Overvoltage protection - Basic Documentation - RMK770-1

Communication via the KNX bus

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Overvoltage protection, secondary protection

Overvoltage protection acts as secondary protection. Surge protectors are installed in the AC 230/400 V mains network and in the KNX network.

AC 230/400 V mains network

Surge arrester (Class B) for the AC 230/400 V mains network:

  • Nominal discharge capacity of at least 5 kA (8/20 µs)

  • Protection level < 2 kV

  • If varistors are used, they must be thermally monitored, and provided with insulating equipment.

KNX network

Surge arresters for the KNX network

  • Nominal discharge capacity of at least 5 kA (8/20 µs)

  • Protection level < 2 kV

  • The surge arrester must be matched to the KNX (TP1) levels.

Effect of surge protectors

The surge protector protects the bus devices from overvoltages (transient surge voltages) which can be picked up by the bus cables. If the protective effect is damaged (e.g. by exceeding the permitted number of surge arresters), this results in a bus fault (bus low resistance). Note that no message is generated by this bus fault. Faulty surge arresters must be replaced immediately.


Using surge arresters can improve the interference immunity of a KNX (TP1) network regardless of any overvoltage protection measures in conjunction with lightning protection.

Surge arresters in bus terminal technology

Surge arresters normally have the same dimensions as bus connection terminals. However, the surge arresters can be distinguished by their color (blue) and the additional earth conductor. Where surge arresters are used instead of bus terminals, they must be connected to the nearest grounding point (e.g. protective conductor).

Surge arresters (manufacturer: Dehn) are available from Siemens as off-the-shelf components in bus terminal format.

  • Overvoltage protection Order no. 5WG1 190-8AD01

During engineering, make sure surge arresters can be installed, and that there is a connection point for them.