Selecting an AC surge protective device for a main distribution board is not simply a matter of choosing the largest kA value. The correct MDB surge protection solution depends on where lightning current may enter, the earthing system, the required protection modes, the operating voltage, the available short-circuit current and the equipment located downstream.
This guide is written for distribution-board manufacturers, electrical contractors, consultants, project procurement teams and OEM buyers who need to approve an AC SPD configuration before panel production or site installation.
Indice dei contenuti
Risposta Veloce
Use a Type 1 SPD at the origin when the installation may carry partial lightning current, such as a building with an external lightning protection system or another confirmed direct-lightning-current exposure condition.
Use a Type 2 SPD when the MDB requires protection against induced or switching surges but the project assessment does not require Type 1 lightning-current capability.
Use a tested Type 1+2 SPD when the main board requires both lightning-current and voltage-limiting performance in one coordinated device.
After selecting the SPD type, confirm the earthing arrangement, protection modes, Uc, Iimp, In, Imax, Up, temporary overvoltage behaviour, short-circuit capability, backup protection and remote signalling requirement. The final model should be approved from the single-line diagram and the SPD manufacturer's installation data, not from a product label alone. [1] [2] [3]
Why the MDB Is a Critical Surge Protection Point
The main distribution board is normally located at, or close to, the supply origin of the low-voltage installation. A surge entering at this point can be distributed through several outgoing circuits before it reaches control systems, drives, communication equipment and other sensitive loads.
Installing an SPD at the MDB creates the first surge-limiting stage. It does not automatically complete the protection design. When downstream equipment is remote, has a lower impulse withstand level or requires a lower residual voltage, additional coordinated Type 2 or Type 3 protection may be necessary. [3] [5]
MDB AC SPD Selection Decision Table
Use the following table as an engineering screening tool. It provides a selection direction, not a substitute for lightning risk assessment, local wiring rules or project approval.
| MDB condition | Direzione di selezione | Cosa deve essere confermato |
|---|---|---|
| External lightning protection system is installed | Evaluate Type 1 or a tested Type 1+2 SPD at the supply origin. | LPS class, expected lightning-current distribution, supply arrangement, required Iimp and coordination with downstream SPDs. |
| Overhead incoming supply or another direct-lightning-current exposure is identified | Type 1 capability may be required. | Risk assessment, local installation rules and whether partial lightning current can enter the board. |
| No Type 1 condition is identified, but surge protection is required at the origin | A Type 2 SPD may be suitable. | Supply system, Uc, Up, In, Imax, protected equipment withstand level and backup protection. |
| One device must provide both Type 1 and Type 2 performance | Use a device tested and declared as Type 1+2. | Do not assume that any Type 1 product provides the required Type 2 protection level. Check the complete declared test data. |
| Critical electronic equipment is supplied far downstream | Coordinate the MDB SPD with additional downstream protection. | Cable route, separation distance, equipment impulse withstand, Up at the equipment terminals and manufacturer coordination guidance. |
Procurement conclusion: the same 400/230 V MDB can require different SPD types when lightning exposure, supply entry and downstream protection requirements are different.
How to Choose Type 1, Type 2 or Type 1+2
Type 1: When Lightning Current May Enter the MDB
Type 1 SPDs are tested with a 10/350 µs lightning impulse current. They are selected where the device may need to conduct partial lightning current at the origin of the installation. [1] [2]
A building with an external lightning protection system is a common reason to evaluate Type 1 protection at the MDB. The required Iimp should be derived from the lightning protection design and current distribution, not copied from another project.
Type 2: When the Main Requirement Is Transient Overvoltage Limitation
Type 2 SPDs are tested with an 8/20 µs current waveform and are commonly used to limit induced lightning surges and switching overvoltages. A Type 2 device can be suitable at the MDB when the installation assessment does not require Type 1 lightning-current capability. [1] [5]
The word “main” in main distribution board does not automatically mean that every project needs Type 1. The decision depends on whether partial lightning current is expected at that location.
Type 1+2: Combined Performance in One Tested Device
A Type 1+2 SPD has declared performance for both test classes. It is often considered at an MDB where the project requires lightning-current discharge and a lower voltage protection level in one unit.
Type 1+2 does not mean that downstream coordination can always be omitted. The final voltage at sensitive equipment is affected by the SPD's Up, conductor inductance, installation layout, cable distance and any additional protective stages. [3] [6]
Do Not Select by Type Marking Alone
Two SPDs marked Type 1+2 can have different Uc, Iimp, In, Up, short-circuit behaviour, protection modes, module widths and backup-protection requirements. The complete datasheet and project conditions must be compared.
How to Judge 3P, 4P and 3P+N Configurations
Pole labels are not always used consistently across suppliers. “4P” may describe four modules, four terminals or a specific protection circuit. “3P+N” may refer to a three-phase arrangement with a dedicated neutral-to-earth protection element. The buyer should therefore verify the internal protection modes and wiring diagram instead of approving the model from the pole name alone.
| Market description | Cosa può descrivere | Approval question |
|---|---|---|
| 3P | Three phase-related protection paths, often used where there is no separate neutral conductor at the installation point. | Is the system TN-C before PEN separation, or is neutral protection being omitted for another documented reason? |
| 4P or 4+0 | Four similar protection paths involving L1, L2, L3 and N, commonly connected toward PE according to the declared circuit. | What are the exact L-PE, N-PE or L-N protection modes, and are they suitable for the earthing system and temporary overvoltage conditions? |
| 3P+N or 3+1 | Three L-N protection paths plus one dedicated N-PE element, often a different technology from the phase modules. | Is the N-PE element correctly rated for total discharge current, follow current and the selected TT or TN-S arrangement? |
Procurement conclusion: specify the required protection circuit, not only “3P,” “4P” or “3P+N” in the purchase order.
TT, TN-S and TN-C-S System Differences
The earthing arrangement determines which conductors are present at the MDB and how surge current should be diverted. It also changes the temporary overvoltage conditions that the SPD may experience.
Sistema TT
In a TT system, the installation protective earth is not the same conductor as the supply neutral. A 3+1 arrangement, with phase-to-neutral protection and a dedicated neutral-to-earth element, is commonly evaluated because it can provide a defined N-PE discharge path while addressing TT-system temporary overvoltage considerations.
This is not a universal instruction to use every product marketed as “3+1.” The N-PE element, Uc values, protection modes, disconnection arrangement and TOV performance must all match the project and local installation rules. [3] [6]
TN-S System
In a TN-S system, neutral and protective earth are separate throughout the relevant part of the installation. The MDB solution may use a four-path arrangement or a 3+1 circuit, depending on the protection concept and the manufacturer's declared wiring.
The correct choice is determined by protection modes, Uc, TOV behaviour, Up and system coordination. “TN-S means 4P” is too broad to use as an approval rule.
TN-C-S System
A TN-C-S installation uses a PEN conductor upstream and separate N and PE conductors after the separation point. The SPD configuration depends on whether it is installed before or after that point.
Before PEN separation, the installation conditions resemble TN-C. After separation, the protected conductors resemble TN-S. The supplier must receive the single-line diagram and the exact SPD location to avoid an incorrect neutral or earth connection.
Critical Wiring Boundary
Never approve a TN-C-S SPD configuration without identifying the PEN separation point. An incorrect assumption can create the wrong protection path or an unacceptable N-PE connection.
Which Parameters Must an MDB Buyer Confirm?
The following parameters answer different engineering questions. None should be used alone as proof that one SPD is better than another.
| Parametro | What it means | What the buyer must confirm |
|---|---|---|
| Uc | Maximum continuous operating voltage for the declared protection mode. | Nominal voltage, maximum normal operating voltage, earthing system, voltage tolerance and TOV conditions. |
| Iimp | Lightning impulse current capability used for Type 1 classification, tested with a 10/350 µs waveform. | Required value per pole or mode, total current distribution and the lightning protection design basis. |
| In | Nominal discharge current, generally associated with repeated 8/20 µs testing. | Declared Type 2 performance and whether the rating applies per mode or per module. |
| Imax | Maximum discharge current declared for an 8/20 µs event under the applicable test method. | Do not use Imax alone. Compare In, Up, Uc, test class and short-circuit performance. |
| Su | Voltage protection level declared during surge testing. | Equipment impulse withstand, lead length and the resulting protection level at the actual equipment terminals. |
| TOV | Behaviour under a temporary power-frequency overvoltage rather than a short surge impulse. | Earthing-system faults, neutral conditions and whether the SPD withstands or safely fails under the declared test. |
| SCCR or short-circuit withstand | The SPD's ability to be safely applied at a point with a specified prospective short-circuit current under declared protection conditions. | Prospective fault current at the MDB, required fuse or breaker, and certificate scope for the complete combination. |
Procurement conclusion: a high Imax does not compensate for an unsuitable Uc, an excessive Up, a missing Type 1 rating or inadequate short-circuit coordination.
Eight-Step Selection Workflow for an MDB Project
- Confirm the supply data. Record nominal phase-to-phase and phase-to-neutral voltage, frequency and maximum normal operating voltage.
- Identify the earthing arrangement at the exact SPD location. State TT, TN-S, TN-C or TN-C-S and mark the PEN separation point where applicable.
- Determine whether Type 1 capability is required. Review the external LPS, overhead supply, lightning-risk assessment and the possibility of partial lightning current entering the installation.
- Define the protection circuit. Specify the required L-N, L-PE, N-PE or L-PEN modes instead of relying only on 3P, 4P or 3+1 terminology.
- Select the voltage-related parameters. Confirm Uc, TOV behaviour and Up against the actual system and protected equipment.
- Confirm discharge-current performance. Check Iimp for Type 1 and In/Imax for Type 2 according to the project requirement.
- Coordinate short-circuit and backup protection. Compare the MDB prospective short-circuit current, upstream protective device and the manufacturer's maximum backup fuse or approved MCB data.
- Approve installation and documentation. Check connection length, conductor route, DIN-rail space, remote contact, replacement module, wiring diagram, datasheet and model-specific certificate scope.
Three Realistic MDB Selection Scenarios
Scenario 1: Commercial MDB, 400/230 V TN-S, No External LPS
A Type 2 SPD may be suitable when the project assessment does not identify partial lightning current at the supply origin. The engineer still needs to confirm the protection circuit, Uc, Up, In, Imax, short-circuit current and downstream equipment requirements.
Selection boundary: the absence of an external LPS does not by itself prove that Type 1 is unnecessary. Overhead supply and other exposure conditions must also be reviewed.
Scenario 2: Industrial MDB, TN-C-S, External LPS Installed
Type 1 or Type 1+2 protection should be evaluated at the origin. The designer must identify whether the SPD is before or after PEN separation and calculate or specify the required Iimp from the lightning protection concept.
Selection boundary: do not automatically copy 12.5 kA or 25 kA per pole from another project. LPS class, current distribution and system configuration can change the requirement.
Scenario 3: Three-Phase TT Main Board Supplying Sensitive Controls
A 3+1 protection circuit is commonly evaluated, with phase-to-neutral modules and a dedicated N-PE element. The final model must be checked for Uc, TOV, total N-PE discharge duty, Up and coordination with downstream control-panel SPDs.
Selection boundary: “3P+N” on a quotation is insufficient. Request the circuit diagram and declared ratings for every protection mode.
Backup Fuse or MCB: How to Confirm It
The SPD's internal thermal disconnector is not automatically a substitute for external short-circuit backup protection. Its purpose and operating conditions are different.
The required external fuse or MCB depends on the SPD design, prospective short-circuit current, upstream protective device and the manufacturer's tested coordination. Some SPDs include integrated backup protection. Others specify a maximum permissible backup fuse or an approved breaker characteristic. [3] [5]
Information Required for Backup-Protection Approval
- Prospective short-circuit current at the MDB installation point
- Upstream fuse or circuit-breaker type and rating
- SPD maximum backup fuse or approved MCB data
- Required discrimination or selectivity with upstream protection
- SPD short-circuit withstand or SCCR under the declared protective arrangement
- Conductor cross-section and terminal capacity
Installation Position and Connection Length
An MDB SPD is normally connected close to the incoming section, either near the incoming isolator or at the closest suitable outgoing connection point permitted by the panel design. The objective is to create a short, direct path from the live conductors through the SPD to PE or PEN.
Long conductors add inductive voltage during a fast surge. This additional voltage appears in series with the SPD's declared Up and can materially increase the voltage reaching the equipment.
IEC-based application guidance commonly targets a combined active-conductor-to-SPD and SPD-to-PE/PEN connection path of no more than approximately 0.5 m where the selected installation arrangement permits it. Local rules, panel construction and the manufacturer's instructions remain the final authority. [3] [5]
Common Panel Layout Mistake
Installing the SPD in an available DIN-rail space far from the incoming busbar can produce long phase and earth leads. The product may have a low declared Up, but the complete installed path can deliver a much higher residual voltage.
When Is a Remote Signal Contact Worth Specifying?
Remote signalling is useful when the MDB is unattended, difficult to inspect or connected to a building-management, SCADA or maintenance system. A volt-free changeover contact can report the SPD status without requiring a technician to open the panel for every inspection.
It is not mandatory for every MDB. The project should confirm the contact logic, terminal rating, normal and alarm states, cable interface and whether the alarm indicates module end-of-life, disconnection or another condition.
What Happens When the Wrong MDB SPD Is Selected?
| Selection error | Possible consequence | Required correction |
|---|---|---|
| Type 2 used where Type 1 lightning-current capability is required | The SPD may not be suitable for the expected 10/350 µs lightning-current duty. | Review the lightning protection design and select a correctly tested Type 1 or Type 1+2 device. |
| Uc is too low for the real operating and TOV conditions | Accelerated ageing, thermal stress, disconnection or premature failure. | Confirm maximum continuous voltage, earthing system and TOV performance. |
| Uc is raised without reviewing Up | The voltage protection level may no longer be suitable for downstream equipment. | Evaluate Uc and Up together with equipment impulse withstand. |
| Wrong 3P, 4P or 3+1 circuit | Missing protection mode or incorrect N, PE or PEN connection. | Approve the protection circuit from the earthing system and single-line diagram. |
| Backup protection is not coordinated | Nuisance tripping, failure to clear a fault safely or non-compliance with the tested SPD combination. | Follow the manufacturer's fuse or MCB coordination data and verify fault current. |
| Connection leads are too long | The installed residual voltage can exceed the product's declared Up. | Rearrange the panel connection to shorten and straighten the surge-current path. |
OEM and Bulk-Order Confirmation
An OEM order should freeze the complete electrical configuration before the label, packaging or production batch is approved. A model name alone is not enough to control technical consistency.
Information the Buyer Should Send
- Single-line diagram
- Tensione nominale e massima di funzionamento
- Frequenza
- TT, TN-S, TN-C or TN-C-S arrangement
- PEN separation point
- External LPS and lightning-risk information
- Required Type 1, Type 2 or Type 1+2 class
- Required protection modes
- Iimp, In, Imax and Up requirements
- Prospective short-circuit current
- Backup fuse or MCB arrangement
- Requisito di segnale remoto
- DIN-rail space and terminal direction
- Quantity and spare-module quantity
- Required standards and target market
- OEM label, model code and packaging files
Information the Supplier Should Return
- Full model code
- Protection-circuit diagram
- Complete technical datasheet
- Uc, TOV and Up by protection mode
- Iimp, In and Imax declarations
- Short-circuit and backup-protection data
- Installation and torque instructions
- Module dimensions and panel width
- Remote-contact terminal diagram
- Replacement-module reference
- Model-specific certificate scope
- Approved OEM label artwork
Need an MDB SPD Configuration Review?
Send the system voltage, earthing arrangement, single-line diagram, external lightning protection information, prospective short-circuit current and required protection class. LEEYEE can use these details to review a suitable AC SPD configuration for panel, project or OEM approval.
Domande frequenti
Does every main distribution board require a Type 1 SPD?
No. Type 1 is selected where partial lightning current may enter the installation. A Type 2 SPD may be suitable at an MDB when the project assessment does not require Type 1 capability. The external LPS, supply route, lightning exposure and local rules must be checked.
Can a Type 1+2 SPD replace every downstream Type 2 SPD?
Not automatically. Additional coordinated protection may be needed when sensitive equipment is far from the MDB, has a lower impulse withstand level or requires a lower voltage protection level at its terminals.
Is a 4P SPD the same as a 3+1 SPD?
No. A 4P or 4+0 arrangement and a 3+1 arrangement can use different protection paths and component technologies. Compare the internal circuit, protection modes, Uc, TOV and N-PE element instead of relying on the number of modules.
Which SPD should be used for a 400/230 V TN-S MDB?
The voltage and TN-S label are not enough to select one model. Confirm whether Type 1 capability is required, then select the protection circuit, Uc, Up, discharge-current ratings, short-circuit coordination and backup protection. Both four-path and 3+1 solutions may exist, depending on the engineering design.
Is a higher Imax always better for an MDB?
No. Imax is only one Type 2 performance value. A suitable SPD must also have the correct test class, Uc, In, Up, protection modes, TOV behaviour and short-circuit capability.
Can an MCB always replace the SPD backup fuse?
No. An MCB should be used only when its type, rating and operating characteristic are accepted by the SPD manufacturer for the available short-circuit current. Otherwise, use the declared fuse arrangement or obtain written technical confirmation.
Should an OEM MDB include remote signalling?
Remote signalling is useful for critical, unattended or centrally monitored panels. Confirm the alarm logic, terminal rating, BMS or SCADA interface and additional panel space before fixing the OEM model.
Riferimenti
- International Electrotechnical Commission, IEC 61643-01:2024 — Low-voltage surge protective devices — Part 01: General requirements and test methods. Official IEC publication page.
- International Electrotechnical Commission, IEC 61643-11:2025 — Low-voltage surge protective devices — Part 11: Surge protective devices connected to AC low-voltage power systems — Requirements and test methods. Official IEC publication page.
- International Electrotechnical Commission, IEC 60364-5-53:2019+AMD1:2020+AMD2:2024 — Low-voltage electrical installations — Selection and erection of electrical equipment. Official IEC consolidated-version page.
- International Electrotechnical Commission, IEC 62305-4:2024 — Protection against lightning — Part 4: Electrical and electronic systems within structures. Official IEC publication page.
- DEHN, Surge Protection in Low-Voltage Switchgear Assemblies, application white paper covering supply-point SPDs, backup protection, coordination and conductor length. Official technical document.
- ABB Furse, Mains Power Protection — Type 1 and Type 2 Surge Protection Series Product Guide, including MDB applications, earthing-system diagrams, remote signalling and installation guidance. Official ABB technical document.
