A surge protective device (SPD) protects electrical systems and equipment from surge events by limiting transient voltages and diverting surge currents.
Maximum continuous operating voltage (Uc)
Lightning impulse current (Iimp)
Nominal discharge current (I)
Voltage protection level (Up)
Short circuit withstand capability (Isccr)
Freewheeling interruption capability at Uc (Ifi)
Transient overvoltage (TOV)
These parameters determine the SPD’s performance under different fault conditions and must be matched carefully to the system’s voltage, installation point, and expected surge environment.
SPDs are generally classified based on internal working principles and standard testing methods.
The two main types of SPDs are:
Voltage limiting components
Voltage switching components
Most modern SPDs incorporate both component types to combine the advantages of each and reduce their respective weaknesses.
Voltage limiting components include:
Metal Oxide Varistors (MOVs)
Transient Voltage Suppression (TVS) diodes
Voltage switching components include:
Gas Discharge Tubes (GDTs)
Spark gaps
These components differ in response speed, energy-handling capacity, and aging characteristics.
According to ANSI/IEEE C62.41, IEC 61643-11, and VDE Classification, there are three standard SPD types based on their tested waveform and application level.
Tested with impulse discharge current Iimp (typically 10/350 µs)
Also tested with 8/20 µs current impulses
Designed for installation at the service entrance
Protects against direct lightning current and high-energy surges
Tested with nominal discharge current in (8/20 µs)
Optionally tested with maximum discharge current Imax (8/20 µs)
Note: Imax is not recommended as a basis for SPD selection
For SPDs with voltage switching components, also tested with 1.2/50 µs voltage impulses.
Installed at sub-distribution boards or equipment inputs
Protects against switching surges and indirect lightning
Tested with a combination wave generator:
Open-circuit voltage Uoc (1.2/50 µs)
Short-circuit current Icw (8/20 µs)
Nominal output impedance: 2 Ω
Installed near sensitive terminal equipment for fine protection
SPDs are used in a wide range of AC and DC systems, each having specific design and selection requirements.
In AC distribution systems, SPDs are mainly used to protect equipment and power networks from transient overvoltages caused by lightning strikes, utility switching, and fault-clearing events.
Type 1 SPDs: Installed at main distribution boards to handle high-energy surges
Type 2 SPDs: Installed downstream for local protection
SPDs for AC must match the system’s nominal voltage and Uc while offering sufficient short-circuit withstand capability (Isccr).
Typical internal components include:
MOVs for fast response and voltage clamping
GDTs for high energy handling and isolation from leakage
The combination of both ensures fast suppression and longevity.
DC systems such as:
Telecom base stations
Industrial control cabinets
Battery storage
EV charging
Solar DC buses
…require SPDs designed specifically for continuous direct voltage, which lacks zero crossing and can sustain arcs.
DC SPDs:
Typically use high-voltage MOVs
Must handle high steady-state DC voltage without overheating
Installed at DC panels, inverter inputs, or battery banks
Help avoid system downtime due to surges or switching noise
PV systems involve high DC voltages (up to 1500 VDC), large array surfaces, and frequent exposure to lightning surges due to rooftop or field mounting.
PV-specific SPDs must comply with:
They are deployed at:
DC side: Between the PV array and the inverter
Inverter input: For internal DC bus protection
AC side: Between the inverter and the grid interface
Requirements include:
High MCOV (Uc)
Low protection level (Up)
Long surge life
Weather and UV resistance (for outdoor DC boxes)
Well-placed PV SPDs reduce equipment replacement costs and improve system reliability and ROI.
Characteristic of Type 1 SPD
Simulates a direct lightning strike
Tested with 10/350 µs waveform
Represents the SPD’s capacity to handle a single extreme surge
Suitable for building entry points or lightning arrestors downstream
Characteristic of Type 2 SPD
Simulates multiple lower-energy surges from switching or indirect lightning
Tested with 8/20 µs waveform
Reflects long-term surge endurance
Suitable for distribution boards and equipment-level protection
In summary:
Iimp = Peak endurance for rare, high-energy events
Imax = Repetitive endurance for frequent lower-energy transients
Both are essential in building a multi-level SPD protection strategy.
Surge protective devices are essential for modern power systems, ensuring equipment safety and power continuity. Their classification into Type 1, 2, and 3, along with the understanding of technical parameters like Uc, Up, Iimp, and Imax, is vital for proper selection and implementation.
Whether protecting AC systems, DC automation, or solar PV, selecting the right SPD with suitable ratings helps reduce damage, lower O&M costs, and extend system life.
A well-engineered SPD plan is a small investment that protects your large-scale infrastructure from irreversible damage.
Optimized by Seraphinite Accelerator
