BS 7671:2018 / 18th Edition. a step ahead for surge protection

(See Mersen at Machine Building North, 13 May 2026, on stand 125)

On one side, the 18th Edition opens the need for installing surge protection devices (SPD) in a very broad spectrum from public, commercial or industrial activities too, even consumer unit applications depending on the circumstances as stated in the section 443.

On a second side, the 18th Edition (based on the EN 62305-4 and EN 61643-12) describes the selection and the application of SPDs in the section 534. The origin of the installation, the main switchboard, is the place to start the design of surge protection on the network.

SPD selection criteria in the main switchboards

The SPD protection design does not depend on the fault ratings given by the transformer, it only depends on the level of exposure in front of surge. So, which SPD do we have to install in the main switchboard?

In accordance to the IEC 61643 standard there are three types of SPDs: Type 1, Type 2 and Type 3. Type 1 SPDs are the ones able to withstand the energy from a direct lightning strike. They have the ability to discharge high currents to earth. These kinds of SPDs are usually installed in the main switchboards (origin of the installation) for those cases where the level of exposure in front of surge is very high: a building contains external lightning rod protection, high exposed locations or insulated installations. Because sometimes it's not possible to know the circumstances of the building at the design stage there is a general practice for considering type 1+2 SPDs in the main switchboard.

What kind of Type 1+2 SPD has to be selected?

There exists in the market different Type 1 SPDs depending on their capacity discharge: 50kA, 25kA, 12.5kA. In the IEC 63205-1 standard, considering the dispersion of the highest lightning considered, 200kA, the minimum type 1 SPD kA rate has to be 12.5kA. Furthermore, based on the IEC 61643-12 standard and even stated in the section 534.4 of the BS 7671: 2018, 12.5 kA is the minimum kA rate when a Type 1 is needed.

The general recommendation is to consider a Type 1 SPD with a discharge capacity of 12.5kA, when a Type 1 is required, as the level of lightning in the UK is typically much lower than the maximum stated by the standard.

Mersen surge-trap Type 1+2 SPDs

STP T12 12.5 is the series of combined Type 1+2 SPD devices for discharging 12.5 kA lightning currents and protecting against voltage surges.

Further SPDs (Type 2 and Type 3) are required to protect sensitive and critical equipment downstream of the origin of the installation when a Type 1 is fitted at the origin of installation.

Mersen SPD and back up fuse combinations

One of the requirements of SPD installation according to IEC61643-12, is the selection, when necessary depending on the circumstances, of the correct back up fuse.

Mersen is a manufacturer and supplier of fuses and fusegear. The company has the expertise and products available from its overcurrent protection product family to offer a complete solution of fuse and fuse holder combination to be installed alongside its range of SPDs.