A single overvoltage spike can shut down a production line, corrupt a PLC program, or quietly weaken power supplies until failures start appearing weeks later. Lightning induced surges and the switching of large electrical loads are common triggers, and a standard circuit breaker is not built to clamp these ultra fast, high energy transients. A surge protective device solves a different problem: the SPD reacts in microseconds and diverts excess surge energy to earth so the downstream distribution board, service entrance equipment, and sensitive loads see a much lower stress level. This article explains what an SPD is, how it works, how to read the key ratings that matter in procurement specs, how Type 1, Type 2, and Type 3 devices differ, and which installation practices most directly affect real world protection.
What Causes Power Surges and Overvoltage?
If you trace surge events in real facilities, most fall into two sources. The first is the lightning surge. A direct strike to an overhead line, rooftop equipment, or nearby ground can inject high energy into the electrical network. Even when lightning does not hit the building, a nearby strike can induce voltage through electromagnetic coupling and ground potential rise, pushing a transient overvoltage onto incoming feeders and long cable runs.
The second source is the switching surge, created inside the installation during normal operation. Motor starts and stops, transformer energization, capacitor bank switching, welding equipment, and elevator drives can generate steep voltage steps. These spikes travel through distribution boards and control wiring, then show up as damaged power supplies, communication faults, nuisance trips, corrupted control signals, or intermittent sensor errors that are difficult to troubleshoot. Over time, repeated surges accelerate insulation aging and raise the risk of arc related damage and fire.
Because surges can enter from outside and also be generated inside your own switchgear, protection is usually staged. The next sections explain where to place SPDs at the service entrance, at downstream distribution points, and near sensitive loads for coordinated, system level protection.
SPD Components and Working Principle
Key Components
Metal Oxide Varistor (MOV): The primary energy handling element. Under normal voltage it stays high resistance, but during a surge it becomes highly conductive, functioning as a MOV surge protector that creates a low impedance path to divert surge current.
Thermal disconnect and or fuse coordination: Prevents a damaged or overheated MOV from remaining connected under sustained stress. This backup protection isolates the MOV to avoid internal overheating and secondary faults in the switchboard.
Status indicator and optional remote signal contact: Enables quick condition checks during routine inspections. Remote signaling can feed SCADA or BMS alarms, so maintenance teams can replace the module before it turns into downtime.
How an SPD Works
An SPD is a parallel connected protective device. It does not sit in series with the load, so it normally carries almost no current. When a surge drives the line voltage above the device threshold, the internal MOV switches into conduction in microseconds and provides a controlled path to shunt to ground through PE or earth. That diversion reduces the voltage seen by downstream equipment and limits insulation stress in cables and control circuits. After the transient passes, the MOV returns to a high resistance state. If the event exceeds the device capability or repeated stress has aged the MOV, the internal thermal disconnect will open so the unit exits service safely and signals that replacement is required. Modular industrial SPDs, including offerings from suppliers such as Weidmüller, often make this replacement process faster by using pluggable protection modules.
Key SPD Ratings and Specifications Explained
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Rating
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Full name
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Test waveform
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What it represents
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How to use it in selection
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In
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Nominal discharge current
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8/20 µs
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Peak 8/20 surge current the SPD conducts in UL testing and must still function after repeated surges.
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Use In to gauge durability for sites with frequent surges; higher In generally supports longer service life.
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Imax
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Maximum discharge current
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8/20 µs
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Maximum 8/20 impulse current the SPD can withstand at least once.
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Use Imax to judge headroom for severe, infrequent events at distribution level.
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Iimp
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Impulse current
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10/350 µs
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Lightning impulse discharge capability for Type 1, Class I test context on 10/350 waveform.
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Use Iimp for service entrance selection in lightning exposed environments; match to risk and lightning protection strategy.
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Uc
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Maximum continuous operating voltage (MCOV)
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Continuous
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Maximum designated RMS power frequency voltage that may be applied continuously without unacceptable degradation.
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Set Uc to cover system nominal voltage plus tolerance; undersized Uc accelerates ageing and failure risk.
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Up
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Voltage protection level
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At In
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Let through level across SPD terminals while diverting surge current under specified test conditions.
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Select Up below the withstand of downstream equipment; evaluate with wiring length since installation inductance increases residual voltage.
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VPR (optional)
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Voltage Protection Rating (UL 1449)
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6 kV, 3 kA combination wave
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Measured limiting voltage result that is rounded up to a standard value, indicating residual voltage passed to equipment.
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Use VPR for North American specs as a comparable clamping metric; conceptually aligns with the same practical goal as Up.
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Types of SPDs: Type 1 vs Type 2 vs Type 3
SPD “types” are not marketing labels. They indicate where the device is used in the electrical system and which standardized surge test it is built to withstand. In most commercial and industrial sites, surge protection works best as a coordinated chain: a high energy device at the incoming supply, a robust device in distribution boards, and a final stage close to sensitive equipment.
Type 1 SPD (Service Entrance / Main Panel)
A Type 1 SPD is installed at the service entrance or main panel where surges first enter the installation. This is the location that sees the highest surge energy, including lightning current effects when the site has external lightning exposure or overhead feeders. In IEC terminology, Type 1 corresponds to IEC 61643-11 Class I testing and is characterized by the 10/350 µs impulse waveform. If the project spec includes lightning current ratings such as Iimp, Type 1 is typically the first stage to evaluate.
Type 2 SPD (Distribution Boards)
A Type 2 SPD is the workhorse for low voltage switchboards and downstream distribution boards. It targets surges caused by indirect lightning coupling and internal events such as motor starts, capacitor switching, drives, and welding equipment. In IEC terms it aligns with Class II testing and is characterized by the 8/20 µs current waveform. Place Type 2 protection where multiple circuits branch out and where sensitive loads are fed, since this is often the best point to reduce plant wide nuisance faults and equipment stress. For AC distribution boards, Type 2 protection is typically mounted at the board incomer so surge energy is diverted before it reaches branch circuits and control power supplies. The WTSP-A40 AC Surge Protection Device 40A SPD is built for this panel level application.
Type 3 SPD (Point of Use Protection)
A Type 3 SPD is installed close to the equipment it protects, often within the same cabinet as PLCs, instrumentation, servers, or control power supplies. It has a lower discharge capacity and is intended as the final clamp when residual voltage remains after upstream stages. In IEC practice, Type 3 aligns with Class III testing using a combination wave, commonly 1.2/50 µs voltage with 8/20 µs current. In facilities with long cable runs, Type 3 protection near critical loads can materially reduce downtime from transient related faults.
How to Select the Right SPD for Your System
Use this SPD selection checklist to align the device rating with the way your electrical system is actually built and operated.
1. Confirm system basics
Start with the supply and earthing details that drive both wiring and stress levels. Confirm whether the network is single phase or three phase, the nominal voltage and frequency, and the earthing system, typically TN, TT, or IT, because the SPD connection mode and return path through PE or earth must match the installation. Application context matters as well. DC systems (PV strings, wind DC circuits, DC power supplies) need DC rated SPDs, since the operating profile differs from AC networks. If you want a concrete spec reference for this scenario, see DC Surge protection device,SPD,WTSP-D40.
2. Define placement and protection stages
Plan protection by location, not by a single datasheet number. Install a first stage at the service entrance or main panel where incoming energy is highest, add Type 2 protection in distribution boards to reduce stress across branch circuits, and apply point of use protection near sensitive loads when cable runs are long or downtime is costly. This staged layout is the practical basis of coordinated surge protection and is often the difference between theoretical and measurable performance.
3. Match ratings to risk and equipment withstand
Tie the ratings to the system limits and the surge exposure profile. Uc must cover the maximum continuous operating voltage including tolerances, otherwise the SPD can age prematurely. Up must remain below the impulse withstand level of the loads you are protecting, since Up indicates the residual voltage that can appear across the SPD during discharge. Use Imax to judge distribution level headroom for 8/20 conditions, and use Iimp when lightning current performance at the service entrance is required.
4. Align to the project standard
Set the compliance target based on where the system is built and how it will be inspected. Specify IEC 61643-11 or UL 1449 according to market and customer requirements, then verify the declared test class and ratings in the product documentation before finalizing procurement.
If you share your panel voltage, earthing type, and where the SPD will be installed, I can map this checklist to a short specification paragraph suitable for a datasheet RFQ and a “surge protection for electrical panel” bill of materials.
Conclusion
Surges come from lightning exposure and switching events, and the damage often shows up as downtime, misoperation, and premature equipment failure. A surge protective device (SPD) addresses this risk by diverting transient energy to earth, then the selection process turns that principle into results by checking Uc, Up, and the discharge current ratings, matching Type 1, Type 2, and Type 3 devices to the right locations, and keeping lead lengths and grounding quality under control. Ongoing inspection matters because surge duty accumulates and protection components eventually reach end of life, so indicators and scheduled checks should be part of routine maintenance. When these steps are applied together, electrical surge protection becomes a reliability measure rather than a reactive repair cost, improving uptime and protecting the assets that run your operation.
Post time: Dec-14-2025