Created by: Glen Zhu | Updated Date: March 11th, 2025
AC Surge Protective Device (SPD) is an electrical device used to protect electrical and electronic equipment from voltage transient overvoltage (such as surges or spikes). Surges are usually caused by lightning, grid switching, or the start-up and shutdown of large equipment, with these transient overvoltages potentially reaching several thousand volts, enough to damage sensitive electronic devices or cause system failures.
The core function of SPD is to detect and limit these overvoltages, guide them to the ground wire, thus protecting connected devices from damage. It is usually installed at key nodes in the electrical system, such as distribution boxes, equipment input terminals, etc., to ensure that surges are effectively suppressed before reaching the equipment.
Simply put, SPD is like the “fuse” of an electrical system, but it specifically targets transient overvoltage rather than overcurrent. It absorbs or transfers excess energy through fast response (usually in nanoseconds) to maintain stable operation of the system and extend the lifespan of equipment.
Surge Protective Devices (SPD) play a crucial role in modern electrical systems, with their main function being to protect equipment and infrastructure from damage caused by voltage surges. The following is a detailed description of the importance of SPD in electrical systems:
1. Preventing damage from voltage surges
Voltage surges are usually caused by lightning strikes, power grid switching, or the start-up and shutdown of large equipment. These transient overvoltages can reach up to several thousand volts, far exceeding the tolerance range of most electronic devices. SPDs can detect and quickly divert these surge energies to the ground wire, preventing them from entering the electrical system and damaging connected devices.
2. Protect sensitive equipment
3. Reduce downtime and economic losses
4. Improve safety
5. Compliance with standards and regulations
6. Protect critical infrastructure
7. Cost-effective investment
8. Adapt to modern technological requirements
9. Extend equipment lifespan
10. Support green energy systems
The core function of the surge protection device (SPD) is to detect and limit transient overvoltages (surges) in the electrical system, thereby protecting connected equipment from damage. The following is a detailed description of the basic working principle of SPD:
1. Surge voltage detection
SPD continuously monitors the voltage levels in the electrical system. When the system voltage is normal, the SPD is in a “high impedance” state and will not affect the circuit. However, when a sudden increase in voltage (surge) is detected, the SPD will respond immediately.
2. Rapid response and energy transfer
The response time of SPD is extremely short, usually in the nanosecond (ns) level. Once an overvoltage is detected, the SPD will quickly switch to a “low impedance” state, providing a low impedance path for excess energy. This excess energy will be guided to the ground wire instead of entering the connected equipment.
3. Voltage limiting function
SPD limits overvoltage to a safe range that the equipment can withstand using voltage limiting technology. For example, common SPDs use components such as metal oxide varistors (MOV) or gas discharge tubes (GDT), which do not conduct electricity under normal voltage conditions but quickly conduct when there is an overvoltage, clamping the voltage at a safe level.
4. Restore normal state
When the surge disappears and the system voltage returns to normal levels, the SPD will automatically return to a high impedance state, stop energy transfer, and continue monitoring voltage changes in the circuit.
5. Multi-level protection mechanism
In some complex electrical systems, SPD may adopt a multi-level protection mechanism. For example:
First level protection: installed in the main distribution cabinet to absorb high-energy surges from external sources (such as lightning).
Second level protection: installed in the distribution cabinet or at the front end of equipment to further limit surge voltage.
Third level protection: directly installed near sensitive equipment, providing fine protection to ensure that the equipment is not affected by minor surges.
6. Thermal protection and fault indication
7. Coordination with the grounding system
The effectiveness of SPD highly depends on a good grounding system. Excess energy needs to be safely directed to the earth through a low impedance path, so the ground resistance must meet relevant standards (usually required to be less than 10 ohms). If the grounding system is not perfect, the protective effect of SPD will be greatly reduced.
The core function of the surge protection device (SPD) relies on its internal key components, which work together to detect, limit, and transfer energy from voltage surges. Here is a detailed description of the main key components of SPD:
Metal Oxide Varistor (MOV)
Function: MOV is the most common component in AC Surge Protective Device, used to limit overvoltage. Under normal voltage, MOV presents a high impedance state, almost non-conductive; but when the voltage exceeds the set threshold, MOV will quickly switch to a low impedance state and guide excess energy to the ground wire.
Characteristics:
Gas Discharge Tube (GDT)
Function: GDT is a component based on the principle of gas discharge, used to handle high-energy surges. When the voltage exceeds a certain value, the gas inside the GDT will ionize and form a conductive channel, transferring surge energy to ground.
Transient Voltage Suppression Diode (TVS Diode)
Thermal protection device (thermal fuse)
Function: The thermal protection device is used to prevent the SPD from overheating or being damaged due to long-term exposure to high energy surges. When the temperature of the SPD is too high, the thermal fuse will disconnect the circuit to prevent fire or further damage.
Features:
Improve the safety of the SPD.
Provide additional protection in case of aging or failure of the SPD.
Fault indicator
Function: The fault indicator (usually an LED indicator light or mechanical symbol) is used to display the working status of the SPD. When the SPD fails due to aging or damage, the indicator will issue a warning, prompting the user to replace it.
Connecting terminals and wires
Shell and Insulation Materials
Multi-level protection circuit
Type 1 SPD, also known as “primary protection” or “B-level protection,” is an important type of surge protective device (SPD) mainly used at the entrance of buildings or in main distribution systems to directly protect electrical systems from external high-energy surges (such as lightning strikes). Here is a detailed description of Type 1 SPD:
Definition and Function
Definition: Type 1 SPD is a high-energy surge protection device specifically designed to handle high-energy surges generated by direct lightning strikes or inductive lightning.
Function: Its main function is to divert lightning strikes or other external high-energy surges to the ground, preventing these surges from entering the electrical system and damaging equipment.
Core components
Gas discharge tube (GDT): Type 1 SPDs typically use gas discharge tubes as core components. GDT can quickly ionize at high voltages and form a low impedance channel to safely divert surge energy into the ground.
Spark gap: Some Type 1 SPDs also use spark gap technology to transfer high-energy surges through air discharge.
High voltage withstand capability: The components of Type 1 SPD have extremely high voltage withstand capability, able to withstand surge currents of several tens of kiloamperes (kA).
Installation location
Component of lightning protection system (LPS): Type 1 SPDs are typically used in conjunction with external lightning protection devices (such as lightning rods, down conductors) to form a complete lightning protection system.
Characteristics
Application Scenarios
Building lightning protection system: Type 1 SPD is an important component of building lightning protection systems, used to protect electrical equipment inside buildings from damage caused by lightning strikes.
Industrial facilities: In industrial environments, Type 1 SPD is used to protect main distribution systems and critical equipment, preventing downtime or damage caused by lightning strikes.
Areas with frequent thunderstorms: In areas with frequent thunderstorm activity, Type 1 SPD is an essential protective device that can effectively reduce the risk of lightning strikes.
Coordination with other types of SPD
Installation and maintenance
Maintenance recommendations: Regularly check the status of Type 1 SPD, especially after thunderstorm seasons, to ensure its normal operation. If the SPD is equipped with a fault indicator, it should be replaced promptly according to the indication.
Type 2 SPD, also known as “secondary protection” or “C-class protection”, is a common type of surge protection device (SPD) used mainly at the distribution board or equipment front end to further limit surge voltage and protect terminal equipment from damage caused by medium-energy surges. Here is a detailed description of Type 2 SPD:
Definition and Function
Function: Its main function is to limit medium energy surges within a safe range, preventing damage to connected electrical and electronic devices.
Core components
Installation Location
Equipment front end: In some applications, Type 2 SPDs are directly installed at the power input of sensitive equipment (such as industrial control cabinets, server racks) to provide close-range protection.
Features
Medium energy handling capability: Type 2 SPD can handle surges caused by grid switching or equipment start-up and shutdown, suitable for protecting most commercial and industrial equipment.
Fault indication: Some Type 2 SPDs are equipped with fault indicators (such as LED indicator lights), making it convenient for users to monitor the status of the equipment.
Application Scenarios
Coordination with other types of SPD
Installation and maintenance
Maintenance suggestions: Regularly check the status of Type 2 SPD, especially after thunderstorm season, to ensure its normal operation. If the SPD is equipped with a fault indicator, it should be replaced promptly according to the indication.
Type 3 SPD, also known as “third level protection” or “D-level protection”, is a type of fine protection in surge protective devices (SPDs), mainly used to directly protect sensitive equipment and provide the last line of defense against low-energy surges. Here is a detailed description of Type 3 SPD:
Definition and Function
Core components
Installation location
Features
Extremely fast response speed: The picosecond response speed of TVS diodes allows them to quickly limit surge voltages and protect sensitive equipment.
Compact design: Type 3 SPDs are typically small in size, making them easy to install near equipment or integrate into sockets.
Plug and play: Some Type 3 SPDs are designed as plug-and-play models, requiring no professional installation and suitable for regular users.
Application Scenarios
Communication equipment: In communication base stations or data centers, Type 3 SPD is used to protect sensitive communication equipment and servers.
Coordination with other types of SPD
When choosing the appropriate surge protection device (SPD), it is necessary to consider multiple factors comprehensively to ensure that it can effectively protect the electrical system and meet specific requirements. The following are key factors that need to be carefully considered when selecting an SPD:
Definition: The voltage protection level refers to the highest value that the SPD can limit the voltage to during a surge.
Considerations:
Choose an Up value lower than the withstand voltage of the protected equipment to ensure device safety.
For sensitive devices (such as electronic devices), choose an SPD with a lower Up value.
Considerations:
For highly sensitive equipment, choose SPD with extremely short response time (such as TVS diodes with response times in picoseconds).
For general equipment, SPD with response times in nanoseconds (such as MOV) are usually sufficient.
Definition: The installation location determines the protection level and type of SPD.
Considerations:
Type 1 SPD: Installed at the entrance of a building or main distribution panel, used for high-energy surge protection.
Type 2 SPD: Installed at the distribution panel or equipment front end, used for medium-energy surge protection.
Type 3 SPD: Directly installed near sensitive equipment, used for low-energy surge protection.
Definition: Environmental conditions include factors such as temperature, humidity, dust, etc. that may affect the performance of SPD.
Considerations:
In high temperature or high humidity environments, choose an SPD with a good protection rating (such as IP65).
Definition: The quality of the grounding system directly affects the performance of SPD.
Considerations:
Ensure that the ground resistance meets standards (usually required to be less than 10 ohms).
Regularly check if the connections of the grounding system are secure to avoid SPD failure due to poor grounding.
Considerations:
Choose certified SPDs (such as IEC 61643, UL 1449, or GB 18802).
Ensure that the SPD has CE, RoHS, and other marks to meet environmental and safety requirements.
Definition: SPD needs regular maintenance and replacement to ensure its long-term effectiveness.
Considerations:
Choose an SPD with fault indicators for easy monitoring of its status.
Regularly test and replace the SPD according to the manufacturer’s recommendations, especially after thunderstorm seasons.
Choose the SPD with the best cost-effectiveness within budget.
Consider long-term costs, including maintenance, replacement, and potential equipment damage costs.
LSP’s reliable surge protection devices (SPDs) are designed to meet the protection needs of installations against lightning and surges. Contact our Experts!
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