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Weidmuller | Questions & Answers Concerning Surge Protection
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Home > Products > MTL Instruments > Arrester > FAQs
The priority you give to overvoltage protection depends on your willingness to take risks! Perhaps you think "it'll never happen to me". Then you won't have lost anything, but will have gained only very little. However, the subject of overvoltage is then a daily worry for you. But if you wish to be on the safe side, you should include overvoltage protection in your corporate strategy. Such an investment brings you operational reliability and can prove invaluable when disaster strikes.
When do I need a class I arrester, when a class II arrester?
In a lightning protection system set up on a building, the class I arrester achieves the lightning protection equipotential bonding for the supply voltage. The class I arrester is used when higher pulses are expected and is installed in the vicinity of the incoming supply. Under DIN VDE 0185 part 1 (Nov 1982), the class I arrester is intended for use in lightning protection equipotential bonding. It also satisfies the requirements of class B to DIN VDE 0675 (draft, Nov 1989) and IEC 61643-1, class I.
The class II arrester serves to protect low-voltage consumer installations and electronic devices against surge that can ensue as a result of atmospheric discharges (lightning) or switching actions. The class II arrester complies with VDE 0675 part 6, class C (draft, Nov 1989), DIN VDE 0675 part 6, A2 (Oct 1996), and ÖVE SN 60 parts 1 and 4, as well as IEC 61643-1-1, class II. High-power metal oxide varistors are employed as a voltage limiter in PU BC class I and PU II class II arresters.
When is a decoupling inductance needed?
When using Weidmuller arresters of class I and II based on varistors, no decoupling inductance is needed. The PU1 TSG operates with a triggered sparkover gap. The fast response and low protection level mean that no decoupling is required here either.
How should the network protection PU be fused?
The terminal compartment of the PU II is designed according to IEC 61643-1 and can accommodate 4 mm2 to 25 mm2. According to DIN VDE 0100 part 430: load capacity, cables or conductors for permanent installations, installation types A, B1, B2 and C as for installation type E, outdoors, and assignment of overcurrent protection devices for protection in case of overload.
According to this, a current rating In of 125 A for single wires (max. 2 wires) and a current rating In of 100 A for single wires (max. 3 wires) applies to 35 mm2 copper when laid on or in walls or under plaster. It is therefore left to the builder of the installation to decide which types of lines are employed. If single-core wiring with 35 mm2 is installed, this can be protected with a 125 A fuse and the PU can be connected with this. If multi-core wiring (NYM 3 x 35 mm2) is chosen, a 100 A fuse must be used. The PU modules do not constitute an electrical load but rather only operate when surge have to be discharged.
Why are there 3- and 4-pole versions?
Various arresters are used depending on the network structure. A widely used network structure is the TN system. In the TN-C system, the electricity supply company routes the potential of the operational earth of the low-voltage source (transformer) to the consumer installation via the integral PEN conductor. The PE conductor has the same potential as the N conductor in this case. A 3-pole arrester is used here. Every rule has an exception: in the TN-S system, PE and N are separate. This means there can be a potential shift between PU and N. A 4-pole PU is used in this case. In addition, a combination of 3 or 4-pole modules reduces the amount of wiring.
What other network structures are available?
In the TT system, class I and class II arresters as surge protection devices are not used between the active conductors and the earth potential as in TN systems, but instead between phases L1, L2 and L3 and the neutral conductor. Why? In the "classic" arrangement of surge protection devices between the phases and the earth potential, these may towards the end of their service life become incapable of suppressing system follow currents, can show signs of ageing or even cause short-circuits. A fault current then flows back to the supplying source depending on the actual earthing resistance of the consumer installation. Generally, the relatively high loop resistances in TT systems mean that fuses carrying operating current do not detect this fault current as a malfunction and thus do not disconnect quickly enough. This can lead to an increase in potential throughout the equipotential bonding system of the building. If this consumer installation supplies buildings that are some distance away, or if consumers outside the effective area of the equipotential bonding system of the building are operated via mobile conductors, dangerous accidental energisation can occur. This is where the 3+1 circuit can be useful.
An IT system is set up in some consumer installations for reasons of availability. A single-phase earth fault practically creates a TN system. The power supply is not interrupted but instead maintained. IT systems are used in medical applications, for example. A device for monitoring insulation provides information on the quality of the insulation of active conductors and connected consumers in relation to the earth potential. Surge protection devices are incorporated between the active conductors and the main equipotential bonding. The fuses, conductor cross-section and conductor routes are handled as for T systems. Likewise, all active conductors are protected against local earth potential in sub-circuit distribution boards. PU D surge protection devices are used to protect sensitive consumers. Arresters must be designed for the phase-to-phase voltage.
What is so good about the 3+1 circuit?
3+1 Circuit
If PU II modules (class II arresters) in a TT system are no longer routed to the local earth but to the neutral conductor, only the line resistance of the neutral conductor limits the arising follow current in the event of the PU modules acquiring a low resistance. After the occurrence of the fault, this is immediately disconnected from the spur line fuses or the main fuses carrying operating current. The earthing system resistance opposing the fault current has turned it into a pure short-circuit current! The connection between neutral conductor and main equipotential bonding circuit is achieved with a sparkover gap. This is capable of carrying the total surge currents occurring at the place of installation without being overloaded (PU II 3+1 280 V). This 3+1 system is also used in the sub-circuit distribution boards. Phase conductors L1, L2 and L3 are connected to the neutral conductor via the PU II 3+1 280 V. A sparkover gap is installed between there and the PE rail. The treatment of local equipotential bonding systems as well as separate systems for diverting voltages to the equipotential bonding system and the arrangement of PU modules before residual-current circuit-breakers is the same as that described for TN systems.
How does monitoring work with PU II modules?
Each individual disc of PU II modules is equipped with a thermal monitoring device. This represents the state of the art and disconnects the ageing arrester from the mains supply, thus avoiding a fire. This thermal monitoring device works with special solder that unsolders within around 30 s when a current of 0.2 A passes through the varistor. This is required under ÖVE SN 60 and other standards. Readiness for operation is usually indicated by a green flag in the status window or, in the case of PU II modules with telecommunication output, via a changeover contact. The PU II modules are plug-in units and are therefore easy to replace.
Does the PU continue working after an overvoltage event?
Yes, if the discharge current for each individual disc remains below 40,000 A. However, each discharge process ages the varistor. This ageing effect accumulates over the service life of the unit and after several years causes the arrester to fail. But this situation can be monitored.
Which standards apply for the testing of PU modules?
PU II and PU BC are tested to IEC 61643-1, which corresponds to DIN VDE 0675, part 6 (Dec 2002). Arresters of the PU II series conform to class II. PU BC arresters conform to classes II and I. The PU III and PU D series was developed and tested in accordance with the requirements of IEC 61643-1 and DIN VDE 0675, part 6 (Dec 2002).
Where are the PU modules installed?
The dimensions of PU modules for distribution boards comply with DIN 43880 A1 (draft, June 1981). Arresters of class I are positioned near the incoming supply and the main equipotential bonding system, arresters of class II in the distribution board and PU III in the sub-distribution boards, near to the object to be protected. Appropriate insulation resistances for the various parts of the system are required according to the insulation coordination specifications of DIN VDE 0110. One way of achieving this is by the graded utilisation of arresters of class I, II and III.
What do I need to watch out for when installing PU modules?
IEC 60364-5-53 describes the selection and installation of surge protection in buildings worldwide. The German prestandard DIN V VDE V 0100-534 describes the selection and installation of equipment for surge protection systems.
What is the difference when using a sparkover gap instead of a varistor?
Modern power networks are fed into the building via underground cables. Part of the lightning energy is absorbed by attenuation of the supply line. The full lightning energy is not expected here. A sparkover gap is characterised by high discharge capacity (approx. 50 kA) and the time to sparkover is in the order of microseconds. To suppress the follow current, sparkover gaps require a special design, sometimes in the form of arcing spaces. A decoupling device must be installed between sparkover gap and downstream varistors. The varistor-based design of the PU BC is capable of discharging a lightning test current (10/350 µs) of up to 16 kA. The response time of the sparkover gap in this case lies in the nanosecond range. The varistors do not draw any power follow current.
What are triggered sparkover gaps?
These sparkover gaps have additional electronics. They "see" the interference pulse and ignite the sparkover gap. This means that the protection level is kept low and the time to sparkover is reduced. This saves on decoupling coils.
When should I use the CL or SL circuit with the MCZ OVP?
The difference between the switching in the CL (current loop) and SL (symmetrical loop) is the integration of the suppressor diodes. The CL circuit has a diode between the lines. This system is used for current loops and offers direct protection at the input or output of the analogue sensor. The SL circuit operates symmetrically to earth, i.e. two Transzorb diodes are connected to earth. If this is used in a current loop instead of the CL circuit, the residual voltage is twice as high because there are two diodes instead of just the one of the CL circuit.
Why are combination circuits available?
The use of GDTs (gas discharge tubes), MOVs (varistors) and TAZs (Tranzorb diodes) besides attenuators such as coils and resistors bring about advantages for different interference pulses. If a high, steep-flanked pulse is present, the GDT responds and diverts the fault. If the pulses are weaker and not as fast, the GDT doesn't operate and the entire load is diverted via the MOV and the TAZ. This means that a combination achieves an optimum protection level depending on the incoming pulse. The use of single components means that only one characteristic can be utilised.