How to Select the Right
Surge Protective Device
A practical selection guide for choosing AC power SPD, DC solar PV SPD and signal line SPD according to system type, installation position, voltage, discharge current and protection level.
01 System type: AC, DC, PV or signal line
02 Risk level: Type 1, Type 2 or Type 1+2 protection
03 Voltage: Uc, Un or Up
04 Electric Current: Iimp, Imax or In
6 Steps to Select an SPD
Start from the application system, then confirm the installation position, SPD type, operating voltage, discharge current and protection level.
1
System Type
AC power, DC solar PV, signal line, data line, telecom or control line.
2
Installation Point
Main distribution board, sub-panel, inverter side, combiner box or equipment terminal.
3
SPD Type
Type 1, Type 2, Type 1+2, Type 3, DC PV SPD or signal line SPD.
4
Operating Voltage
Confirm Uc, Ucpv or working voltage according to the actual system.
5
Discharge Current
Check In, Imax and Iimp according to lightning risk and protection level.
6
Protection Level
Check Up value and coordinate protection close to sensitive equipment.
SPD Selection Steps
Use the following steps as a practical checklist before selecting a surge protective device.
1
Confirm the System Type
Different systems require different SPD structures, voltage ratings and standards.
AC Power System
Use AC SPD for low-voltage distribution systems, main panels and sub-panels.
Solar PV DC System
Use DC PV SPD for PV strings, combiner boxes and inverter DC input side.
Signal / Data Line
Use signal line SPD for RS485, telecom, CCTV, PoE, sensors and control lines.
2
Confirm the Installation Position
Different systems require different SPD structures, voltage ratings and standards.
Main Distribution Board
Usually requires Type 1 or Type 1+2 SPD for entrance protection.
Sub-Distribution Board
Usually requires Type 2 SPD for downstream electrical equipment protection.
Terminal Equipment
Use Type 3 or signal SPD close to sensitive electronic equipment.
3
Choose the Correct SPD Type
Type 1, Type 2 and Type 1+2 SPDs are used at different protection levels.
Type 1 SPD
Used at power entrance or high-risk locations where lightning current may occur.
Type 2 SPD
Used in sub-distribution boards and electrical cabinets for induced surge protection.
Type 1+2 SPD
Combines Type 1 and Type 2 protection for main distribution and high-risk applications.
4
Confirm Operating Voltage: Uc / Un
The SPD voltage rating must match the actual AC or DC system voltage.
AC SPD Voltage
Common Uc values include 275V, 385V and 420V AC depending on system configuration.
DC PV SPD Voltage
Common Ucpv values include 1000V DC and 1500V DC for solar PV systems.
Signal SPD Voltage
Signal SPD should match the nominal voltage and interface type of the communication line.
5
Check Discharge Current: In / Imax / Iimp
Discharge current capacity should be selected according to lightning risk and installation level.
Iimp
Impulse current. Important for Type 1 SPD and direct lightning current applications.
Imax
Maximum discharge current. Indicates the maximum surge current the SPD can withstand.
In
Nominal discharge current. Commonly used for Type 2 SPD performance reference.
6
Check Protection Level: Up
Lower Up value generally means lower residual voltage reaching protected equipment.
Equipment Sensitivity
Sensitive electronics need lower residual voltage and coordinated protection.
Protection Coordination
Use multi-level SPD protection from main panel to equipment side when needed.
Installation Quality
Short wiring, correct grounding and proper installation affect actual protection performance.
SPD Selection Reference by Application
Use this table as a quick reference. For final product selection, confirm actual project voltage, wiring method and installation conditions.
| Application | Installation Position | Recommended SPD | Key Parameters | Related Standard |
|---|---|---|---|---|
| Low-Voltage AC Distribution | Main Distribution Board | Type 1 / Type 1+2 AC SPD | Uc, Iimp, In, Imax, Up, Pole Configuration | IEC 61643-11 |
| Sub-Distribution Panel | Branch Panel, Electrical Cabinet | Type 2 AC SPD | Uc, In, Imax, Up, 1P / 2P / 3P / 4P | IEC 61643-11 |
| Solar PV System | Combiner Box, Inverter DC Input | DC Solar PV SPD | Uc, In, Imax, Type 1+2 / Type 2, DC 1000V / 1500V | IEC 61643-31 |
| Industrial Automation | PLC Cabinet, Sensor Line, Control Line | Signal / Control Line SPD | Signal Voltage, Interface Type, Response Time, Residual Voltage | IEC 61643-21 |
| CCTV and Security System | Camera Power Line, PoE Line, Video Line | CCTV / PoE SPD | Power Voltage, Connector Type, Data Rate, Protection Level | IEC 61643-21 |
Common Mistakes When Selecting SPD
Avoid these common selection mistakes before confirming the final SPD model.
Mistake 01
Using AC SPD on DC PV Circuits
Solar PV systems require DC-rated SPD with suitable Ucpv and DC disconnection structure.
Mistake 02
Selecting Uc Too Low
If Uc is lower than the actual operating voltage, the SPD may age quickly or fail.
Mistake 03
Ignoring Signal Lines
RS485, CCTV, PoE and telecom lines can also be damaged by transient surges.
Mistake 04
Only Checking Imax
SPD selection should also consider In, Iimp, Up, installation position and coordination.
Mistake 05
Poor Grounding Connection
Even a suitable SPD cannot perform well without proper earthing and short connection wiring.
Mistake 06
No Coordination Between Levels
Main panel, sub-panel and terminal equipment may need coordinated multi-level protection.
Choose SPD by Product Category
After confirming your system type and installation position, browse the related product category.
AC Power Protection
AC Power SPD
For low-voltage AC distribution systems, main panels and sub-panels.
Type 1 / Type 2
Common Uc: 385V / 420V / 440V AC
IEC 61643-11 reference
AC Power Protection
Selecting Uc Too Low
If Uc is lower than the actual operating voltage, the SPD may age quickly or fail.
AC Power Protection
Type 1 / Type 2
Common Uc: 385V / 420V / 440V AC
AC Power Protection
Ignoring Signal Lines
RS485, CCTV, PoE and telecom lines can also be damaged by transient surges.
AC Power Protection
Type 1 / Type 2
Common Uc: 385V / 420V / 440V AC
Not Sure Which SPD to Choose?
Send us your system parameters and installation position. We will help recommend suitable SPD product categories or models.
System type: AC, DC PV, signal line or telecom
Operating voltage: AC voltage, DC voltage or signal voltage
Installation position: main panel, sub-panel, inverter side or equipment side
Grounding system and wiring method if available
Quantity requirement or project background
FAQ
What information is needed for automation SPD selection?
You need to provide power voltage, signal type, communication protocol, cable length, installation environment, grounding condition, and protected equipment type.
Where should SPD be installed in industrial control systems?
SPD should be installed at power input points, cabinet cable entry points, communication interfaces, sensor line entrances, and near sensitive control equipment.
What type of SPD is used for RS485 communication?
RS485 systems require signal SPD designed for low-voltage communication lines. The SPD should match the working voltage, number of wires, transmission speed, and communication structure.
Can surge protection protect PLC systems?
Yes. Properly selected SPD can help protect PLC power supplies, communication modules, I/O modules, and connected field devices from surge damage.
What parts of an automation system need SPD?
SPD may be needed for AC power supply lines, DC power circuits, PLC input/output lines, RS485 communication lines, sensor lines, Ethernet networks, and control cabinets.
Why do industrial automation systems need surge protection?
Industrial automation systems include PLCs, sensors, control cabinets, communication networks, and power supplies. These components are sensitive to transient overvoltage caused by switching operations, lightning-induced surges, and electrical interference.
Can CRESIN provide SPD for PoE security systems?
Yes. CRESIN can provide Ethernet and PoE surge protective devices for IP cameras, network switches, and security monitoring systems.
Where should CCTV SPD be installed?
SPD should be installed near outdoor cameras, at cable entry points, near network switches, and close to protected monitoring equipment when necessary. When choosing CCTV SPD, both power and signal lightning protection must be taken into consideration simultaneously.
Do analog CCTV systems need different SPD?
Yes. Analog CCTV systems may require video signal SPD, power SPD, or control line SPD depending on the wiring structure.
What type of SPD is used for IP cameras?
IP cameras usually require Ethernet SPD, especially when installed outdoors or connected through long network cables. If PoE is used, the SPD should support PoE transmission.
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