A three-phase surge protective device is not sized from panel amperes and it should not be selected by choosing the largest kA number. The correct specification starts with the voltage across each connected protection mode, the earthing system and the installation boundary. Only then can you select SPD Type, topology, Uc, Up, Iimp/In/Imax and short-circuit protection.
Use the nine gates below in order. Each gate can disqualify a product; passing only the current-rating gate is not enough.
Three-Phase SPD Sizing: The Required Inputs
Collect this information before comparing models:
| Vstup | Record from the project |
|---|---|
| Napájení | Nominal and maximum L-L voltage, L-N voltage where present, frequency |
| Vodiče | Three-wire or four-wire; L1/L2/L3, N, PE or PEN |
| Zemnění | TN-S, TN-C-S, TN-C, TT, IT or project-defined arrangement at this panel |
| Umístění | Service origin, main switchboard, distribution board or near final equipment |
| Exposure | External lightning protection system, overhead supply, lightning risk and upstream SPD |
| Zařízení | Required impulse withstand/protection objective and criticality |
| Poruchové zatížení | Prospective short-circuit current and upstream fuse/breaker |
| Instalace | Space, environment, lead route, status and remote-alarm requirement |
| Normy | IEC/EN, UL or other project basis; do not mix classifications |
The output is a specification, not just “a 40 kA SPD.”
Gate 1: Define Voltage by Protection Mode
Record both line-to-line and line-to-neutral voltage. A 400/230 V system and a 480 V system may both be called “three phase,” but the voltage continuously applied to an SPD element depends on whether that element is connected L-N, L-PE, L-L or N-PE.
Also identify the highest expected normal power-frequency voltage at the installation point. Do not confuse this with a surge. Voltage tolerance, transformer regulation, generation modes and system faults can change the stress across a protection mode.
Specification output: nominal system voltage, maximum normal voltage and voltage across every proposed protection mode.
Gate 2: Identify the Earthing System at This Panel
Do not select topology from the building’s general description alone. Identify whether neutral and PE are separate or combined at the actual SPD location and where any PEN separation or N-PE bond occurs.
- TN-S/TN-C-S downstream of separation: L, N and PE are distinct at the panel; L-N and N-PE coordination is commonly relevant.
- TN-C: the conductor arrangement includes PEN; use only a topology approved for that point and do not create an artificial neutral-earth separation.
- TT: N-PE behavior and temporary overvoltage stresses require explicit consideration; 3+1 arrangements are commonly used in IEC-oriented designs, but the exact choice remains project- and product-specific.
- IT: first-fault conditions can raise voltage to earth on healthy conductors. Uc and topology must be verified for the real IT configuration and fault condition.
Použijte TN, TT and IT earthing guide for system definitions.
Specification output: earthing system, N/PE/PEN arrangement and bond location.
Gate 3: Select the SPD Type from Location and Exposure
In the IEC framework, Type identifies standardized surge duty, not pole count:
| Typ | Main duty parameter | Typical design role |
|---|---|---|
| Typ 1 | IMP, commonly associated with 10/350 µs testing |
Lightning-current handling at an origin or LPZ boundary where the risk design requires it |
| Typ 2 | Na adrese and, where declared, Imax, commonly with 8/20 µs current |
Distribution-level limitation of induced and switching surges |
| Typ 3 | Combination-wave/terminal-equipment coordination | Fine protection close to sensitive equipment, coordinated with upstream SPDs |
| Typ 1+2 | Declared performance for both duties | Combined application where both sets of requirements are satisfied |
An external lightning protection system, lightning-current risk assessment, overhead service and installation boundary may drive a Type 1 requirement. A downstream distribution board does not automatically need Type 1 merely because it is industrial.
IEC Type 1/2/3 and UL 1449 SPD Types are not interchangeable labels. If the project is UL-based, specify the UL category, nominal system configuration, MCOV, VPR, SCCR and installation location under that framework. See the detailed Průvodce typem 1 vs. typem 2 vs. typem 3.
Specification output: standards framework, SPD Type and installation boundary.
Gate 4: Choose Topology and Protection Modes
Topology must fit the conductors and earthing system:
- 3+0: three protection paths, with the exact connected modes defined by the product diagram.
- 4+0: four voltage-limiting paths in common market terminology; supplier notation and modes must be verified.
- 3+1: commonly three phase-to-neutral voltage-limiting paths plus a coordinated N-PE switching path.
Do not equate four physical modules with a universal “four-pole” circuit. Read the internal circuit diagram and declare the required modes: L-N, L-PE, L-L and/or N-PE. The Průvodce topologií SPD a 3-pole vs 4-pole guide cover this decision in depth.
Specification output: conductor count, topology and required modes of protection.
Gate 5: Select Uc or MCOV—and Check TOV Behavior
Univerzita v Utahu is the maximum continuous operating voltage that may be applied to the SPD under its declared conditions. Select it for the voltage across each mode, not from the line-to-line system label alone.
Uc must be high enough to avoid operation or accelerated aging during normal voltage conditions, but choosing an unnecessarily high Uc can result in a less favorable protection level. Use manufacturer tables for the specific earthing system.
Also verify the SPD’s behavior under temporary power-frequency overvoltage (TOV). A TOV is not simply an ordinary long-term supply deviation. Its significance depends on magnitude, duration, earthing system and fault, equipment withstand, SPD response and local installation rules. A generic voltage protector is not a substitute for checking declared SPD TOV behavior.
For a deeper distinction, read Uc vs Up u svodiče přepětí (SPD).
Specification output: Uc/MCOV for each protection mode and required TOV behavior.
Gate 6: Coordinate Up with the Protected Equipment
Nahoru is the declared voltage protection level under specified tests. It must support coordination with the equipment’s impulse withstand and upstream/downstream protection plan.
Do not simply select the lowest number on a catalog page:
- Compare Up only on the same standard and protection-mode basis.
- Allow for voltage added by connection-lead inductance in the real installation.
- Consider cable distance and coordinated downstream SPDs where required.
- Confirm that Uc and TOV suitability remain correct; a low Up cannot compensate for the wrong continuous-voltage rating.
Specification output: maximum acceptable declared Up by mode, equipment withstand basis and installation coordination note.
Gate 7: Specify Iimp, In and Imax Correctly
These values are not interchangeable kA ratings:
| Hodnocení | Selection use | Comparison rule |
|---|---|---|
IMP |
Zatížení bleskovým proudem typu 1 | Compare per pole/mode with the same 10/350 µs test basis and charge/energy context |
Na adrese |
Nominal discharge-current duty | Compare with the same 8/20 µs basis, mode and standard |
Imax |
Maximum discharge current for products that declare it | Treat as a maximum test value, not proof of repeated duty or Type 1 capability |
The required duty comes from the risk assessment, lightning protection design, installation level, coordination and applicable rules. There is no defensible universal kA value for every commercial or industrial panel. The In vs Imax guide explains the ratings without turning them into a false sizing shortcut.
Specification output: required Iimp for Type 1 duty and/or In plus Imax where relevant, each with waveform, mode and per-pole/total basis.
Gate 8: Verify Short-Circuit and Backup Protection
An SPD can reach end of life or fail under abnormal system conditions. The branch must be able to disconnect safely at the available fault level.
Zaznamenejte a porovnejte:
- Prospective short-circuit current at the installation point.
- SPD
SCCRor applicable short-circuit withstand/current declaration such asIsccr. - Maximum permitted upstream fuse or breaker.
- Manufacturer-specified external backup fuse/breaker, if required.
- Coordination and selectivity expectations for the upstream distribution system.
- Follow-current extinguishing capability where relevant to the SPD technology.
An internal thermal disconnector and an external short-circuit protective device perform different functions. Never choose the backup device from panel current alone or replace it with a larger rating without the manufacturer’s coordination data.
Specification output: available fault current, applicable short-circuit declaration, upstream device and dedicated backup device.
Gate 9: Complete the Industrial Specification
Electrical ratings are only part of a usable RFQ. Add:
- DIN-rail or panel-mounted format and available modules
- Pluggable or fixed cartridges
- Visual status indication and remote changeover contact
- Operating temperature, altitude and derating requirements
- Enclosure/pollution/environmental conditions
- Terminal range, conductor requirements and torque data
- Required certification and test documentation
- Replaceable-module coding and maintenance access
- Coordination with upstream/downstream SPDs
- Maximum practical connection-path objective
Worked Example: 400/230 V TN-S Distribution Board
Assume a project records a 400/230 V, 50 Hz, three-phase four-wire TN-S distribution board downstream of the service. There is an upstream Type 1 SPD at the origin, no separate direct-lightning-current boundary at this board, and sensitive PLC and drive equipment is supplied downstream.
The correct result is not an invented catalog model. It is a requirement set:
- IEC-oriented Type 2 duty at this distribution level, subject to the project risk assessment.
- Topology approved for 3P+N TN-S, with exact L-N/N-PE or other required modes declared.
- Uc selected from the manufacturer’s TN-S table against the actual voltage in each mode and maximum normal condition.
- TOV behavior compatible with the system and local rules.
- Up coordinated with downstream equipment withstand and corrected for the intended lead arrangement.
- In and Imax, where declared, selected from the project’s surge-duty requirement on a matched 8/20 µs basis.
- Available fault current checked against SCCR/Isccr and the manufacturer backup-protection table.
- Remote indication included because the board is maintained centrally.
Only after these statements are complete should a model be selected.
Copy-Ready Three-Phase SPD RFQ Worksheet
Application / panel:
Standards framework and market:
Nominal voltage: ____ V L-L / ____ V L-N
Maximum normal voltage and frequency:
Conductors: L1 / L2 / L3 / N / PE or PEN
Earthing system and bond/PEN separation location:
Installation boundary and upstream SPD:
External LPS / lightning risk result:
Required SPD Type:
Required topology and protection modes:
Uc / MCOV by mode:
Required TOV behavior:
Maximum acceptable Up by mode:
Iimp requirement with waveform and per-pole/total basis:
In requirement with waveform and mode:
Imax requirement, where used, with waveform and mode:
Prospective short-circuit current:
Required SCCR or applicable declaration such as Isccr:
Upstream protective device:
Required external backup fuse/breaker:
Mounting, environment and module count:
Visual / remote status requirement:
Lead-route constraint and coordination notes:
Required certificates and documentation:Použijte průvodce technickým listem SPD to verify supplier responses line by line. Physical installation then follows the SPD wiring guide.
Pravidlo konečného výběru
A defensible three-phase SPD specification must pass every gate:
system voltage → earthing → installation Type → topology → Uc/TOV → Up → Iimp/In/Imax → fault/backup protection → project features
If a supplier quote omits protection modes, test waveforms or short-circuit coordination, it is not yet comparable. Send the completed worksheet and single-line diagram to the tým produktů VIOX SPD for model-level verification.