Lightning protection - Basic Documentation - RMK770-1

Communication via the KNX bus

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The notes on lightning and overvoltage protection are based on the "KNX Handbook": Volume 4, Part 4 "Installation Safety Requirements".

Lightning protection measures

  • The KNX bus must be included in the safety measures for the mains network.

  • Lighting protection systems must be engineered and installed by specialists who understand and comply with the relevant regulations and standards.

External lightning protection

External lighting protection consists of permanently active lightning conductors (an air-termination system, a down conduction system and an earth termination system).

External lightning protection is required for buildings where lightning strikes are common due to the location, type of building or its use, or where the consequences may be severe. Lightning protection is mandatory for public buildings and buildings with assembly rooms. The necessity of erecting a lighting protection system is determined by the building codes of the related country.

Internal lightning protection

Internal lightning protection is based on consistent equipotential bonding for lightning protection, i.e. all metal components such as water, heating and gas pipes, metal walls etc. must be connected to the equipotential bonding rail.

Internal lightning protection is required in buildings with electronic devices susceptible to transient surges. This protection consists of measures to counter the effect of the lightning current and its electric and magnetic fields on metal and electrical conductors and equipment.

Figure: Lightning equipotential bonding (primary protection)

Engineering of lightning protection equipment

VDI 6004, Sheet 2, Edition July 2007, "Protection of building services - Lightning protection systems and surge protection" applies.

If lightning protection is required in a building, the active cores (see figure on previous page) must be connected with lightning arresters. This is also recommended in the following cases, for example, if:

  • The building is connected to a low voltage overhead cable.

  • The construction of the building includes metal parts which can be struck by lightning, e.g. metal chimneys or aerials

  • There is another building with a lightning protection system in the vicinity.


During engineering, selection of the lightning arrester must be coordinated with overvoltage protection equipment (note the information supplied by the manufacturers on the use of their products).

Lightning arresters for primary protection

Lightning arresters can discharge high-energy lightning current repeatedly without damage. They are installed in the AC 230/400 V mains network and KNX network.

230/400 V mains network

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

  • Nominal leakage current of at least 10 kA (10/350 µs)

  • Protection level < 4 kV

KNX network

Lightning arrester for the KNX network

  • Nominal leakage current of at least 1 kA (10/350 µs)

  • Protection level < 4 kV

Routing cables
between buildings

Variant 1 with lightning arresters

When routing cables between buildings, lightning arresters must be installed where the bus cable enters the building. They must be connected to the nearest equipotential bonding point.

Routing cables
between buildings

Variant 2 with surge arresters

With Alternative 2, the bus cable is protected with surge arresters and routed in a metal conduit or duct which is part of the equipotential bonding system.

The minimum cross-section of the conduit or duct must be sufficient to allow a substantial part of the lightning current to pass across it (min. cross-section: Cu 16 mm2, Al 25 mm2, Fe 50 mm2).


With Variants 1 and 2, a surge arrester must be installed as secondary on the bus device closest to the entry to the building. In relation to the bus cable length, the bus device should be installed some meters away from the lightning arrester, so that the surge arrester is not also partly responsible for primary protection.