SPD selection rule
This calculator uses a practical selection rule for the starting SPD type. It first checks whether the circuit is PV/DC, then checks lightning exposure and the installation point.
T = DC/PV SPDT = Type 1+2 SPD when LPS = yes or supply = overhead/mixedT = upstream SPD + Type 3 local protectionwhere:
- T
- Recommended SPD type family
- LPS
- External lightning protection system status
- supply
- Incoming supply exposure, such as underground, overhead, or mixed
- Type 3
- Local fine protection for sensitive equipment
The result is a product-family starting point. Final SPD selection still requires Uc, Up, In/Imax, earthing system, coordination distance, and local standard checks.
How to select an SPD type
Surge protective device selection begins with the source of surge exposure and the installation point. Type 1 devices are intended for the service entrance or locations exposed to partial lightning current, Type 2 devices are commonly used in distribution boards, and Type 3 devices provide coordinated fine protection close to sensitive equipment.
- Identify whether the system is AC, DC, or photovoltaic.
- Confirm whether an external lightning protection system is present.
- Assess overhead, underground, or mixed supply exposure.
- Choose the incoming board, sub-board, or equipment panel location.
- Verify voltage, discharge-current ratings, earthing system, and coordination using product data.
SPD selection example
For a building with an external lightning protection system and an overhead supply, the calculator starts with a Type 1+2 SPD at the main distribution board. If a sensitive equipment panel is downstream, coordinated Type 3 protection may also be appropriate near the equipment.
| Installation condition | Starting SPD family |
|---|---|
| Incoming board with higher lightning exposure | Type 1+2 |
| Normal downstream distribution board | Type 2 |
| Sensitive equipment near point of use | Coordinated Type 3 |
| PV array or DC circuit | DC/PV-rated SPD |
Type 1 vs. Type 2 vs. Type 3 SPD
SPD types describe different test duties and installation roles; they are not interchangeable labels for stronger and weaker products. A coordinated installation can use more than one type so that high-energy surges are handled upstream and residual voltage is limited closer to equipment.
- Type 1: incoming locations exposed to lightning-current duty.
- Type 2: distribution-level transient overvoltage protection.
- Type 3: local protection coordinated with upstream SPDs.
- Type 1+2: combined duties in one tested device family.
Ratings to check after choosing the SPD type
The type result is only the first selection step. Final selection must match the maximum continuous operating voltage, voltage protection level, nominal or maximum discharge current, impulse current where applicable, prospective short-circuit conditions, protective backup device, earthing arrangement, and installation conductor length.
Coordinate surge protection with the installation
Check the SPD together with the distribution board, upstream protective device, downstream equipment withstand, and local lightning-risk assessment. Product-family support can follow after the electrical conditions are known.
Assumptions
- Low-voltage power system
- Preliminary product family selection only
- Final SPD rating must match system voltage, earthing, short-circuit conditions, and installation standard
Important Warnings
- Do not use this output as proof of IEC 61643 compliance.
- PV and DC SPDs must be DC-rated and selected against maximum array voltage.