Key Takeaway
A surge protection device (SPD) is a modular component that protects electrical installations against transient overvoltages caused by lightning. It diverts lightning current to earth within nanoseconds. Installation is mandatory in certain cases under the NF C 15-100 standard (AQ2 zones, buildings with lightning rods, overhead supply lines). There are three types: Type 1 (direct lightning current, main switchboard), Type 2 (induced overvoltage, distribution board) and Type 3 (fine protection, equipment level). Total wiring length between the SPD and the protected point must not exceed 50 cm at the main switchboard and 30 m downstream.
What Is a Surge Protection Device (SPD)?
A surge protection device is a protective apparatus installed in the electrical panel that limits transient overvoltages caused by lightning strikes or network switching operations. When an overvoltage occurs, the SPD conducts and diverts the excess energy to earth, thereby protecting all downstream connected equipment: PLCs, variable speed drives, IT systems, home automation, and fire safety systems.
Without an SPD, a single overvoltage event can instantly destroy sensitive electronic equipment whose replacement cost far exceeds that of the protection device.
The Three Types of Surge Protectors
| Characteristic | Type 1 (T1) | Type 2 (T2) | Type 3 (T3) |
|---|---|---|---|
| Test parameter | Impulse current Iimp (10/350 µs wave) | Discharge current In (8/20 µs wave) | Combined wave (1.2/50 – 8/20 µs) |
| Typical values | Iimp = 12.5 to 25 kA per pole | In = 5 to 40 kA per pole | Uoc = 6 kV |
| Location | Main switchboard (MSB) — service entrance | Distribution board | Immediate proximity of equipment |
| Usage | Direct lightning, building with lightning rod, overhead line | Induced overvoltages — most common | Fine protection for sensitive equipment |
| NF C 15-100 requirement | If lightning rod or exposed overhead line | AQ2 zone or distance > 30 m from T1 | Recommended (not mandatory) |
When Is SPD Installation Mandatory?
The NF C 15-100 standard (section 443 and guide C15-443) defines the cases where an SPD is mandatory:
1Building Equipped with a Lightning Rod
If a lightning rod is installed on the building or on a neighboring building within 50 m, a Type 1 SPD is mandatory at the main switchboard. The partial lightning current flowing through the power conductors must be absorbed before entering the installation.
2Overhead or Mixed Overhead-Underground Supply
When the low-voltage supply line is overhead (fully or partially), atmospheric overvoltages propagate directly along the conductors. A Type 1 or Type 2 SPD is mandatory depending on the keraunic level of the area.
3High Lightning Density Zone (AQ2)
Areas classified as AQ2 (lightning density Ng > 2.5 strikes/km²/year) require SPD installation when the installation supplies sensitive or safety-critical equipment (fire safety systems, alarms, medical equipment, industrial processes).
4Photovoltaic Installations
PV installations, often roof-mounted and therefore particularly exposed, require an SPD on the DC side (between panels and inverter) and an SPD on the AC side (connection board). The UTE C 15-712-1 standard specifies the requirements.
⚠️ Regulatory reminder
The absence of an SPD in cases where the standard requires one constitutes an electrical non-compliance. In the event of a claim, the insurer may refuse coverage if overvoltage protection was not compliant with the NF C 15-100 standard.
Essential Installation Rules
Wiring length: the 50 cm rule
The total length of conductors between the network, the SPD, and the earth terminal (L1 + L2 + L3) must not exceed 50 cm. The residual voltage at the equipment terminals increases proportionally to cable length according to the formula U = L × di/dt. Excessive wiring length negates the SPD's effectiveness.
Cable separation
Incoming cables (from the network) and outgoing cables (to the installation) must be physically separated within the panel. Phase, neutral, and PE conductors must be grouped on the same side to reduce the ground loop area and limit overvoltages induced by magnetic coupling.
Maximum protection distance: 30 meters
An SPD protects equipment located up to 30 m downstream. Beyond this distance, oscillation and reflection phenomena on the conductors can generate local overvoltages exceeding the SPD's protection level Up. An additional SPD (Type 2 or 3) must then be installed near the equipment.
Coordination between SPDs
When multiple SPDs are installed in cascade (T1 at the main switchboard, T2 at the distribution board), they must be energy-coordinated. The minimum distance between a T1 and a T2 is typically 10 m (unless specific coordination is validated by the manufacturer). This distance ensures each SPD absorbs its share of energy.
Earthing and Interconnection
An SPD's effectiveness depends directly on the quality of the earthing system:
- Without lightning rod: earth resistance compliant with NF C 15-100 (according to the earthing scheme) is sufficient.
- With lightning rod: earth resistance must be ≤ 10 Ω.
- Mandatory interconnection: all earth electrodes within the same building must be interconnected (SPD earth, lightning rod earth, installation earth, telecommunications earth).
- Multiple buildings: if several buildings are served within 50 m, interconnecting earth electrodes is recommended to achieve equipotential bonding.
Disconnection Device and Maintenance
Each SPD must be associated with a short-circuit protection device (dedicated circuit breaker or fuse) that automatically disconnects it at end of life. Modern SPDs include a visual indicator (green/red LED or mechanical flag) showing their status.
Plug-in models allow replacement without power interruption: the base remains wired and the cartridge can be removed and replaced in seconds. A dry contact alarm relay (available on most Surtelec models) enables status reporting to the BMS or supervision system.
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Frequently Asked Questions — Surge Protectors
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Key Takeaways
- An SPD protects the electrical installation against transient overvoltages by diverting lightning current to earth.
- Type 1 absorbs direct lightning (main switchboard), Type 2 handles induced overvoltages (distribution board), Type 3 provides fine protection (equipment).
- Installation is mandatory per NF C 15-100: lightning rod, overhead line, AQ2 zone, PV installation.
- Total wiring length at the main switchboard must not exceed 50 cm (L1+L2+L3 < 50 cm rule).
- An SPD protects up to 30 m downstream — beyond that, an additional SPD is required.
- Plug-in models with alarm reporting facilitate maintenance without power interruption.
- Optim-Elec distributes Surtelec SPDs: Types 1, 2, 1+2, and PV DC solutions.
