| Lightning current entering the installation | Impulse discharge capability | Type 1 testing uses a 10/350 μs current wave; Type 2 testing uses an 8/20 μs current wave. | Use a Type 1 or Type 1+2 device where a direct lightning current may enter through the service or through an external lightning protection system. | The required impulse current rating depends on the lightning protection level, current-sharing arrangement, service configuration, and installation standard. |
| Induced and switching surges | Nominal discharge current, In | Common Type 2 values are 5 kA, 10 kA, 15 kA, or 20 kA with an 8/20 μs waveform. | Select a Type 2 metal-oxide surge arrester with an In suitable for the exposure and prospective surge environment. | A higher In rating generally provides greater surge-current capability, but it does not replace correct voltage selection or coordination. |
| Maximum continuous system voltage | Maximum continuous operating voltage, Uc or MCOV | For a 230/400 V low-voltage system, commonly used phase-to-earth MCOV values are around 255–275 V, depending on the earthing system and product design. | Choose an MCOV that remains continuously stable at the highest normal voltage and at permitted temporary overvoltages. | Do not select only from nominal voltage. Verify phase-to-earth voltage, neutral behavior, fault conditions, and the manufacturer’s temporary-overvoltage data. |
| Protection of sensitive equipment | Voltage protection level, Up | Typical low-voltage SPD protection levels are approximately 1.2–2.5 kV, depending on device class, rated voltage, and test current. | Select an arrester with a Up below the impulse withstand voltage of the protected equipment. | Lower Up is desirable only when the device can withstand the expected surge energy and the installation remains properly coordinated. |
| Direct lightning protection zone boundary | SPD test class or type | Type 1: 10/350 μs impulse current; Type 2: 8/20 μs surge current; Type 3: combination-wave protection near sensitive loads. | Use Type 1 at the incoming boundary when required; use Type 2 downstream; use Type 3 close to particularly sensitive equipment. | A coordinated cascade can reduce residual voltage and distribute surge energy across multiple protection stages. |
| Earthing arrangement | Connection mode and pole configuration | TN-S, TN-C, TT, and IT systems require different conductor connections and voltage considerations. | Use an arrester configuration designed specifically for the supply system, such as L–PE, L–N, or N–PE protection paths. | For TN-C systems, the PEN conductor must be considered; for TT systems, neutral-to-earth protection and residual-current-device coordination are important. |
| Thermal and short-circuit safety | Short-circuit withstand and backup protection | The arrester’s rated short-circuit withstand must be equal to or greater than the prospective short-circuit current at its installation point. | Choose a device with an internal thermal disconnector, status indication, and a specified backup fuse or circuit-breaker arrangement. | Confirm coordination between the arrester, upstream overcurrent protection, conductor cross-section, and local wiring regulations. |
| Installation lead inductance | Connection length and conductor routing | At high-frequency surge fronts, each metre of conductor can add a significant inductive voltage component. | Use short, straight, separated conductors with a low-inductance routing layout. | Keep the total connecting conductor length as short as practical; many installations target approximately 0.5 m or less for the complete connection path. |
| Equipment coordination | Impulse withstand voltage and protection distance | Low-voltage equipment commonly has impulse withstand categories from approximately 1.5 kV to 6 kV, depending on the equipment location and overvoltage category. | Ensure the arrester’s effective protection level, including wiring effects, is below the equipment’s impulse withstand level. | Install additional downstream protection when cable length, separation, or electrical conditions could allow the surge voltage to increase again. |
| Monitoring and maintenance | Status indication and remote signaling | Many modular arresters provide a mechanical end-of-life indicator and optional remote alarm contact. | Use visible status indication for accessible panels and remote contacts where continuity of operation is critical. | Include periodic visual inspection, checking of wiring and backup protection, and replacement after confirmed high-energy operation. |
| Applicable standards | Design, product, and installation requirements | IEC 60099-4 covers metal-oxide surge arresters for AC power systems; IEC 61643-11 covers low-voltage surge protective devices; IEC 62305 addresses lightning protection. | Select equipment with verified test data and conformity to the standard applicable in the installation location. | National wiring rules and utility requirements may add conditions for testing, disconnection, clearances, and installation practices. |