SPDs and TN-C-S earth configurations
The minimum SPD connection requirements at or near the origin of the installation for a TN-C-S system requires further clarification as Section 534 of BS 7671 illustrates (see Figure 3 below) a Type 1 SPD being required between the live and PE conductors – the same as required for a TN-S system.
Figure 3 – Installation of Types 1, 2 and 3 SPDs, for example in TN-C-S systems
The term ‘at or near the origin of the installation’ creates ambiguity given the fact that the word ‘near’ is not defined. From a technical point of view, if SPDs are applied within a 0.5m distance of the PEN split to separate N and PE, there is no need to have an SPD protection mode between N and PE as shown in the figure.
If BS 7671 would allow the application of SPDs to the TN-C side (utility side) of the TN-C-S system (observed in some parts of Europe), then it may be possible to install SPDs within 0.5m of the PEN split to N and PE and omit the N to PE SPD protection mode.
However as SPDs can only be applied the TN-S side (consumer side) of the TN-C-S system, and given SPDs are typically installed at the main distribution board, the distance between the SPD installation point and the PEN split will almost always be greater than 0.5 m, so there is a need to have an SPD between N and PE as required for a TN-S system.
As Type 1 SPDs are specifically installed to prevent the risk of loss of human life (to BS EN62305) through dangerous sparking which could present a fire hazard for example, in the interests of safety alone, the engineering judgement is that an SPD should be fitted between N and PE for a TN-C-S system as it would in a TN-S system.
In summary, as far as Section 534 is concerned, TN-C-S systems are treated the same as TN-S systems for the selection and installation of SPDs.