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What Is a Surge Protective Device (SPD)? How It Works, Types and Applications

Learn how a surge protective device works, compare SPD types, read real CRESIN ratings, and understand selection, backup protection and replacement.

CRESIN · Surge protection fundamentals

A circuit breaker can stay switched on while a voltage surge damages an inverter or control board. The breaker responds to excessive current; the electronics may be exposed to a damaging voltage spike without a sustained overload.

A surge protective device (SPD) addresses that gap. This guide explains what it does, how to read its ratings and where it belongs in a protection system, using CRESIN products and drawings as examples.

What Does a Surge Protective Device Do?

An SPD limits transient overvoltage and diverts surge current through designated protection paths. Its purpose is to reduce the voltage stress reaching connected equipment during a surge.

Lightning is one source. A strike near a building can couple a surge into its wiring without hitting the building itself. Switching motors, transformers or other electrical loads can also produce transients. Power cables, outdoor communication lines and connections between buildings all deserve attention.

An SPD has limits. It does not maintain the supply during a power cut, regulate a persistently high mains voltage or replace overload and electric-shock protection. A lost neutral causing prolonged overvoltage is a different problem from a brief surge.

This basic function is described in IEC 61643-01, the general requirements and test-methods standard for low-voltage SPDs.

How Does an SPD Work?

In normal operation, a typical power SPD presents a high impedance across its protection paths. It draws little current and leaves the supply operating normally. When a surge arrives, its nonlinear components conduct, diverting surge current and limiting voltage. After a surge within its capability, it returns to its normal state.

Common components include metal-oxide varistors (MOVs), which change resistance with voltage, and gas-discharge tubes or spark gaps, which become conductive when their firing conditions are reached. Different designs combine these components for different protection duties.

CRESIN 3+1 SPD schematic showing three line-to-neutral varistor paths and a neutral-to-earth discharge path — CRESIN's CSMS-B40 3+1 schematic: three line-to-neutral paths and a neutral-to-protective-earth path. This is one configuration; other versions use a different internal circuit.

The drawing also explains why “it sends everything to earth” is an incomplete description. An SPD can limit voltage between live conductors as well as between a live conductor and protective earth. The required paths depend on the supply and earthing arrangement.

For a typical shunt-connected power SPD, the load current does not pass through the protection modules continuously. The device is connected alongside the supply. Signal protectors may have a different connection arrangement.

Type 1, Type 2 and Type 3: Different Duties

The type marking describes a tested protection duty. It is more useful than comparing the largest kA number on the front.

Type	Typical role	Test reference to recognize
Type 1	At the installation origin where the design must account for partial lightning current	I_imp, commonly a 10/350 μs current impulse
Type 2	Distribution-level protection against induced lightning and switching surges	I_n and I_max, with an 8/20 μs current impulse
Type 3	Additional protection near equipment, coordinated with upstream SPDs	Combination-wave testing: 1.2/50 μs voltage and 8/20 μs current

Common IEC SPD classifications for power circuits

A Type 1+2 device meets both test duties. It can combine functions at one location, but the need for further downstream protection still depends on the installation.

For example, CRESIN's CSMS-A15 product page lists I_imp of 15 kA at 10/350 μs. The CSMS-B40 AC model lists I_max of 40 kA at 8/20 μs. Those numbers cannot rank the two products by strength: the waveforms represent different stresses.

These descriptions use IEC terminology. Do not assume a UL 1449 type designation has exactly the same meaning; check the standard required by the project.

Match the SPD to the Circuit

An AC distribution board, a PV string and an RS485 link can all need surge protection. They cannot use the same device simply because each is exposed to lightning.

AC power SPDs are selected around supply voltage, earthing system and protection modes. In a three-phase board, the neutral arrangement matters as much as the number of phases.

CRESIN CSMS-B40 PV DC SPD with positive and negative terminals, PE connection and 670 V DC Uc marking — PV DC version: polarity and voltage markings identify its intended circuit.

A similar housing does not mean the same duty

The CSMS-B40 PV DC range has separate DC ratings and references IEC 61643-31. For PV selection, check the array's maximum open-circuit voltage under the lowest design temperature, together with the manufacturer's configuration requirements.

Signal and data SPDs require another set of checks: interface voltage, bandwidth, connection type and, where applicable, PoE compatibility. Protecting a control cabinet's power supply does not protect an outdoor data cable entering the same cabinet.

Think about the equipment's connections

Consider an outdoor control cabinet with an AC supply and an RS485 cable running to a remote sensor. An AC SPD addresses the power connection. The sensor cable needs its own assessment and, where required, a protector suitable for that interface. Both protection arrangements depend on effective bonding. Buying a larger AC SPD does not close the unprotected signal path.

How to Read an SPD Label

Start with voltage and protection mode, then examine surge-current ratings. Choosing by “40 kA” alone leaves out much of the information needed to select a device.

Marking	Meaning	What to check
U_c	Maximum continuous operating voltage	Suitability for the voltage across the relevant protection mode, including supply tolerances
U_p	Declared voltage protection level	Compatibility with equipment impulse withstand and the installed wiring arrangement
I_n	Nominal discharge current, commonly 8/20 μs	The declared current and the associated test duty
I_max	Maximum discharge current at 8/20 μs	Read alongside I_n; it is not a continuous-current rating
I_imp	Impulse discharge current for Type 1 duty	The waveform and associated charge and specific-energy requirements

The five ratings you will encounter most often

CRESIN CSMS-B40 four-pole AC SPD label showing Uc 385 V, In 20 kA, Imax 40 kA and Up no greater than 1.8 kV — CSMS-B40 AC product photograph. The version shown uses a four-pole configuration.

Read a real product label

This CSMS-B40 AC version shows U_c 385 V, I_n 20 kA, I_max 40 kA and U_p ≤1.8 kV. The current values describe impulse duties. They do not mean it can feed a 40 kA load.

A higher U_c is not automatically a better choice. It must suit the system, while U_p must still support the required equipment protection. Check temporary-overvoltage behavior separately: U_c alone does not describe performance during every abnormal supply condition.

Also establish whether a quoted current applies to one protection mode, one pole or the complete assembly. Adding the numbers printed on several cartridges can produce an impressive total without giving a meaningful comparison. Ask for ratings on the same basis and under the same test conditions.

Installation and Backup Protection Matter

The voltage at the equipment depends on more than the SPD's declared U_p. Connecting conductors add voltage during a rapidly rising surge. Short, direct connections and effective bonding therefore form part of the protection, rather than being finishing details.

Use the wiring drawing for the exact version. The 3+1 arrangement below differs from the four-pole product photographed above. Terminal labels and the internal circuit determine the connections; housing size alone does not.

CRESIN three-phase 3+1 SPD wiring example with an upstream SCB on the phase conductors, neutral and PE connections, and remote signal contacts — CRESIN CSMS-B40 3+1 wiring example with an SCB and remote signaling. It illustrates one arrangement, not a universal circuit or a device-rating recommendation. Installation must follow the selected model's instructions and applicable electrical rules.

Backup protection deals with a different event: a fault in the SPD branch. The arrangement may use a coordinated fuse, circuit breaker, dedicated backup protector or integrated protection. Existing upstream protection may be sufficient where the manufacturer's conditions are met.

The CSMS-B40 AC table lists a maximum backup fuse of 125 A gL/gG. That is not an instruction to fit 125 A in every board, nor permission to substitute a 125 A MCB. The permitted combination depends on the device characteristics and available fault current.

CRESIN's CSCB backup protector has published 220/380 V AC ratings. PV DC combinations require separate confirmation.

What to Check Before Ordering

A useful enquiry starts with the installation conditions. Send the supplier:

The AC or DC voltage, earthing arrangement and installation location.
The lightning exposure and whether an external lightning protection system is present.
The equipment to protect, including incoming signal or communication cables.
The prospective short-circuit current and existing upstream protective devices.
The destination market, required standard and monitoring requirements.

Then request the exact model's datasheet, connection diagram, backup-protection conditions and relevant test evidence. A standard number on a webpage does not establish every rating or certification for every variant.

For a replacement, photograph the complete part number, front label and terminal arrangement before ordering. “CSMS-B40” identifies a family with AC and PV DC versions; the family name alone is not a complete specification. Include the voltage variant and configuration so a physically similar device is not mistaken for an interchangeable one.

Lightning surge generator and measurement equipment pictured in CRESIN's testing laboratory — Lightning surge test equipment shown on CRESIN's laboratory page. For procurement, request the test report applicable to the model and rating being ordered.

Our testing laboratory overview describes the checks behind the products. The SPD test-report checklist explains what buyers should request before approving a bulk order.

Inspection and Replacement

An SPD can reach end of life while the connected equipment remains powered. Do not use “the machine still runs” as evidence that surge protection is available.

Check the model's status indication during routine maintenance and after a known significant surge event. On common green/red designs, red indicates that replacement is required according to the product instructions. A green window does not verify the earthing, wiring or complete protection system.

Remote contacts can bring an alarm to a monitoring system. They report a defined status; they do not measure remaining surge capacity. CRESIN's failure-indicator and remote-signaling guide covers this maintenance function.

If a replacement fails again, investigate the cause before installing another cartridge. Check the supply voltage, selected variant, connections and backup-device coordination. There is no universal replacement interval that makes these checks unnecessary. Any inspection or replacement involving exposed conductors belongs with qualified personnel using the required isolation procedure.

Start with the panel details

Send CRESIN your system voltage, earthing arrangement and panel diagram to discuss a suitable SPD and its backup protection. These details are more useful than a request for the highest kA rating.

Frequently Asked Questions

Do I need an SPD if I already have a circuit breaker?

A breaker provides overcurrent protection; an SPD limits transient overvoltage. One does not replace the other. Whether and where an SPD is required depends on the installation design and applicable rules.

Can one SPD protect an entire building?

An incoming SPD is one protection stage. Cable lengths, equipment sensitivity and incoming data lines may require additional coordinated devices. Review every relevant entry path, not just the main power supply.

Does an SPD need replacing after every surge?

No. SPDs are designed for specified surge duties. Replacement depends on condition, indication and manufacturer instructions. Severe stress can cause failure, while service life varies with the electrical environment.

Can an AC SPD be used on a solar DC circuit?

Only if that exact device is explicitly rated and documented for the DC application. An AC marking, similar appearance or matching kA value does not establish PV suitability.

Is a surge-protected power strip enough?

It may provide protection at a connected appliance, but it does not establish protection for the distribution board, fixed equipment or incoming communication lines. Check its actual surge-protection rating and how it fits with upstream protection. An ordinary extension strip may have no surge protection at all.

What is the most important thing to check when choosing an SPD?

Begin with the circuit voltage and required protection paths. Then match the surge protective device type, voltage protection level, discharge ratings and backup protection to the installation. The complete combination matters more than any single number on the label.

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