Protective, isolation and switching devices explained

By: Calum Mansell, IEng, MIET 

Having a means to isolate and/or switch a complete, or parts of an, electrical installation is essential for any designer looking to comply with BS 7671:2018+A4:2026 Requirements for Electrical Installations, IET Wiring Regulations¹ (referred to as BS 7671 from here on in).  

Isolation is required for activities such as maintenance, while switching can be applied as a safety measure to immediately disconnect the electrical supply in an emergency, as well as a means to conveniently control sections of the installation.

This article reviews the key changes to Table 537.4 of BS 7671 (see Table 1).

Table 1: Guidance on the selection of protective, isolation and switching devices (based on Table 537.4 of BS 7671)

(1) Function provided if the device is suitable and marked with the symbol for isolation (see IEC 60617 identity number S00288 and Figure 1).
(2) See Regulation 537.3.3 of BS 7671.
(3) Device is suitable for on-load isolation, i.e. disconnection whilst carrying load current.
(4) In an installation forming part of a TT or IT system, isolation requires disconnection of all the live conductors. See Regulation 462.2 of BS 7671.
(5) Circuit-breakers, arc fault detection devices (AFDDs) and residual current devices (RCDs) are primarily circuit protective devices and, as such, they are not intended for frequent load switching. Infrequent switching of circuit-breakers on-load is admissible for the purposes of isolation or emergency switching. For a more frequent duty, the number of operations and load characteristics according to the manufacturer’s instructions should be taken into account or an alternative device from those listed as suitable for functional switching in Table 537.4 should be employed.
(6) The switch of a socket-outlet is not required to be suitable for isolation, as isolation is achieved by withdrawal of the plug. See Regulation 537.1.3 of BS 7671.
(7) Marked with the switch-disconnector symbol (see Figure 2).  
(8) Isolation provided by opening device means of isolation, and optionally by withdrawal of plug. 

Firefighters’ switches 

The BS EN 60669 series covers switches for household and similar fixed electrical installations, and the standard BS EN 60669-2-6:2012² contains requirements for firefighters’ switches for exterior and interior signs and luminaires. 

In England and Wales, the Regulatory Reform (Fire Safety) Order 2005³ contains several statutory requirements relating to firefighters' switches for luminous tube signs, etc. In Northern Ireland, the Fire Safety Regulations (Northern Ireland) 2010⁴ apply, and in Scotland, the Fire Safety (Scotland) Regulations 2006⁵ apply.

In respect to BS 7671, firefighters’ switches are required in low-voltage circuits supplying outdoor lighting, and indoor discharge lighting installations operating at a voltage exceeding 1,000 V AC and 1,500 V DC. Additionally, they are required on certain premises for specific equipment, subject to the fire and rescue authority’s requirements, or those subject to licensing conditions such as petrol station forecourts. Portable discharge lighting luminaires, or signs not exceeding 100 W and supplied from an accessible socket-outlet, are excluded from this requirement.  

Table 537.4 of BS 7671 now lists BS EN 60669-2-6 as a switching device (see Table 1). This identifies their capability for on-load isolation and emergency switching off, although, as specialized safety devices, they are not permitted to be frequently switched as is expected of a functional switch. 

It should be noted that devices conforming to the BS EN 60669 series are typically intended for use in household and similar installations. Firefighters’ switches conforming to BS EN IEC 60947-3⁶ are typically used in industrial installations and petrol station forecourts. 

Residual direct current protective device (RDC-PD) 

BS IEC 62955:2018⁷ is a standard that covers residual direct current detection devices (RDC-DDs) for Mode 3 charging of electric vehicles. Mode 3 charging utilizes a dedicated circuit supplying a charger unit termed ‘electric vehicle supply equipment (EVSE)’, ranging from 7 kW for single-phase to 22 kW for three-phase, with additional means of control and monitoring. 

An RDC-DD is an umbrella term for devices designed for the purpose of detecting DC fault currents in ^Mode 3 charging equipment. The term is referenced in indent (b) of Regulation 722.531.3.101 of BS 7671 for use in conjunction with Type A or Type F RCDs. 

As per BS IEC 62955, RDC-DDs are categorized as the following: 

• Residual direct current protective device (RDC-PD) 
• Residual direct current monitoring device (RDC-MD) 

Within the confines of one unit, the RDC-PD provides integrated AC residual current detection, pulsating DC and 6 mA DC detection, evaluation and mechanical switching, as well as isolation (see Table 1). RDC-PDs must conform to BS EN 61008⁸ or BS EN 61009⁹ and contain a Type A RCD with a rated residual current not exceeding 30 mA. As well as being referenced in Table 537.4 for on-load isolation and both switching functions, they provide an additional option for protection of EVSE (see Regulation 722.531.3.101, indent (c) of BS 7671). 

RDC-MDs are further broken down into types of construction, although all are capable of 6 mA DC residual current detection (see Table 2). BS IEC 62955 refers to terms and definitions given in IEC 62873-2¹⁰ and provides two definitions for RDC-MDs constructions: 

1. BS IEC 62955 Clause 3.1.4 - RDC-M-unit 
   Unit that includes detection and evaluation of residual currents and provides a mechanical operation to an RCD or circuit-breaker to cause switching of the monitored circuit. 
2. BS IEC 62955 Clause 3.1.5 - RDC-M-module  
   Module to detect residual direct currents and to provide a signal to an electrically coupled switching device to cause switching of the monitored circuit. 

Table 2: RDC-MD types of construction 

(1) Capable of 6 mA DC residual current detection and evaluation.
(2) For AC and pulsating DC protection, selection of either a Type A RCD to BS EN 61008 or BS EN 61009 with a rated residual current not exceeding 30 mA or a Type F RCD to BS EN 62423¹¹ with a rated residual current not exceeding 30 mA.
(3) For AC and pulsating DC protection, a circuit-breaker to BS EN 60898-1 in series with a Type A RCD to BS EN 61008 with a rated residual current not exceeding 30 mA or BS EN 61009 or a Type F RCD to BS EN 62423 with a rated residual current not exceeding 30 mA.
(4) For AC and pulsating DC protection, a contactor or relay in series with a Type A RCD to BS EN 61008 or BS EN 61009 with a rated residual current not exceeding 30 mA or a Type F RCD to BS EN 62423 with a rated residual current not exceeding 30 mA. 

Isolating switches  

As the name would suggest, these devices are suitable for isolation (on-load), and common applications include the isolation and switching of fans and water heaters.

Where a switch is selected for the purpose of functional switching only, conformance with BS EN 60669-1:2018¹² is required. However, BS EN 60669-2-4:2005¹³ contains requirements for isolating switches in household and similar fixed electrical installations, and while not new to Table 537.4, isolating switches are now only referenced as conforming to BS EN 60669-2-4  (see Table 1). 

The standard requires isolating switches to be rated for a voltage not exceeding 440 V and a current not exceeding 125 A, with the capability to make and break the current in all line current-carrying poles. 

Disconnectors and switch-disconnectors  

While disconnectors and switch-disconnectors are separate devices, they both originate from the same standard, BS EN IEC 60947-3:2021. BS EN IEC 60947-3 is part of the BS EN (IEC) 60947 series, with Part 3 specifying requirements for low-voltage switchgear and controlgear, including switches, disconnectors, switch-disconnectors and fuse-combination units.  

So, what is the difference? 

Disconnectors are referred to as 'off-load' switching devices, whereas a switch-disconnector is suitable to be operated under load, termed 'on-load'. Disconnectors are designed to be operated when the circuit or installation has already been de-energized by another on-load device. Disconnectors lack mechanisms to extinguish arcs, for example, arc chutes, and where a circuit is opened while carrying current, an arc is created. Attempting to open a disconnector while the circuit is under load can cause an arc flash or explosion. Therefore, Table 537.4 does not permit disconnectors to be used for functional switching, as they are unable to break current under load. 

The superscript numbers 1 and 7 to Table 537.4 (see Table 1) provide a useful aid for a designer selecting an appropriate device; these refer to the isolation and switch-disconnector symbols from Table 1 of BS EN IEC 60947-3 (see Figures 1 and 2). 

Figure 1: Disconnector symbol (not suitable for on-load operation)  

Figure 2: Switch-disconnector symbol (suitable for on-load operation) 

Isolators 

It is common for electricians to refer to both disconnectors and switch-disconnectors as ‘isolators’; however, the sole term ‘isolator’ is now considered deprecated, or slang, and has been removed from BS 7671. While it is appreciated that the term will likely continue to be used in the wholesalers and building sites across the UK for some time, it is deemed too broad a term when considering the need for clarity within BS 7671; as the term does not qualify if a device is suitable for on-load isolation or not. 

Switched socket-outlets with or without interlocks 

Switched socket-outlets with an interlock prevent the socket from being energized until the plug is fully inserted, while conversely, preventing the plug from being removed until the switch is off. The BS EN IEC 60309-4:2022¹⁴ standard is specific to switched socket-outlets with or without interlock. It is important to note that as well as incorporating an interlock, these products may also include protective devices. 

Table 537.4 of BS 7671 now includes references to the following: 

• BS EN IEC 60309-4 socket-outlets with switches 
• BS EN IEC 60309-4 socket-outlets with switch-disconnectors 

A BS EN IEC 60309-4 socket-outlet with a switch is not required to be suitable for isolation, as isolation is achieved by the withdrawal of the plug. However, a BS EN IEC 60309-4 socket-outlet with a switch-disconnector is required to be marked with the switch-disconnector symbol (see Figure 2) and permit isolation by opening the isolation device or by withdrawing a plug (see Table 1). 

Summary 

The additions and modifications to Table 537.4 of BS 7671 give designers a wider choice of devices to select from, as well as providing clarity on the application of different devices. However, it should be noted that this table provides guidance only, and it is for the designer to consider the requirements for their specific installation, such as the environment the device will be installed in. 

While this article covers some of the changes to Table 537.4 of BS 7671, users of the standard are encouraged to familiarise themselves with all updates to BS 7671 to ensure continued safety and compliance. 

Acknowledgements 

• BEAMA 
• Joe Cannon 
• Michael Peace 
  
  

Further reading 

• BEAMA Guide - RCD selection for protection of Electric Vehicle charging installations 
• IET Guidance Note 2: Isolation & Switching  
• Wiring Matters, Issue 94: Isolation and switching for mechanical maintenance 
  
  

References 

(1) BS 7671:2018+A4:2026 Requirements for Electrical Installations, IET Wiring Regulations, Eighteenth Edition.
(2) BS EN 60669-2-6:2012 Switches for household and similar fixed electrical installations - Particular requirements. Fireman's switches for exterior and interior signs and luminaires.
(3) The Regulatory Reform (Fire Safety) Order 2005.
(4) The Fire Safety Regulations (Northern Ireland) 2010.
(5) The Fire Safety (Scotland) Regulations 2006.
(6) BS EN IEC 60947-3:2021 Low-voltage switchgear and controlgear - Switches, disconnectors, switch-disconnectors and fuse-combination units.
(7) BS IEC 62955:2018 Residual direct current detecting device (RDC-DD) to be used for mode 3 charging of electric vehicles.
(8) BS EN 61008-1:2012+A12:2017 Residual current operated circuit-breakers without integral overcurrent protection for household and similar uses (RCCBs) - General rules. 
(9) BS EN 61009-1:2012+A13:2021 Residual current operated circuit-breakers with integral overcurrent protection for household and similar uses (RCBOs) - General rules.
(10) IEC 62873-2:2016 Residual current operated circuit-breakers for household and similar use - Part 2: Residual current devices (RCDs) - Vocabulary.
(11) BS EN 62423:2012+A11:2021 Type F and type B residual current operated circuit-breakers with and without integral overcurrent protection for household and similar uses.
(12) BS EN 60669-1:2018 Switches for household and similar fixed-electrical installations - General requirements.
(13) BS EN 60669-2-4:2005 Switches for household and similar fixed electrical installations - Particular requirements - Isolating switches.
(14) BS EN IEC 60309-4:2022 Plugs, fixed or portable socket-outlets and appliance inlets for industrial purposes - Switched socket-outlets and connectors with or without interlock.