A surge protection device (SPD) protects electrical equipment from transient overvoltages caused by lightning, switching, or other disturbances. Choosing the right SPD involves understanding the type of surge, the location in the installation, and the equipment to be protected. This guide explains the main types, standards, and selection criteria.
What Is a Surge Protection Device?
A surge protection device is a component that limits transient overvoltages by diverting surge current to earth or by clamping the voltage to a safe level. SPDs are installed in parallel with the supply and protect downstream equipment from damage or malfunction.
Sources of transient overvoltage include lightning strikes (direct or nearby), utility switching, large motor starts, capacitor bank switching, and short-circuit events. Without SPDs, sensitive electronic equipment is exposed to these events, which can degrade or destroy components over time.
Types of SPDs
The international standard IEC 61643-11 classifies low-voltage SPDs into three types based on their location and discharge capacity.
Type 1 SPD
Type 1 SPDs are installed at the origin of the electrical installation, typically at the main distribution board. They are designed to handle direct lightning strikes and partial lightning currents, with a discharge capacity of at least 10/350 µs waveform. Type 1 SPDs must be installed when the building has an external lightning protection system or is supplied by overhead lines.
Features: High discharge capacity, robust, installed at the main board. Typical use: Buildings with lightning protection or overhead supply. Limitation: Voltage protection level (Up) may still be too high for sensitive equipment.
Type 2 SPD
Type 2 SPDs are installed at sub-distribution boards, downstream of Type 1. They handle the residual switching surges and indirect lightning effects, with a discharge capacity of 8/20 µs waveform. Type 2 is the most common SPD in commercial and industrial installations.
Features: Medium discharge capacity, balanced cost and protection. Typical use: Sub-distribution boards in any installation. Limitation: Must be coordinated with upstream Type 1 if both are present.
Type 3 SPD
Type 3 SPDs are installed close to sensitive equipment, typically at socket outlets or near the equipment terminals. They handle low-energy transients that pass through Type 1 and Type 2, with a combined discharge capacity.
Features: Fine protection, low discharge capacity, complementary. Typical use: Servers, medical equipment, control systems. Limitation: Cannot handle large surges alone; must be downstream of Type 1 and/or Type 2.
Comparison of SPD Types
| Type | Location | Waveform | Discharge Capacity | Typical Use |
|---|---|---|---|---|
| Type 1 | Main board | 10/350 µs | High (25 kA+) | Lightning protection entry |
| Type 2 | Sub-board | 8/20 µs | Medium (20–40 kA) | General distribution |
| Type 3 | Near load | Combined | Low (5–10 kA) | Sensitive equipment |
Key Specifications
- Maximum continuous operating voltage (Uc) — The voltage at which the SPD remains inactive. Typically 275 V or 320 V for 230 V systems.
- Voltage protection level (Up) — The maximum voltage that passes through the SPD during a surge. Lower Up means better protection.
- Nominal discharge current (In) — The current the SPD can handle repeatedly, typically 8/20 µs waveform.
- Maximum discharge current (Imax) — The peak current the SPD can handle once, typically 8/20 µs.
- Impulse current (Iimp) — For Type 1, the 10/350 µs lightning current.
- Short-circuit withstand (Isccr) — The fault current the SPD can withstand without creating a hazard.
How to Choose the Right SPD
Use this short process to select the correct SPD for your installation:
- Assess the surge environment — Is there a lightning protection system? Overhead supply? Heavy switching loads? This determines whether Type 1 is required.
- Plan the cascade — Use Type 1 at the main board, Type 2 at sub-boards, Type 3 near sensitive loads. Coordinate the Up levels to ensure proper discharge sharing.
- Select Uc — Choose Uc above the nominal system voltage by 10% to 20% to avoid nuisance operation.
- Select discharge capacity — For Type 1, 25 kA per pole is a common starting point. For Type 2, 20 to 40 kA Imax is typical.
- Select Up — For sensitive electronic equipment, Up below 1.5 kV is recommended. Coordinate with the equipment’s rated impulse voltage.
- Add backup protection — Provide a backup fuse or circuit breaker to disconnect a failed SPD.
Backup Protection and Coordination
SPDs must be coordinated with upstream protective devices. A short-circuit inside a failed SPD must be cleared by a backup fuse or breaker. Without backup protection, a failed SPD may cause a fire or a sustained arc.
Coordination between Type 1 and Type 2 SPDs is essential. If the distance between them is too short, the upstream SPD may not operate and the downstream SPD may be overloaded. A minimum distance of 10 m of cable, or a dedicated decoupling inductor, is typically required.
Installation Best Practices
- Keep the SPD connection leads as short and straight as possible. Long leads add inductance and increase the effective Up.
- Connect the SPD to the main earthing terminal with the shortest practical conductor.
- Use a dedicated backup fuse or breaker, sized per the SPD manufacturer’s instructions.
- Install Type 1 and Type 2 with proper coordination distance or decoupling.
- Provide a status indicator or remote contact for maintenance.
- Mark the SPD clearly on the single-line diagram and the panel label.
How to Inspect SPD Quality
- Visual check — Look for discoloration, cracks, or tripped indicators.
- Status indicator — Verify the green/red status window is in the correct state.
- Thermal disconnector — Verify the internal disconnector has not opened.
- Backup fuse — Check continuity and rating.
- Connection torque — Verify terminal tightness per the manufacturer’s torque table.
- Earth continuity — Verify the SPD earth conductor is connected and continuous.
Frequently Asked Questions
Do I need a Type 1 SPD?
Type 1 is required when the building has an external lightning protection system (LPS) or is supplied by overhead lines. Otherwise, Type 2 at the main board is usually sufficient.
Can I install a Type 2 SPD without Type 1?
Yes. In installations without a lightning protection system and without overhead supply, Type 2 at the main board and Type 3 near sensitive equipment provide good protection.
What does Up mean, and why does it matter?
Up is the voltage protection level — the maximum voltage that reaches the equipment during a surge. Equipment with a low rated impulse voltage requires an SPD with a low Up.
How long does an SPD last?
SPDs degrade with each surge they handle. Most have a status indicator and some have a counter. In environments with frequent surges, plan to replace SPDs every 5 to 10 years. In quiet environments, they may last longer.
Do SPDs need a backup fuse?
Yes. A backup fuse or breaker protects the installation in case the SPD fails short. The fuse rating depends on the SPD’s Isccr and the manufacturer’s instructions.
Can I install an SPD on a DC system?
Yes, but use a DC-rated SPD. Solar PV, battery storage, and DC traction systems all require dedicated DC SPDs. AC SPDs must not be used on DC systems.
What is the difference between Class I, II, and III in the old terminology?
Class I, II, and III in the older IEC 61643-1 are the predecessors of Type 1, Type 2, and Type 3 in the current IEC 61643-11. The concepts are similar; the new “Type” terminology is more common in current specifications.
Source Surge Protection Devices from BANGE Electric
BANGE Electric provides integrated electrical distribution and cable system solutions, including distribution boxes, switchgear, and complementary surge protection devices. We support specification customization, consistent quality control, export packaging, and reliable international supply.
To receive a quotation, send us your installation type, surge environment, and required SPD specifications.