About Surge Protection Devices

Surge Protection Devices and our Partner Companies have been solving problems in Lightning and Surge Protection for over 50 years.

ISO 9001 European designed and manufactured, state of the art products which exceed the requirements of the latest Standards and our competitors’ specifications.

Because we specialise in Lightning and Surge protection we can offer you the very best, most cost-effective solution of any company in this field.

We carry out many seminars with consultants and specialist training organisations as well as attending exhibitions and conferences both in the UK and overseas where our specialist knowledge is renowned.

We also hold regular contractor training sessions online, if you are interested in participating please contact us. We also use our specialist knowledge of Lightning and Surge protection to assist the industry in the preparation of new regulations or the modification of existing regulations. SPD Ltd stands for innovation, quality and excellent customer service.

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.