Surge Protection for KNX Systems

Surge Protection for KNX Systems

Created by: Glen Zhu | Updated Date: December 06th, 2023

KNX Systems Surge Protection

The KNX system is frequently used for smart home and building automation as a common bus system in accordance with EN 50090, sensors and actuators easily combine the most diverse smart building functions.

The great advantage of KNX is its decentralization which means no main unit such as control panels that manages the operation of the whole system. The rest of the system will continue to operate without interruption. A faulty module can be replaced using the same or even another device with similar capabilities. Another strength is the interoperability, allowing KNX devices from different manufacturers to work seamlessly together within the same system.

Today and future smart home automation is indispensably equipped with an installation of a KNX system, the concern for smart home surge protectors must be addressed in the case of lightning strikes and power surges.

KNX Bus Topology

The basic unit of a KNX installation is the line, and it serves as the backbone for connecting various devices within a building automation system.

Each line can support up to 256 devices, a significant increase from the previous limit of 64 devices. This expanded capacity allows for more extensive and intricate automation setups within a single line.

Line couplers are used for connecting to main lines, thus, a maximum of 255 devices can be installed in each of the secondary lines. However, the lines could not be over 15.

For a very complex large building automation, the structure also can be extended by installing backbone couplers on 15 backbone lines that can be connected (Figure 1).

Figure 1 – KNX bus topology

With a single line, a tree topology can be built, with branches at arbitrary points. The number of all line couplers operating in an area must be deducted.

The couplers provide galvanic isolation between areas and lines, which protects them in the event of an overvoltage. All lines, including the backbone and main lines for individual areas, have their own power supply. Consequently, the failure of one power supply unit will not cause the entire network to stop working.

Equipotential Bonding

Lightning protection measures are required for smart homes and buildings that are vulnerable to lightning, not only physical damage but also sensitive electrical equipment.

The lightning protection zones concept standardized the protective measures against lightning electromagnetic impulses, the KNX system is applicable the regulation.

Induction loops may occur during cable installation when data cables are installed too close to low-voltage power lines or metal construction or pipe connected to the main earthing busbar, these loops can induce unwanted currents in the data cables, potentially leading to signal interference, data corruption, or equipment malfunctions.

Therefore, it is necessary to install cables as close to each other as possible. The loops can be prevented by means of the equipotential bonding strips.

Lightning equipotential bonding is mandated for all cables in LPZ 0A to 1, necessitating the use of lightning current arresters. All conducting elements or systems must be connected to the equipotential bonding strip, and all cables must be buried in the ground (Figure 2).

Figure 2 – Lightning equipotential bonding at the entrance point of the KNX bus cable into the building

If the bus cables extend over more than two buildings, special measures must be taken, the bus cable should be installed in-between the buildings in metal conduit or duct, which should be connected to the earth on both sides at the entrance to the building. In order to discharge parts of the lightning current, a minimum cross-sectional area of 16mm2 Cu is required.

Lightning equipotential bonding does not have to be established when a shield duct or metal pipe is connected to the main earthing terminal on both ends. Shielding helps protect against electromagnetic interference and can provide some level of protection against induced surges.

Figure 3 – Lightning equipotential bonding is not required for the KNX cable

Power Supply Unit

KNX’s support for various transmission media, including twisted-pair cable (KNX TP), Ethernet (KNX IP), radio frequency (KNX RF), and powerline (KNX PL), provides a versatile and adaptable platform for building automation.

One of the necessary components of any KNX TP system is the power supply, the output voltage of KNX power supplies is 30V DC while devices on the network can be supplied with a voltage from the range of 21 to 30V DC. Such a wide range makes it possible to compensate for possible voltage drops on the wires.

In order for a smooth operation of the system, the length of the cable between a single bus device and the power supply must not exceed 350 m, and the cable between two bus devices not be longer than 700 m. The max length of a single line can be up to 1000 m. The minimum distance between two power supply units working in one line should be 200 m. The distance between two power supply units is specified by the manufacturer (Figure 4).

Figure 4 – The distance between the power supply and the bus device

Each line has its own power supply unit for the bus devices, both main and secondary. The power supply unit has an integrated voltage and current control and therefore resistant to short circuits. To prevent static charges on the bus side, the power supply unit should be connected to the earth point of the low voltage section.

The main power supply is always necessary to be protected.

FLP7-275/3S+1 is mounted in the main distribution board to safeguard the overvoltage from the power lines as the primary protection.

Install surge protection devices on the power supply lines of the KNX system. This helps prevent voltage surges from reaching the power supply units of the devices connected to the KNX bus.

Figure 5 – Surge protective devices installed in the main power supply system

Surge Protection for KNX Systems

In structures with external lightning protection systems, surge protectors are recommended due to the higher electromagnetic field during direct lightning strikes. The use of shielded ducts or metal pipes, earthed at both ends, for bus cables between buildings eliminates the need for lightning equipotential bonding, requiring only surge arresters.

To safeguard low-voltage overhead lines, combined arresters are advised at the point of entry into the building. SLP40-275/3+1 is recommended at the distribution board of the KNX system. Type 3 SPD is installed close to the protected load as a supplement to Type 2 SPD.

For buildings with buried power cables, surge protection is essential downstream of the meter and advisable at the KNX system’s distribution board, irrespective of the strike point.

Figure 6 – Surge protective devices installed at the main distribution board and the KNX system

Additional surge protection should be provided when bus cables and powerful mains are installed in parallel or bus services are connected to conductive sections. Surge arrester with KNX bus terminal design is a symmetrical safety device discharging both bus wires, thus preventing large voltage differences. It adapted to the immunity of KNX/EIB systems, however, this bus overvoltage arrester cannot be used to branch the bus cable.

Installation Guidelines of KNX Systems

The KNX system is a standardized protocol for home and building automation, it is important to follow the guidelines by the KNX Association. The right installation of bus cable and bus devices are significant for the performance of the whole smart home automation system.

Distance between the power supply unit and bus device must be ensured as it is limited to avoid voltage drops and telegram run times (Figure 4). Bus cables from different lines should not be linked together and make sure cables are marked clearly. Clearance space must be observed between the insulated bus cables and sheathed main cables.

Avoid installing bus devices above high-power-consuming mains devices, as this could lead to excessive heat generation. Excessive heat can negatively impact the performance and reliability of electronic components. The heat generated by high-power-consuming mains devices, such as transformers or power supplies, can affect the temperature of nearby equipment.

Check the polarity of all bus devices. This involves inspecting each individual device connected to the bus system to ensure that the electrical polarity is correctly configured. Pay close attention to the positive and negative terminals of each device, confirming that they align with the specified polarity requirements of the bus system.

Once all bus devices are mounted, verify the bus voltage at the termination point of each bus cable. The voltage should be at least 21V.

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