.’ For distribution boards, this is the BS EN 61439 series. Part 3 is a specific part of the BS EN 61439 suite and it includes requirements for Distribution Boards intended for use by ‘ordinary persons’.

The requirements for compliance are varied and include the performance of the distribution board under internal and external fault conditions, internal creepage distances, temperature rise limits, short-circuit withstand strength and more. In order to prove compliance, manufacturers will populate a distribution board with their own protective devices, before testing the assembly as a whole.

When purchasing a consumer unit or distribution board, the manufacturer’s instructions will state which devices can be used in the unit – and don’t forget according to BS 7671 (regulation 510.3) there is an obligation on designers and installers to follow the manufacturer’s instructions when selecting and erecting equipment.

The regulation in question (536.4.203) specifies requirements for integrating devices and components into low voltage assemblies to the BS EN 61439 series and the regulation highlights:

  • The need to ensure conformity with the relevant part of BS EN 61439 series.
  • That just because individual components conform to their respective product standards and are UKCA marked, it does not indicate their compatibility for integration into an assembly.
  • The person introducing a modification/alteration becomes the original manufacturer with the corresponding obligations for that assembly.

As an example of the need for this regulation, consider a circuit-breaker clearing a substantial fault of several thousand amps. When the contacts open, there is a corresponding arc that needs to be safely dissipated. Circuit-breakers typically have a vent from an internal arc suppression chamber to the outside in order to help the hot air and gasses to escape during the disconnection.

If devices from different manufacturers are used together, the venting characteristics may not be coordinated which could result in significant further damage to adjacent devices or other parts of the distribution board.

On a slightly more benign note, circuit-breakers are designed to run warm in normal conditions when carrying load current. The thermal performance of the assembly is considered in BS EN 61439 to ensure that the internal temperature is not excessive and the operation of protective devices is not impaired – when excessively warm for example, the thermal overload operating current of a device will reduce, potentially causing premature operation. Excessive temperatures may affect the control circuits of electronic equipment mounted in the enclosure.

For this reason, regulation 536.4.203 states that ‘The relevant part of the BS EN 61439 series shall be applied to the integration of mechanical and electrical devices and components, e.g., circuit-breakers, control devices, busbars into an empty enclosure or existing low voltage assembly.’

Of note here is that a protective device may conform to the appropriate product standard (e.g. BS EN 60898) and be UKCA marked. If the devices are from different manufacturers, they are likely to be designed differently and will perform differently – though both may comply with the appropriate safety requirements of BS EN 60898.

In other words, adding together CE marked products from one manufacturer with another’s CE marked products does not necessarily equal a CE marked assembly which is compliant with BS EN 61439. This may also be true when mixing product ranges from the same manufacturer. This is reinforced in Note 1 of regulation 536.4.203 which states that ‘The use of individual components complying with their respective product standards does not indicate their compatibility when installed with other components in a low voltage switchgear and controlgear assembly.’

Going back to the Fundamental Principles, regulation 133.1.3 states that ‘Where equipment to be used is not in accordance with Regulation 133.1.1 or is used outside the scope of its standard, the designer or other person responsible for specifying the installation shall confirm that the equipment provides at least the same degree of safety as that afforded by compliance with the Regulations. Such use shall be recorded on the appropriate electrical certification specified in Part 6.’

This is developed in Note 2 to regulation 536.4.203, which states that ‘If an assembly deviates from its original manufacturer’s instructions, or includes components not included in the original verification, the person introducing the deviation becomes the original manufacturer with the corresponding obligations’.

In summary, can you mix devices in distribution boards (including consumer units)? Yes, you can. But you need to seek assurance from the manufacturer of the original assembly that the devices will be compatible, or conduct your own study to ensure the requirements are met. In the words of BEAMA, ‘The installer has responsibility to act “with due care”. If this is not done then there is a probability that, in the event of death, injury, fire or other damage, the installer would be accountable under Health and Safety legislation.’

For more information the following guides are worth reading:

BEAMA technical bulletin on the safe selection of devices for installation into assemblies [PDF]

BEAMA Consumer unit safety checklist [PDF]


GAMBICA Technical Guide to the requirements of BS EN 61439 Parts 1 and 2 [PDF] (Note that the standard has been updated since this guide has been published, though the general information about the design and characteristics of distribution boards is still relevant).