Figure 7.49 presents an example of the connections in the use of lightning current
arresters and SPDs in TN–C–S systems. It can be seen that SPDs class III are used
downstream of the residual current device (RCD). In this context, please note the
following. As a result of the frequency of switching surges in terminal circuits,
SPDs class III are primarily employed to protect against differential mode (DM)
voltages. These surges generally arise between L and N. A surge limitation between
L and N means that no impulse current is discharged to PE. Thus, this process can
also not be interpreted as residual current by the RCD. In all other cases, SPDs
class III are designed for a nominal discharge capacity of 1.5 kA. These values are sufficient
in the sense that upstream protective stages of SPDs class I and II take over the
discharge of high-energy impulses. When using an RCD capable of withstanding
impulse currents, these impulse currents are not able to trip the RCD or cause mechanical
damage. The following diagrams illustrate the use of SPDs as part of the LPZs
concept, and the required lightning and surge protective measures for a TN–C–S
system. Figure 7.50 illustrates the use of SPDs in a TN–S system