Overvoltage protection
Under surge protector to protect electrical and electronic equipment is understood from excessive voltages. Overvoltages can be caused by lightning , capacitive or inductive coupling of other electrical systems. Also electrostatic discharge (ESD), which can occur even in simple manipulations can cause surges.
The surge protection is part of the DIN lightning protection standard VDE 0185. This standard was revised in 2002 and published in 4 parts. This standard has been converted into VDE 0185-305 (2007) with 3 parts.
Causes of overvoltages
One of the main causes of critical overvoltages are lightning strikes in and near power and signal lines. The capacitive and inductive effects of lightning (LEMP from English lightning electromagnetic pulse ) induce impermissible voltages in conductor loops around 200 meters. Up to approx. 2 km, dangerously high potential differences can occur due to ohmic effects (earth resistance).
Switching operations in the medium or low voltage network in the house can also cause overvoltages (SEMP for switching electromagnetic pulse ). Switching overvoltages of up to several kilovolts occur in cables in addition to fluorescent lamps with conventional ballasts (chokes) or when motors are switched off.
Aboveground atomic bomb explosions causing by the nuclear electromagnetic pulse (NEMP of Engl. Nuclear electromagnetic pulse ) is extremely energy overvoltages.
Low-energy, but very steep overvoltage pulses are caused by electrostatic discharges - they particularly endanger sensitive electronic components and assemblies and are caused by improper handling and transport.
Countermeasures
Lightning protection
Buildings that are particularly important or at risk are equipped with lightning protection systems. This includes external lightning protection with its air-termination conductors, arresters and earth electrodes as well as internal lightning protection. Internal lightning protection includes all measures against the effects of lightning current. This mainly includes equipotential bonding and overvoltage protection.
In Europe, lightning protection is defined by the EN 62305 standard. This deals with the risk of direct and indirect lightning strikes. The standard provides for lightning protection for buildings, systems and people and specifies the necessary protective measures (down conductor, interception device, etc.). The series of standards was also included in the VDE set of regulations (VDE 0185-305) and the national standard. The VDE lightning protection standard (VDE 0185 as of 10/2006) specifies that external lightning protection must be connected to the potential equalization of the building. In the event of a strike, the earth potential is sharply increased or decreased in relation to the external conductors (in the TN-C and TT networks ), which can result in insulation damage and fires. Therefore, in every building with external lightning protection, the internal lightning protection must also be implemented consistently in order to compensate for the potential differences that arise when impacting the ground or power supply lines (e.g. roof stands ).
Lightning and surge protection is only fully effective if all accesses to the system are secured. In buildings, this includes the power supply, data cables (cable TV, telephone), metal parts of the building and piping systems.
Another measure would be the establishment of a shield against inductively and capacitively coupled overvoltages on electrical cables and lines. For this purpose, lines or cables can be laid in metal or shielded ducts, for example. Another possibility would be laying with a double screen.
Device protection
The mains power supply and data line connections (LAN, antenna cables, modem connections) in devices must be protected against overvoltages. Since some network devices are already very inexpensive, it does not always make sense to equip these areas with surge protection. In addition, the transformer potential isolation in LAN and Ethernet offers a certain protection. Fiber optic networks are not at risk. WLAN is only at risk with exposed antennas.
Gas-filled surge arresters insulate as long as the voltage remains below approx. 450 V and do not interfere because of their low capacitance of only approx. 2 pF. If the ignition voltage is exceeded, the resistance drops to very low values within microseconds, whereby current peaks of up to 20 kA can be diverted. They are thermally overloaded during continuous operation.
The decision as to which plants or systems should be protected is based on the following focal points:
- System parts that are principally particularly at risk should be protected. External antennas, long data lines and lines in the vicinity of energy transmission facilities are particularly at risk.
- Systems that are particularly expensive to purchase should be well secured against overvoltage. These can be computers, custom-made products or high-performance network routers.
- Hazard for buildings or people: If there is an increased risk of injury in the event of overvoltage, additional measures are taken to ensure lightning current equipotential bonding and to avoid overvoltage. This is also regulated by law, especially in public buildings, via standards or requirements. Examples are medical technology and the electronics of elevators and cranes.
Classification
A general distinction is made between high-energy (direct or nearby lightning strikes) and low-energy (distant lightning strikes or switchovers in the power grid).
Surge protection can be divided as follows:
- Protection of signal lines (lightning arresters)
- Protection of the power supply lines at low voltage level (<1000 V) (surge arrester)
- Protection of distribution networks at medium and high voltage level, especially of overhead lines and their connection points (arresters as device protection)
Depending on how high the protection is aimed for and how severe the overvoltage events are to be expected, graduated overvoltage protection devices are used, which are referred to as coarse, medium and fine protection. They differ in their discharge capacity (energy absorption capacity, maximum current), the shutdown behavior (upstream fuse trips or not) and the protection level (maximum remaining overvoltage when responding).
Low voltage and signal voltage
Low voltages are often protected with a suppressor diode (similar to a Zener diode ). Also varistors are now being manufactured for low voltage applications. Both elements are characterized by the fact that they lock again automatically after the overvoltage event - the internal network is not disrupted.
The following graphic shows the basic function of the overvoltage limitation with a suppressor diode:
In low-voltage power supplies, a so-called clamping circuit ( thyristors ) is often used to protect against internal defects that lead to overvoltage , which triggers the fuse in the supply line from a certain overvoltage by actively shorting the supply.
Another possibility are protective diodes , which can also be used to protect signal inputs of integrated circuits (fine protection).
Gas arresters and four-layer diodes are often used in telephone networks (central protection).
Antenna cables are often protected with spark gaps (coarse protection) and gas discharge tubes.
Mains voltage
Mains supply lines in devices or upstream adapter plugs are often protected with varistors. If gas arresters are installed, the upstream fuse is always triggered because the arc discharge continues to burn when the overvoltage pulse has already passed and the voltage has fallen back to the nominal voltage.
Varistors or spark gaps are also used in house inlets. These protective devices (see picture above) have a significantly higher discharge capacity than device protective measures.
The fuse is installed after the LS switch so that the sockets behind it are protected.
Medium and high voltage
Spark gaps are used to protect the insulators of overhead lines . Varistors are used to protect transformers connected to overhead lines. These can be produced for any high voltage. They consist of a stack of discs made of metal oxide ceramic (e.g. zinc oxide).
Coarse, medium and fine protection
A complete overvoltage protection concept takes into account all external and internal electrically conductive connections and is often structured in three stages, which for building protection are essentially based on the rated surge voltages for the overvoltage categories according to DIN VDE 0110 / IEC Publication 664:
- Class A falls within the scope of the energy supplier.
Coarse protection
The coarse protection (type 1, formerly class B) in the building feed should divert the energy content of lightning and limit the remaining residual voltage to values below 1300 to 6000 V (depending on the technology used). Currents of 50/100 kA with a pulse shape of 10/350 µs are calculated, which corresponds to the typical values of a direct lightning strike. A few years ago, 3000 V was considered a good value for coarse protective arresters. Thanks to new technologies, the protection level could be reduced to 1300 V for coordinated (combined) arresters.
Fund protection
The medium protection (type 2, formerly class C) is usually located in the floor distributors in buildings and limits the remaining overvoltages to less than 600 to 2000 V and relies on the overvoltages to be intercepted not exceeding 4000 V.
Fine protection
Fine protection (type 3, formerly class D) protects the respective sockets and the plug connections of all other lines. It reduces the remaining overvoltages to the level that the connected devices, assemblies or components can handle. In most countries, manufacturers of electrical and electronic devices are obliged to equip their devices with the fine protection required for safe operation (CE mark indicates this). In Germany this is regulated by the law on the electromagnetic compatibility of devices (EMVG). Still, there are significant differences; Fine protection installed ex works does not automatically have to be of the same quality level as good external fine protection. Overvoltage protection adapters (adapter plugs) and overvoltage protection socket strips in particular often do not have any (fusible) fuses that disconnect them from the mains if the varistors overheat or if they are destroyed, which in extreme cases can lead to fires. In any case, these protective devices only divert overvoltages to the protective conductor running in parallel, which is usually not sufficient.
The protective effect of each level builds on the previous one. This means that the previous stage reduces the energy content of the overvoltage in order to avoid thermal overloading of the subsequent protection module (energetically coordinated overvoltage protection). Dispensing with a stage can make the surge protection almost ineffective; this also applies to long cable lengths for dissipating energy or incorrect positioning and selection of products. The heart of every surge protection is the medium protection. It is the most important component and must be supplemented as required by fine protection arresters (for sensitive electronic devices) and coarse protection (if there is external lightning protection, if the grid is fed in via roof stands, if there are extensive outdoor facilities and other factors). Often, several medium protection and coarse protection units are necessary: lines leading to the outside (path lighting, swimming pool, etc.) must usually be secured in the same way as the power supply, but vice versa: First the medium protection, then the coarse protection, since the interference current comes from the other Direction is initiated. Leads from B or C arresters must not be routed through conventional residual current circuit breakers (and also LS, depending on the arrester and LS type), as the high leakage currents would destroy the FI or the miniature circuit breaker if triggered.
Web links
- Overvoltage protection of sensitive amplifier inputs
- Installation standards for overvoltage protection devices
Individual evidence
- ↑ Lightning rods on the house - lightning protection classes and surge protection. Retrieved December 19, 2016 .
- ↑ Surge protection in the Local Area Network (LAN) - FAQ on lightning protection . VDE website . Retrieved November 17, 2013.