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How to stay safe when working on domestic electrical installations

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Colin McAhren (MIET), technical business development manager at BG Electrical, part of the Luceco plc group, focuses on how to stay safe when working with domestic installations, from selecting appropriate surge devices through to inspection and testing.

Designing, selecting, installing, verifying, inspecting, testing and certifying: The seven words that cover the process of all household electrical installations. 

In order to ensure the protection for the safety of persons, livestock and property, fundamental principles from BS7671 must be respected for both old and new dwellings. 

This begins with choosing the right surge protection device (SPD), although, while many people still discuss this in terms of the need to install, the questions that should be asked are what to install and where? Additionally, attention should be taken as to understanding what we are protecting against as stated in BS7671. 

So, let’s start with Regulation 443, which states that you should protect against transient overvoltages or atmospheric origin and due to switching.

What does this mean?

The Scope and Object characterises this in 443.1.1

  1. Potential direct strikes to the supply system, ‘overhead cables’ as well as lightning strokes passing close to cables and inducing a transient voltage into the supply system.
  2. Switching overvoltages generated by the equipment within the installation; power supply generators turning on and off is one such switching transient.  

This section does not look at direct lightning strokes to the building, which is covered in BS EN 62305-2. 

534.4.1.1 tells us we need to install Type 1 and Type 2 SPDs at the origin of the installation. In the majority of cases, this will be a Type 2 device, as we will be dealing with transient overvoltage(s) from either lightning strokes to the supply installations or switching transients.

If the SPD is pre-assembled in the consumer unit and connected in parallel across the main incoming supply cables, there is no extra work or time required over that of a consumer unit with no SPD fitted, so just the cost of the SPD.

534.4.1.1 goes on to state, Type 1 SPDs are specifically designed to protect against dangerous sparking that could lead to a fire. They do not protect against failure of sensitive electrical and electronic systems, as an example, they would not protect modern TVs, Hi-Fi systems, washing machines, dishwashers; all of which have sensitive electronic components. It will also not protect against those parts of the actual installation that have been fitted as part of the fixed equipment, safety elements of smoke and heat alarm equipment, or USB socket-outlets, as Type 1 SPDs allow transient overvoltages to enter the electrical system.

Type 2 and Type 3 are designed to protect against these transient voltages that damage sensitive and critical equipment that are part of the fixed system, alarms and USBs, as well as any future connected loads like TVs and Hi-Fis.

We should be all familiar with regulation 443.4. This regulation provides mandatory guidance for protection of human life, public services, commercial and industrial installations, as well as large numbers of co-located people.

Single dwelling units consider the cost of the installation of SPDs to decide if they are required. Considering how transient voltages arrive along the supply network, what they can damage, if not instantly, then over time, the cost of installing an SPD is modest. Especially if we consider protection of safety elements like smoke and heat alarms, where failure could result in serious consequences for the people inside.

Changes coming on April 1, 2021

Electrical Safety Standards in the private sector from April 1, 2021, require all existing tenancies to be tested by a person who is ‘qualified and competent’, at least every five years. As a result, many landlords will need to consider the effects of voltage surges within their properties and consider installing SPDs in order to protect occupants, as well as fire alarm systems, the sensitive electronics of smoke and heat alarms, and the expensive equipment that may be connected by residents.

Older rental buildings may also have electrical installations where no RCDs are installed or only partially installed, and cables in walls would not be protected from the occasional nail being introduced to the circuit. 

Back to surge protection

Section 443.5 provides guidance for the risk assessment method to determine if an SPD is required to be installed due to lightning strokes, and if you do not want to do the risk assessment, just install an SPD.  

Once we have made the decision to include an SPD, let’s take a look at how they are connected to the installation.

According to 534.4.8, connection of SPDs tells us that all conductors and interconnections are to be protected, as well as the connections between SPD and any external OCPD, and shall be kept as short as possible. That consideration shall be given to limit the total wiring length of conductor between connection points of the SPD assembly – and should preferably not exceed half a metre, and in no case exceed one metre.

534.4.5.1 says that SPD installations shall be protected against overcurrent with respect to short circuit current.

This protection may be internal and/or external to the SPD, according to the manufacturer’s instructions. Most manufacturers/suppliers install an external protection MCB within the enclosure close to the SPD it is protecting, by doing this we meet regulations 534.4.8 and 534.4.5.1. For any other configuration contact your supplier for advice.


641.1 notes that every installation shall, during erection and on completion, be inspected and tested for verification, so far as is reasonably practicable, to ensure the requirements of the regulations have been met. So, performing an insulation test on cables after the first fix could prevent issues later.

At every stage of our design, selection and erection process, as designers and installers, we must ensure 511.1 compliance with standards, and every item of equipment shall comply with applicable British or harmonised standards, appropriate to its intended use. 133.5 states that any departure shall not be less than obtained by compliance to regulation.

Initial verification

Inspection 642.1 shall precede testing and shall be performed with the part being inspected disconnected from the supply. Safe isolation must be observed. BS7671 part six and GN3 provide the order and guidance on the tests to be performed.

Initial verification on a new install would require testing the circuits where an SPD is connected.          A 500VDC insulation resistance test may activate the SPD, as it will see this voltage as a spike, and also influence the verification test result. Regulation 643.3.2 states it is okay to test at 250VDC where it is not reasonable to disconnect the equipment.

Disconnection may be a viable option where there are a limited number of ‘at risk’ equipment, socket-outlets to be tested are an example. However, it becomes less of an option as the number of these socket-outlets increases, because disconnection and reconnection takes time and introduces the possibility of the socket-outlets being incorrectly reconnected. In such instances, the tester can decide that it is not reasonably practicable to disconnect the socket-outlets.

This is true for any part of the fixed wiring system, where vulnerable equipment could be damaged by a test voltage of 500VDC. Examples of equipment installed that this could apply to are socket outlets containing SPDs, USBs, RCDs, smoke and heat alarms. SPDs installed within the enclosure can be easily disconnected and the circuit tested at 500VDC, so long as the rest of the circuit/s can be tested at 500VDC as well.

We are not advocating testing all circuits at 250VDC, 500VDC stress tests the circuit beyond normal operational voltage and adds a degree of safety margin for the nominal 230VAC used within domestic buildings. It does, however, indicate how vulnerable electronics embedded within the fixed wiring system are becoming, and how they can be damaged by testing at 500VDC. What will transient voltages of potentially 6kV do to any of these accessories, devices and equipment if not protected by a surge protection device?

Colin McAhren
Colin McAhren
‎Technical Business Development Manager at BG Electrical

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