Created by: Glen Zhu | Updated Date: March 15th, 2025
In power systems and electronic equipment, surge protective devices (SPD) are the core defense against transient overvoltages.
According to the IEC 61643 standard, SPDs are divided into Type 1, Type 2, and Type 3. Among them, Type 2 surge protection devices have become the “invisible guardians” in the industrial, construction, and data center fields due to their wide range of applications and efficient energy dissipation capabilities. However, many engineers still have misconceptions about their working principles, selection logic, and maintenance methods.
This article will combine international standards with practical experience to provide an in-depth analysis of the core knowledge points of Type 2 surge protection devices.
Type 2 surge protective device (SPD) is a voltage-limiting device designed based on the transient overvoltage protection grading theory. Its core task is to release the energy of transient overvoltage generated by indirect lightning strikes or switch operations while limiting the residual voltage within the tolerance range of the protected equipment.
Indirect lightning strike: refers to the high-energy surge (typical waveform is 8/20μs) generated in the circuit through electromagnetic induction or resistive coupling when lightning does not directly hit the line.
Switching operation: Transient overvoltage caused by operations such as circuit breaker opening and closing, motor starting and stopping, capacitor switching, etc. (waveforms are mostly ringing waves or fast pulses).
According to the International Electrotechnical Commission (IEC) three-level protection model, Type 2 SPD belongs to the second level (intermediate protection):
1. Level 1 (Type 1 SPD): Installed at the building entrance, directly discharges direct lightning current (10/350μs waveform), withstands energy up to 100kA.
Type 1 SPDs are designed to handle the most severe surge conditions, making them essential for buildings with external lightning protection systems or overhead power supply lines.
They are typically equipped with spark gaps or high-energy varistors to safely divert extreme surges to the ground, preventing damage to downstream electrical installations.
2. Level 2 (Type 2 SPD): Located in main distribution boards or sub-distribution boards, handles residual energy not completely discharged by level 1, and further reduces residual voltage.
Type 2 SPDs are designed to protect electrical installations from transient overvoltages caused by indirect lightning strikes and switching operations.
They primarily use metal oxide varistors (MOVs) or gas discharge tubes (GDTs) to absorb and dissipate surges, ensuring that the residual voltage remains within safe limits for connected equipment.
3. Level 3 (Type 3 SPD): Near terminal equipment, used for fine protection (such as signal ports), with minimal energy discharge (usually <5kA).
Type 3 SPDs are typically installed close to sensitive terminal equipment, such as communication devices, computers, and other electronic systems, to provide fine protection against residual surges.
These devices are designed to handle low-energy transients that may still exist after the primary protection stages (Type 1 and Type 2 SPDs).
Key difference:
Waveform difference:
Direct lightning strike (10/350μs): high energy, long duration, requires Type 1 SPD handling.
Iimp 10/350 µs Waveform of Type 1 Surge Protective Device
Inductive lightning (8/20μs): moderate energy, requires Type 2 SPD discharge.
Imax 8/20 µs Waveform of Type 2 Surge Protective Device SPD
Energy distribution:
Type 1 releases over 80% of the total energy, Type 2 handles the remaining 20%, and Type 3 handles less than 5%.
Using IEC 61643-11 as an example, Type 2 SPDs must meet the following key test requirements:
Impulse the SPD with an 8/20μs waveform 20 times (with a 60-second interval), and after each impulse, the change in voltage protection level (Up) of the SPD must be ≤10%.
Apply a single Imax value (such as 40kA) of an 8/20μs impulse, and after testing, the leakage current of the SPD must remain stable within normal range.
Thermal stability test:
Simulate MOV aging failure, and the thermal disconnect device of the SPD must cut off the circuit within 30 seconds to prevent fire.
The core components of Type 2 SPD are metal oxide varistors (MOV) and gas discharge tubes (GDT), which operate in three stages:
Metal Oxide Varistor (MOV)
Gas Discharge Tube (GDT)
1. Normal state:
2. Surge invasion stage:
When transient overvoltage exceeds the MOV threshold, MOV switches to low impedance state within nanoseconds, diverting surge current to PE grounding line and limiting residual voltage (Up) within equipment tolerance range through clamping effect.
3. Energy dissipation and self-recovery:
Gas discharge tube (GDT) serves as a backup protection, providing additional discharge path when MOV fails.
Microscopic mechanism of Metal Oxide Varistor (MOV):
Synergy of Gas Discharge Tube (GDT):
1. Industrial facilities:
Special requirements in industrial settings:
For example, when installing a Type 2 SPD at the front end of a variable frequency drive, considerations include:
2. Commercial buildings:
3. Data centers and communication base stations:
Precise protection in data centers:
Type 2 SPDs for server racks should meet the following criteria:
Precautions:
1. Nominal discharge current (In):
Definition: The value of the surge current that SPD can continuously discharge 20 times in 8/20μs (such as 20kA).
Selection recommendation: Choose according to the lightning risk level at the installation location, usually requiring In≥20kA in medium-risk areas.
2. Maximum discharge current (Imax):
The maximum surge current that an SPD can withstand in a single event (such as 40kA), reflecting its ultimate discharge capacity.
3. Voltage protection level (Up):
Must be lower than the withstand voltage of the protected equipment (such as Up≤1.5kV for servers).
4. Continuous operating voltage (Uc):
Needs to be higher than the system’s maximum continuous operating voltage (such as choosing Uc≥440V for a 380V three-phase system).
5. Response time (tA):
High-quality SPD should be ≤25ns, ensuring action at the beginning of a surge voltage rise.
6. Failure indication and remote signaling function:
Visual indicator lights or dry contact signal for easy operation and monitoring.
7. Certification and brand:
Preferably choose products certified by TUV, such as LSP.
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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