About Surge Protection Devices Ltd

Founded in 2006, we have spent the last 20 years helping electrical wholesalers, contractors, consultants and engineers protect electrical installations from the damaging effects of lightning and transient overvoltages.

At Surge Protection Devices Ltd, we believe that great technical solutions start with great people. Whilst our products protect some of the UK’s most critical installations, including commercial, industrial and public sector projects, we remain a close-knit, family-run business at heart. Our size allows us to do something many larger manufacturers struggle to achieve – provide genuine technical support from people who truly care about getting the right solution for every customer.

Our reputation has been built on honesty, integrity and technical expertise. We don’t believe in simply selling products; we believe in helping our customers understand surge protection and ensuring devices are specified and installed correctly.

Over the years, we have become recognised throughout the electrical industry for our educational approach. Through CPD seminars, contractor training sessions, technical guidance and industry involvement, we are committed to raising standards and improving knowledge across the sector.

Today, our products are protecting electrical installations across the UK and beyond, but our philosophy remains unchanged: provide outstanding technical advice, exceptional customer service and reliable surge protection solutions that customers can trust.

Whether you’re a contractor looking for installation advice, a wholesaler seeking expert support, or a consultant working on a complex specification, our team is here to help.

Because protecting electrical installations is what we do.

The relevance of different earthing systems

Throughout our website you will notice different SPD’s for use on various earthing arrangements (TNC-S, TN-S and TT). We offer these different products because it is very important to get the correct device for the earthing arrangement you are currently working on. This is discussed in BS7671, in regulation 534.4.3 (page 162 & 163) where we consider connection types. Some people consider these to be wiring diagrams, when in fact they are the internal designs of SPD’s designed for different earthing systems.

Connection type 1 (CT1) shown in fig 534.3 is for a TNC-S or TN-S system. These are systems that have a mains earth connection. Connection type 2 (CT2) shown in fig 534.4 shows the internal connections of an SPD device suitable for TT systems. These are systems that rely on earth rods as opposed to a mains earth connection.

When reviewing both figures it can be seen that the layout is slightly different. CT1 shows an SPD component between each phase and earth and also an SPD component between neutral and earth, whereas CT2 shows an SPD component between each phase and earth but then shows the neutral SPD component in line with the live SPD components. The reason for this is that SPD’s function on sending excess voltage to earth, so we need a low resistance earth path.

When reviewing both figures it can be seen that the layout is slightly different. CT1 shows an SPD component between each phase and earth and also an SPD component between neutral and earth, whereas CT2 shows an SPD component between each phase and earth but then shows the neutral SPD component in line with the live SPD components. The reason for this is that SPD’s function on sending excess voltage to earth, so we need a low resistance earth path.

For this reason SPD’s designed to CT1 are not suitable for TT systems, but devices designed to CT2 will be suitable for all earthing arrangements as the reconfiguration of the components will not cause any issues on a TNC-S or TN-S system.