設置場所： Building service entrance, main low-voltage switchboard, and the boundary between LPZ 0 and LPZ 1.
Main Function: Discharges high-energy lightning currents that may enter the electrical installation through the incoming power supply.
Key Parameters: Tested with a 10/350 μs waveform. Common Iimp ratings include 12.5 kA, 25 kA, and higher values. The voltage protection level, Up, is commonly around 2.5 kV, depending on the product design.
Typical Applications: Industrial plants, outdoor facilities, large buildings, and installations with external lightning protection systems or a high risk of direct lightning currents.
A Type 1 SPD is mainly intended to handle high lightning current energy. Additional downstream Type 2 or Type 3 protection may still be required to reduce the residual voltage reaching sensitive equipment.
Type 2 SPD – Distribution-Level Surge Protection
設置場所： Main distribution boards, sub-distribution boards, workshop power panels, and industrial control cabinets.
Main Function: Protects against induced lightning surges, switching transients, and residual surge energy passing through an upstream Type 1 SPD.
Key Parameters: Tested with an 8/20 μs waveform. Common ratings include In of 20 kA and Imax of 40–80 kA. The voltage protection level, Up, is commonly between 1.5 and 1.8 kV.
Typical Applications: Residential buildings, commercial facilities, and general industrial power distribution systems.
Type 2 SPDs protect electrical systems against induced lightning surges, switching transients, and residual surge energy passing through an upstream Type 1 SPD.
Type 3 SPD – Final Protection for Sensitive Equipment
設置場所： Close to PLCs, variable frequency drives, servers, instruments, socket outlets, and other sensitive terminal equipment.
Main Function: Further limits residual overvoltage and provides final-stage protection for sensitive electronic equipment.
Key Parameters: Tested with a combination wave. The main parameters are Uoc and Up, although some products may also specify a nominal discharge current.
Typical Applications: Automation systems, communication equipment, servers, measurement instruments, and other sensitive electronic loads.
Type 3 SPDs have limited energy-handling capability and should normally be coordinated with an upstream Type 1, Type 2, or Type 1+2 SPD.
Type 1+2 Combined SPD – Lightning Current and Surge Protection
設置場所： Building service entrances, main switchboards, solar PV AC distribution panels, compact distribution boards, and retrofit installations.
Main Function: Combines lightning-current discharge capability and surge-voltage limitation in a single device.
Key Parameters: Iimp, In, Imax, and Up should all be considered. Common Iimp ratings include 12.5 kA and 25 kA per pole, while In is commonly 20 kA or higher. Final values should be confirmed according to the product specifications and project requirements.
Typical Applications: Industrial facilities, commercial buildings, solar PV systems, compact switchboards, and projects where installation space is limited.
A Type 1+2 SPD meets both Type 1 and Type 2 protection requirements in a single device. It combines high-energy lightning-current discharge capability with voltage-limiting protection.
SPD Selection Guide by Application
Application Condition
Recommended SPD Type
Typical Installation Location
主要パラメータ
Building with an external lightning protection system
Type 1 or Type 1+2
Service entrance or main switchboard
Iimp、Up
General residential or commercial distribution system
タイプ2
主配電盤または分電盤
In、Imax、Up
Sensitive electronic equipment
Type 3
保護対象機器の近く
Uoc、Up
Industrial facility with high lightning exposure
Type 1+2 with downstream Type 2 protection
Main switchboard and sub-distribution boards
Iimp, In, Up
Solar PV DC system
DC Type 1+2 or DC Type 2
PV combiner box or DC distribution board
Ucpv, Iimp or In, Up
Sensitive loads located far from the main distribution board
Additional Type 2 or Type 3
Near the downstream equipment
Up, coordination
ステップ3：Iimp、In、Imaxを決定する
The discharge current rating of an SPD should be selected according to the expected surge energy, installation location, lightning exposure, and SPD type. The three main current parameters are Iimp, In, and Imax.
Iimp – Impulse Discharge Current
Iimp represents the lightning impulse current that a Type 1 or Type 1+2 SPD can discharge under a 10/350 μs waveform. It is mainly used to evaluate the SPD’s ability to withstand high-energy lightning currents at the service entrance or LPZ 0–LPZ 1 boundary.
Common In ratings include:
10 kA for light-duty terminal protection
20 kA for general residential and commercial distribution
30–40 kA or higher for industrial and high-exposure installations
For most main and sub-distribution boards, an In rating of 20 kA or higher is commonly selected.
Imax – Maximum Discharge Current
Imax represents the maximum 8/20 μs surge current that an SPD can discharge once or a limited number of times without failing. Imax is normally higher than In, but it should not be used as the only selection criterion because it does not represent the SPD’s repeated operating capability.
Typical Imax ratings include:
20–40 kA for general distribution systems
40–80 kA for industrial installations
Higher values for severe surge environments
Practical Selection Guide
Installation Condition
Recommended Parameter Focus
Building service entrance with lightning protection system
Prioritize Iimp
主配電盤または分電盤
Prioritize In and Imax
High lightning exposure industrial facility
Higher Iimp and In
Sensitive terminal equipment
Lower Up is usually more important than a very high current rating
タイプ1+2 SPD
Check Iimp, In, Imax, and Up together
エンジニアリングノート A higher discharge current rating does not automatically mean better protection. SPD selection should balance discharge capacity, voltage protection level, installation location, backup protection, and energy coordination. An SPD with a very high Imax but an unsuitable Up or Uc may still fail to provide effective protection.
ステップ4：電圧保護レベルUpを確認する
The voltage protection level, Up, indicates the SPD’s ability to limit surge voltage under specified test conditions. It is a declared value provided by the manufacturer and represents the protection level achieved when the SPD discharges the specified impulse current. A lower Up generally means that less surge voltage is allowed to pass toward downstream equipment. However, Up should not be evaluated alone. It must be considered together with the system voltage, maximum continuous operating voltage Uc, discharge-current capacity, temporary overvoltage withstand capability, and the impulse withstand voltage of the protected equipment.
Compare Up with the Equipment Impulse Withstand Voltage
The selected SPD should limit the surge voltage to a level below the impulse withstand voltage of the downstream equipment. A practical selection principle is:
The effective protection level at the equipment should remain below the equipment’s rated impulse withstand voltage.
A sufficient safety margin should be allowed because the actual voltage reaching the equipment may be higher than the Up value stated in the datasheet.
Consider Installation Cable Length
The Up value is measured at the SPD terminals under specified laboratory conditions. In an actual installation, the inductance of the connecting conductors can add extra voltage during surge discharge.
Therefore:
Keep the connecting conductors as short and direct as possible.
Avoid unnecessary loops and sharp bends.
Position the SPD close to the protected circuit.
Install additional downstream protection when the protected equipment is far from the main SPD.
Long connecting cables can reduce the effectiveness of even a low-Up SPD. Correct installation is therefore just as important as product selection. Phoenix Contact also notes that excessively long connection conductors can create installation problems and recommends attention to cable length and layout.
Practical Selection Focus
Protection Position
Main Selection Focus
Building service entrance
Balance Iimp and Up
主配電盤または分電盤
Check In, Imax, and Up
Sensitive electronic equipment
Select a sufficiently low Up and install the SPD close to the load
Type 1+2 combined SPD
Check Iimp, In, and Up together
Long distance between distribution boards
Consider an additional downstream SPD
エンジニアリングノート： Do not select an SPD based only on the lowest Up value. An SPD with a low Up but insufficient discharge capacity may be damaged by the expected surge current. Conversely, an SPD with high discharge capacity but an excessively high Up may not adequately protect sensitive equipment. The final selection should balance: Uc + Up + Iimp/In/Imax + equipment withstand voltage + installation distance
ステップ5：極数、断路装置、遠隔信号、バックアップ保護を確認する
After selecting the SPD type and electrical ratings, confirm that the device matches the system configuration and installation requirements. The number of poles, internal disconnection function, remote signaling option, and backup protective device all affect the safety and reliability of the installation.
Confirm the Number of Poles
The SPD pole configuration should match the power system, conductor arrangement, and protection mode.
電力システム
Common SPD Configuration
Single-phase L–N system
1P+N or 2P
Three-phase three-wire system
3P
Three-phase four-wire system
3P+N or 4P
TN-C system
Protection between phase conductors and PEN
TN-S or TN-C-S system
Protection between L–N and N–PE, depending on the selected circuit
TT system
Commonly uses a 3+1 or 1+1 circuit
DC or PV system
Select the pole configuration according to system voltage, polarity, and grounding arrangement
The number of poles alone is not enough to determine suitability. The protection circuit must also match the system earthing arrangement and the voltage that may appear between conductors.
Check the Disconnection Device
Most SPDs include an internal thermal disconnection device that separates the protective component from the power system when it reaches the end of its service life or overheats. The disconnection device helps reduce the risk of:
Thermal runaway
Overheating
Short circuit
Fire caused by a failed MOV or other protective component
The SPD should also provide a clear status indicator so that maintenance personnel can identify when the protection module needs replacement.
Determine Whether Remote Signaling Is Required
A remote signaling contact allows the SPD operating status to be monitored from a control panel, alarm system, PLC, or building management system.
Remote signaling is particularly useful in:
Unattended substations
Industrial production lines
Data centers
Telecom facilities
Solar PV plants
Critical power systems
When remote monitoring is required, check the contact type, terminal arrangement, rated voltage, and rated current of the signaling contact.
Select the Backup Protective Device
An SPD may require an upstream fuse or circuit breaker to protect the circuit if the SPD fails or develops an internal short circuit.
The backup protective device should be selected according to:
The manufacturer’s specified maximum backup fuse or circuit breaker
The available short-circuit current at the installation point
The SPD’s short-circuit current rating
The type and cross-sectional area of the connecting conductors
Coordination with the upstream protective device
Do not select the backup fuse or circuit breaker only according to the SPD discharge-current rating. Follow the manufacturer’s coordination table and installation instructions.
Practical Selection Guide
Selection Item
Main Point to Confirm
Number of poles
Match the phase conductors, neutral conductor, and earthing system
Disconnection device
Confirm internal thermal disconnection and visible status indication
Remote signaling
Confirm whether remote alarm or system monitoring is required
Backup protection
Follow the manufacturer’s recommended fuse or circuit-breaker rating
Replaceable module
Check whether the protection cartridge can be replaced without changing the base
Short-circuit rating
Confirm suitability for the prospective short-circuit current
エンジニアリングノート An SPD with the correct Uc, Up, and discharge-current rating may still be unsuitable if the pole configuration or backup protection is incorrect. Always verify the complete installation arrangement rather than selecting the SPD only from its front-label parameters.
ステップ6：複数のSPD間のエネルギー協調を確認する
When several SPDs are installed at different levels of the electrical system, they must operate in a coordinated sequence. The upstream SPD should discharge the highest surge energy, while the downstream SPD further reduces the residual voltage before it reaches sensitive equipment. Proper energy coordination prevents a downstream SPD from being overloaded before the upstream device has diverted the main surge current.
Typical Cascaded Protection Arrangement
Protection Level
Typical SPD Type
Typical Installation Location
Main Function
First stage
Type 1 or Type 1+2
Service entrance or main switchboard
Discharge high-energy lightning currents
Second stage
タイプ2
Sub-distribution board or control cabinet
Limit residual surges and switching overvoltages
Final stage
Type 3
Close to sensitive equipment
Further reduce residual voltage
Check the Distance Between SPDs
Cable length between two SPDs affects how surge energy is shared. The inductance of the connecting cable can help create the voltage difference required for the upstream and downstream SPDs to operate in the correct sequence. However, there is no single spacing rule that applies to every SPD combination. The required distance depends on:
SPD technology and internal design
Upstream and downstream Up values
Discharge-current ratings
Cable routing and conductor length
Manufacturer-tested coordination data
Some installations use cable length to provide natural decoupling. Where the available distance is insufficient, a coordinated SPD combination or a specified decoupling element may be required.
Use Manufacturer-Verified Combinations
The safest method is to select upstream and downstream SPDs that have been tested and confirmed as an energy-coordinated combination by the manufacturer. Check the manufacturer’s documentation for:
Permitted upstream and downstream SPD combinations
Minimum separation distance
Whether an additional decoupling element is required
Maximum discharge-current capability
Backup protective device requirements
Effective voltage protection level at the load
Do not assume that two SPDs are coordinated simply because one is Type 1 and the other is Type 2.
Consider the Distance to the Protected Equipment
When sensitive equipment is located far from the main SPD, an additional downstream SPD may be necessary. Long cables can pick up new induced surges and can also increase the voltage reaching the equipment. For long distribution circuits, consider installing:
Type 2 protection in downstream distribution boards
Type 3 protection close to sensitive terminal equipment
Additional SPDs where required by the project design or manufacturer’s instructions
Practical Coordination Checklist
Item to Verify
Selection Requirement
SPD sequence
Type 1 or Type 1+2 upstream, followed by Type 2 and Type 3 where required
Energy capacity
Upstream SPD handles the highest surge energy
Voltage limitation
Downstream SPD provides a lower effective protection level
Separation distance
Follow manufacturer-tested requirements
Cable layout
Keep all SPD connections short and direct
Product compatibility
Use verified coordinated combinations where possible
Protected-equipment distance
Add downstream protection when the load is far from the main SPD
エンジニアリングノート Energy coordination is not achieved simply by installing several SPDs in series. The devices must share surge energy correctly without overloading the downstream unit. A poorly coordinated system may cause the Type 2 or Type 3 SPD to operate too early and absorb more energy than it can withstand. Final coordination should therefore follow the manufacturer’s tested combination data and installation instructions.
Where Are Surge Protective Devices Commonly Used?
Surge protective devices are widely used in residential, commercial, industrial, communication, renewable energy, transportation, and outdoor electrical systems. Their main purpose is to protect electrical and electronic equipment from transient overvoltages caused by lightning, switching operations, and power system disturbances. The required SPD type depends on the system voltage, installation location, earthing arrangement, equipment sensitivity, and expected surge exposure.
Application
Typical Equipment Protected
Common Protection Approach
Residential buildings
Household appliances, computers, security systems, and smart home devices
Type 2 at the main distribution board; Type 1 or Type 1+2 may be required for buildings with external lightning protection
Commercial buildings
Elevators, HVAC systems, fire alarms, CCTV, servers, and building management systems
Type 1 or Type 1+2 at the service entrance, followed by Type 2 protection in downstream boards
Industrial facilities
PLCs, variable-frequency drives, sensors, automation equipment, and production lines
Coordinated Type 1, Type 2, and Type 3 protection
Data centers and communication systems
Servers, UPS systems, network equipment, communication devices, and signal lines
Power-line SPDs combined with dedicated data and signal-line protection
太陽光発電システム
PV modules, combiner boxes, inverters, and AC/DC distribution circuits
DC SPDs on the PV side and AC SPDs on the inverter output side
EV charging stations
AC chargers, DC fast chargers, controllers, payment modules, and communication equipment
SPD protection at the incoming supply and charging equipment
Wind power and energy storage systems
Converters, inverters, battery management systems, control cabinets, and monitoring equipment
AC and DC SPDs selected according to the system architecture
Outdoor equipment and LED lighting
Streetlights, traffic systems, cameras, electronic signs, and outdoor control cabinets
Local SPD protection close to exposed outdoor equipment
In practice, larger or more complex systems usually require multi-stage surge protection. A high-energy SPD may be installed at the main service entrance, while additional Type 2 or Type 3 SPDs are installed closer to downstream distribution boards and sensitive equipment.
Where Should a Surge Protective Device Be Installed?
The effectiveness of a surge protective device depends not only on selecting the correct SPD type, but also on installing it at the appropriate location within the electrical system. Proper installation helps ensure that surge energy is safely diverted to earth before it reaches sensitive electrical or electronic equipment. In most low-voltage installations, SPDs are installed in stages to provide coordinated protection from the service entrance to the final load.
Typical SPD Installation Locations
設置場所
Recommended SPD Type
Primary Purpose
サービス入口
Type 1 or Type 1+2
Divert high-energy lightning currents entering the building
Main distribution board
タイプ2
Protect downstream electrical circuits from switching surges and residual lightning surges
Sub-distribution board
タイプ2
Provide additional protection for branch circuits
Close to sensitive equipment
Type 3
Reduce the remaining surge voltage before it reaches sensitive electronic equipment
PV combiner box or DC distribution board
DC Type 1+2 or DC Type 2
Protect photovoltaic DC circuits and inverters
Control cabinets and communication systems
Type 2, Type 3, or signal-line SPDs
Protect PLCs, communication equipment, and control electronics
Installation Recommendations
To achieve the best protection performance, SPDs should be installed as close as possible to the equipment or distribution point they protect. The connecting conductors should be short and direct to minimize additional inductive voltage during surge discharge. When the distance between the main SPD and the protected equipment is relatively long, an additional downstream SPD may be required to maintain an effective protection level.
When planning an SPD installation, the following principles should always be considered:
Install the appropriate SPD type at each protection level.
Keep all connecting conductors as short and straight as possible.
Ensure reliable bonding to the earthing system.
Follow the manufacturer’s recommendations for backup protection and coordination.
Use coordinated multi-stage protection for large or complex electrical installations.
Common SPD Installation Mistakes
Even a correctly selected surge protective device may fail to provide effective protection if it is installed incorrectly. Common installation mistakes can increase the residual voltage, reduce the SPD’s service life, or prevent the surge current from being safely diverted.The following issues should be checked during installation and commissioning.
Common Mistake
Why It Is a Problem
Recommended Practice
Installing the wrong SPD type
A Type 2 or Type 3 SPD may not be able to withstand the surge energy expected at the service entrance
Match the SPD type to the installation location and surge exposure
Selecting an incorrect Uc
A Uc value that is too low may cause premature operation or damage; a value that is too high may reduce protection effectiveness
Select Uc according to the system voltage and earthing arrangement
Using excessively long connecting conductors
Long conductors add inductive voltage during surge discharge and increase the effective protection level
Keep the connection path short, direct, and free from unnecessary loops
Poor earthing or equipotential bonding
A high-impedance earth path can prevent surge current from being discharged effectively
Ensure reliable earthing and equipotential bonding
Ignoring backup protection
An internal SPD fault may not be safely disconnected from the power system
Install the fuse or circuit breaker specified by the manufacturer
Installing only one SPD in a large system
A single SPD may not provide sufficient protection for distant distribution boards or sensitive loads
Use coordinated multi-stage protection where required
Using an incorrect pole or circuit configuration
The SPD may not protect all required conductors or may be unsuitable for the earthing system
Match the SPD circuit to the phase, neutral, polarity, and grounding arrangement
Failing to coordinate multiple SPDs
A downstream SPD may absorb more surge energy than it can withstand
Use manufacturer-verified coordinated combinations
Installing the SPD too far from the protected equipment
Long downstream cables may allow additional induced surges or increase the voltage reaching the equipment
Add downstream Type 2 or Type 3 protection when necessary
Failing to inspect or replace a failed SPD
Once the SPD reaches end of life, the circuit may no longer be protected
Check the status indicator and replace failed modules promptly
To avoid these mistakes:
Select the SPD according to the system voltage, earthing arrangement, and installation location.
Keep the total connection path as short and direct as practical.
Use the correct backup protective device.
Ensure reliable earthing and equipotential bonding.
Verify coordination between upstream and downstream SPDs.
Inspect the SPD status indicator during routine maintenance.
エンジニアリングノート The SPD itself is only one part of the surge protection system. Its actual performance also depends on conductor routing, earthing quality, backup protection, installation distance, and coordination with other SPDs. A high-performance SPD can still provide poor protection if the installation layout is incorrect.
Frequently Asked Questions About Surge Protective Devices
Can an SPD Be Used Without a Backup Fuse or Circuit Breaker?
It depends on the SPD design and the manufacturer’s installation requirements. Some SPDs require an external backup fuse or circuit breaker, while others may include integrated overcurrent protection. The backup protective device is used to disconnect the SPD safely if it develops an internal short circuit or reaches the end of its service life. Its rating should not be selected only according to the SPD’s discharge-current rating. Always follow the manufacturer’s specified maximum backup fuse or circuit-breaker rating, short-circuit current requirements, and coordination instructions.
How Should a Surge Protective Device Be Maintained?
SPD maintenance mainly involves regular inspection rather than internal repair. During routine electrical maintenance, check the status indicator, remote signaling contact, terminal tightness, signs of overheating, and the condition of the backup protective device. The SPD should also be inspected after severe thunderstorms, lightning events, major power-system faults, or repeated tripping of the upstream protective device. Replace the SPD or plug-in protection module if:
The status indicator shows failure.
A remote alarm is activated.
The enclosure is discolored, cracked, or overheated.
The backup fuse or circuit breaker has operated because of an SPD fault.
The manufacturer’s replacement criteria have been reached.
How Long Does a Surge Protective Device Last?
An SPD does not have a fixed service life. Its lifespan depends on the number, magnitude, and duration of the surges it absorbs, as well as system voltage, temporary overvoltages, environmental conditions, and product quality. An SPD may remain operational for many years in a low-surge environment, but a single severe surge can significantly reduce its remaining capacity or cause immediate failure. The status indicator and remote signaling contact should therefore be used to determine whether the SPD is still operational. Replacement should be based on actual condition and manufacturer guidance rather than a fixed number of years alone.
Do I Need Both AC and DC Surge Protection?
Both AC and DC surge protection may be required when a system contains separate AC and DC circuits. An AC SPD cannot automatically replace a DC SPD because the two devices are designed for different voltage characteristics, arc-extinguishing requirements, and system configurations. For example, a solar PV system may require:
A DC SPD on the PV array, combiner box, or inverter DC input.
An AC SPD on the inverter output or AC distribution board.
Energy storage systems, EV charging systems, and industrial DC control circuits may also require separate AC and DC protection. The selected SPD must be rated for the actual system voltage, polarity, grounding arrangement, and maximum continuous operating voltage.
Can a Damaged SPD Continue Protecting Equipment?
A damaged SPD should not be assumed to provide effective protection. Some SPDs contain an internal disconnection device that isolates the failed protection component while allowing the electrical circuit to remain energized. In this condition, the connected equipment may continue operating, but surge protection may already be partially or completely lost. If the status indicator shows failure or the remote alarm is activated, replace the SPD or protection module as soon as possible. Continuing to operate without replacement may leave downstream equipment unprotected.
Are SPDs Required for Solar PV Systems?
SPD requirements for solar PV systems depend on the applicable electrical standard, lightning protection design, installation risk, cable routing, and local regulations. PV systems are particularly exposed to induced surges because modules and DC cables are usually installed outdoors and may cover a large area. SPDs are commonly installed on the DC side, AC side, or both. Typical locations include:
PV combiner boxes
Inverter DC inputs
Inverter AC outputs
Main AC distribution boards
The required SPD type depends on whether the building has an external lightning protection system, the separation distance, the system voltage, and the lightning exposure level. Therefore, SPDs are not selected simply because the system is photovoltaic. The need, type, and installation position should be determined by the applicable project requirements and risk assessment.
LOOKING FOR A SUITABLE SPD FOR YOUR PROJECT?
After understanding the different SPD types, selection parameters, installation locations, and coordination requirements, you may also explore our complete range of surge protective devices for AC and DC applications. Visit our Surge Protective Devices (SPD) product category to compare available models, voltage ratings, discharge current levels, pole configurations, and application options.
ABOUT THIS GUIDE
This guide was prepared to help engineers, panel builders, electrical contractors, system integrators, and industrial buyers better understand surge protective devices (SPDs), their operating principles, selection criteria, installation practices, and common applications.
Electrical Engineer | Low-Voltage Power Distribution
Hello, I’m Evan.
I am an electrical engineer with 10 years of experience in low-voltage electrical equipment, circuit protection, and power distribution systems. I specialize in product selection, application engineering, and technical support for industrial, commercial, and renewable-energy projects.
For technical inquiries, please contact me at evan@jutrion.com.
