Surge Protective Device

Type 2 Surge Protective Device

DIN-rail AC SPD for single and three phase - SLP40 series

An 8/20 µs lightning current waveform characterizes type 2 surge protective device SPD SLP40 series.

These Type 2 surge protective devices SPD SLP40 series are designed to be installed at the service entrance of low voltage systems or close to sensitive equipment to protect against transient overvoltages.







Type 2 DC Surge Protective Device SPD

Pluggable DC SPD for Photovoltaic PV Solar Panel Inverter - SLP-PVxxx series

This DC surge protective device SPD Type 2, isolated DC voltage systems with 600V 1000V 1200V 1500 V DC have a short-circuit current rating up to 1000 A.

Type 2 surge protective device SPD is characterized by an 8/20 µs lightning current waveform.

for 1500V DC

for 1200V DC

for 1000V DC

for 600V DC

What is Surge Protective Device?

SPD Definition

A surge protective device, also known as a surge protector or SPD, is designed to safeguard electrical components against surges in voltage that might happen in the electrical circuit.

Whenever a sudden increase in current or voltage is produced in the electrical circuit or communication circuit as a consequence of outside interference, the surge protection device may conduct and shunt in a very short period of time, preventing the surge from damaging other devices in the circuit.

Surge protective devices (SPDs) are a cost-effective method for preventing outages and enhancing system reliability.

They are typically installed in the distribution panels and play an important role in ensuring the smooth and uninterrupted operation of electronic devices in a wide range of applications by limiting transient overvoltage.

What is Transient Voltage?


In electrical distribution systems, transient voltages occur due to a sudden increase in the amplitude of the voltage or current of the circuit. It is also known as spikes or voltage surges.

These voltage fluctuations can be caused by lightning strikes, switching operations, or the operation of large motors due to high inrush starting current or other equipment.

Voltage Spikes and Transients
Transient by lightning strike
Switching transient
Transient by switching operation

One common type is a lightning transient, which occurs when lightning strikes nearby electrical lines or equipment. This can cause a sudden spike in voltage, which can damage equipment and cause power outages.

Another type of transient voltage is called a switching transient, which occurs when a large electrical load is switched on or off. This can cause a sudden surge in voltage, which can damage nearby equipment.

Therefore, surge protection equipment is required in electrical systems to lessen the effects of transient voltage, they redirect excess voltage away from the connected system.

Why Do We Need Surge Protective Devices?

Surge protective devices (SPDs) are essential in protecting electronic equipment from the harmful effects of transient overvoltage that can cause damage, system downtime, and data loss.

In many cases, the cost of equipment replacement or repair can be significant, particularly in mission-critical applications such as hospitals, data centers, and industrial plants.

Circuit breakers and fuses are not designed to handle these high-energy events, making additional surge protection necessary.

While SPDs are specifically designed to divert transient overvoltage away from the equipment, protecting it from damage and prolonging its lifespan.

In conclusion, SPDs are essential in the modern technological environment.

How Does Surge Protective Device Work?

SPD Working Principle

The basic principle behind SPDs is that they provide a low impedance path to ground for excess voltage. When voltage spikes or surges occur, SPDs work by diverting the excess voltage and current to the ground.

In this way, the magnitude of the incoming voltage is lowered to a safe level that doesn’t damage the attached device.

To work, a surge protection device must contain at least one non-linear component (a varistor or spark gap), which under different conditions transitions between a high and low impedance state.

Their function is to divert the discharge or impulse current and to limit the overvoltage at the downstream equipment.

Surge protection devices function under the three situations listed below.

A. Normal Condition (absence of surge)

In case of no surge conditions, the SPD has no impact on the system and acts as an open circuit, it remains in a high impedance state.

B. During voltage surges

In case of voltage spikes and surges, SPD moves to the conduction state and its impedance decreased. In this way, it will protect the system by diverting the impulse current to the ground.

C. Back to normal operation

After the overvoltage has been discharged, SPD shifted back to its normal high impedance state.

To learn more about how SPD functions, please visit our webpage:

The Different Types of Surge Protective Devices

SPD Types

There are three main categories of surge protective devices based on their level of protection defined by the current IEC 61643-11:2011/EN 61643-11:2012 standards:

  • Type 1 / Class I / Class B
  • Type 2 / Class II / Class C
  • Type 3 / Class III / Class D

Understanding the various SPD kinds and the corresponding applications is made easier by the table below:

Surge Protective Device Types





Type 1


Install in the primary distribution board at the origin of the electrical


Critical infrastructure, healthcare facilities, data centers, high lightning frequency areas

Type 2


Install at the sub-panel or Distribution board level. They provide protection against surges that may have passed through a Type 1 SPD but can still cause damage to sensitive electronic equipment.


Residential & commercial applications

Type 3


Protect individual electronic devices plugged into a wall outlet. Type 3 SPDs are typically built into power strips or plug-in surge protectors

combination of voltage waves (1.20/50μs) and current waves (8/20μs)

Power strips, plug-in surge protectors, electronic devices

Common characteristics

Surge protective devices (SPDs) have a number of features in common that make them efficient at shielding electronic equipment from power surges.

Uc: Maximum continuous operating voltage

Up: Voltage protection level

In: Nominal discharge current of Type 2 SPDs with 8/20 μs waveform.

Imax: Maximum discharge current of Type 2 SPDs with 8/20 μs waveform.

Iimp: Impulse current of Type 1 SPDs with 10/350 μs waveform.

You may choose the suitable surge protection for your unique needs by being aware of the various types and classifications of surge protectors, whether you need them for a large building, a distribution board, or a single device.

For more information about SPD types, visit our webpage:

How to Choose the Ideal Surge Protective Device?

Surge Protective Devices (SPDs) are essential components of electrical networks. However, choosing a suitable SPD for your system might be a difficult issue.

Key considerations to keep in mind include the following:

EN 61643-11:2012

IEC 61643-11:2011

In (80/20μs)

Imax (8/20μs)

Iimp (10/350μs)

Uoc (1.2/50μs)

Type 1

Class I

25 kA

100 kA

25 kA


Type 1+2

Class I+II

20 kA

50 kA

7 kA


12.5 kA

Type 2

Class II

20 kA

40 kA



Type 2+3

Class II+III (or III)

10 kA

20 kA


20 kV

5 kA

10 kA

10 kV

Maximum continuous operating voltage (UC)

The rated voltage of SPD should be compatible with the electrical system voltage to offer appropriate protection to the system. A lower voltage rating will damage the device and a higher rating will not divert transient properly.

Response Time

It is described as the time of SPD reacts to the transients. The quicker SPD responds, the better the protection by the SPD. Usually, Zener diode based SPDs have the fastest response. Gas-filled types have a relatively slow response time and fuses and MOV types have the slowest response time.

Nominal discharge current (In)

SPD should be tested at 8/20μs waveform and the typical value for residential miniature-sized SPD is 20kA.

Maximum impulse discharge Current (Iimp)

The device must be able to handle the maximum surge current that is expected on the distribution network to ensure that it does not fail during a transient event and the device should be tested with 10/350μs waveform.

Clamping Voltage

This is threshold voltage and above this voltage level, SPD starts to clamp any voltage transient that it detects in the power line.

Manufacturer and Certifications

Selecting an SPD from a well-known manufacturer that has certification from an impartial testing facility, such as UL or IEC, is crucial. The certification guarantees that the product has been examined and passes all performance and security requirements.

Understanding these sizing guidelines will enable you to select the best surge protection device for your needs and guarantee effective surge protection.

For additional information about choosing a single-phase or three-phase SPD, visit the website listed below:

How To Install a Surge Protective Device?

Installation Guide for AC SPD
How to install surge protective device SPD for Home
  • Single-phase Surge Protector in a Sub-distribution Box
  • Three-phase Surge Protective Device in a Sub-distribution Box
  • Three-phase Surge Protection Device in a Main distribution Box
Installation Guide for DC SPD
Installation and Wiring Diagram 1000V DC Combiner Box for PV Solar Inverter with Surge Protection Device - 1 String Input 1 String Output
  • 1000V DC Solar PV Combiner Box with Surge Protective Device
  • 1500V DC Solar PV Combiner Box with Surge Protective Device

To learn more about correct surge protective device installation and wiring diagram, please visit our webpage:

Installation Rules to Follow

Despite how easy it is to install a surge protector in a power distribution system, it is crucial to follow the appropriate procedures to assure safety and reduce any potential dangers.

Following these steps when installing an SPD in a distribution system:

  • Turn off the power: Make sure to shut off the power before beginning any electrical repair and engage off-load isolator to avoid any unwanted event to occur.
  • Installation location: Choose an appropriate location for the SPD. For best protection, the SPD should ideally be located as close as feasible to the main breaker. The surge protection device diagram from the manufacturer should be consulted for details on location requirements.
  • Mount the SPD: Install the SPD in the desired location on the DIN rail. Verify that the mounting screws are firmly in place.
  • Connecting to the earth connection: In accordance with the manufacturer’s recommendations, ground the SPD. Typically, this requires joining a ground wire from the SPD to the grounding bus bar.
  • Test the SPD: Restarting the device after installing the SPD will let you to check that everything is working as it should. For details on particular testing techniques, consult the surge protection device installation manual or the designer’s instructions.

Only electricians with a license or other technicians with the necessary education and training should install SPDs.

To maintain continuous security, the SPD should also go through routine testing and maintenance.

Surge Protective Device Coordination

Coordination between SPD & SPD

Surge protective device (SPD) coordination refers to selecting and coordinating multiple SPDs, to ensure comprehensive protection against voltage surges in an electrical system.

When it comes to coordination of surge protecting devices, keep the following points in mind:





Determine the location of all essential equipment and systems in the grid firstly, then determine the type of SPD necessary


Voltage Levels

SPDs need to be rated based on the voltage level that they can effectively protect to ensure optimal protection.

Nominal Discharge Current (In)

The In value must match the expected surge level

Maximum Continuous Operating Voltage (Uc)

The Uc rating should be higher than the system voltage


Coordination Devices

Using coordination devices like fuses and circuit breakers can also be used to ensure optimal protection.

Coordination between SPD & its disconnect circuit breaker or fuse

In electronic circuits, if the surge protection device exceeds its limitation capacity, it may be destroyed by short-circuiting itself. SPDs along with circuit breakers or fuses are arranged in such a way as to minimize the risk of overvoltage and interference effects.

It is essential for effective protection against overload caused by SPD degradation, high-intensity short circuits brought on by transient voltages, and low-intensity short circuits brought on by overload.

A circuit breaker for SPD performs the following functions:

  • Allows power to the surge protector to be removed without interrupting power to other loads.
  • If a component fails inside the protector, only the circuit breaker will trip, and the power of any other load will not be disturbed.
How do I match a surge protector to a circuit breaker?

We list a table below for your general reference:

Main Circuit Breaker

Dedicated Circuit Beaker

Surge Protective Device

SPD Selection Reference

< 40A or 63A

20A – 32A

10-20kA (Type 2)

SLP20 series

63A or 100A

32A – 40A

20-40kA (Type 2)

SLP40 series



20-50kA (Type 2)

7kA (Type 1)

FLP7 series



20-50kA (Type 2)

12,5kA (Type 1)

FLP12.5 series



25-100kA (Type 2)

25kA (Type 1)

FLP25 series

What Causes Surge Protective Devices (SPD) Failure?

Surge protective devices (SPDs) are engineered to provide reliable protection against transient overvoltages, but certain factors can lead to their failure. Following are some of the underlying reasons behind SPDs failure:

  • Excessive power surges

One of the primary causes of SPD failure is overvoltage, overvoltage can occur due to lightning strikes, power surges, or other electrical disturbances. Make sure to install the right type of SPD after proper design calculations according to location.

  • Ageing factor

Due to environmental conditions including temperature and humidity, SPDs have a limited shelf life and might deteriorate over time. Furthermore, SPDs can be harmed by frequent voltage spikes.

  • Configuration issues

Misconfigured, such as when a wye-configured SPD is linked to a load that is connected via a delta. This may expose the SPD to greater voltages, which could result in SPD failure.

  • Component failure

SPDs contain several components, such as metal oxide varistors (MOVs), that can fail due to manufacturing defects or environmental factors.

  • Improper grounding

For an SPD to operate properly, grounding is necessary. An SPD can malfunction or possibly become a safety concern if it is improperly grounded.

How Much Are Surge Protection Devices?

The cost of a surge protection device may vary depending on the elements mentioned in the subsection above, such as the device’s kind, the desired level of protection, and the application.

The price range for AC SPDs typically lies between $10 and $150 per unit. The type, brand, and features of the particular device affect the pricing.

It is critical to take the necessary amount of protection parameters into account while choosing an SPD. The highest level of protection is provided by a Type 1 SPD, however, it may cost more than a Type 2 SPD.

There might be additional expenses of installation, other than the price of the item itself. To guarantee that the device is placed correctly and adjusted for maximum safety, it is crucial to make sure that the installation is carried out by a certified electrician.

Despite the fact that initially they may appear to be an additional investment, the price of fixing or replacing broken equipment may be far more than the price of setting up an SPD.

Surge Protective Device Guidelines

Surge Protective Devices (SPDs) play a critical role in protecting electrical and electronic equipment and to ensure proper protection, it is essential to follow guidelines for selecting and installing SPDs.

Here are a few guidelines for using surge protective devices:

  • Always select the suitable type of surge-protected device. As there are many types in the market. One should use the appropriate type as per the requirements to protect the devices connected with it.
  • Always check the voltage, energy and current ratings of the SPDs.
  • Avoid overloading effects to reduce the risk of failure of surge-protected devices. Never connect too many devices to handle the high risk involving it.
  • Properly install the SPD device as per standard and protection measures
  • Maintain and test the SPDs periodically to avoid the effects of wear and tear. It ensures its long life and reduces the end of service before time.

For more information about surge protective device guidelines visit our webpage:

Applications of Surge Protection Devices

Surge Protection Devices (SPDs) find extensive applications in a wide range of industrial, commercial, and domestic areas.

They protect electrical and electronic equipment against voltage surges and transients that can damage or degrade their performance.

Low voltage SPD for industrial, commercial, residential

In industrial settings, Low Voltage SPDs are commonly used to protect sensitive equipment such as computer systems, PLCs, and other electronic devices against surges and transients.

These SPDs are also used to protect motors and other heavy machinery from power surges and voltage spikes.

Commercial areas such as shopping malls also rely on Low Voltage SPDs to protect critical equipment against electrical disturbances. SPDs are installed in residences to safeguard electronic devices such as computers, TV, and home appliances from voltage surges.

SPD for EV charging application

In the emerging market of Electric Vehicle (EV) charging applications, SPDs play a critical role in ensuring the safety and reliability of EV charging systems. These SPDs protect the charging station from voltage spikes and surges that can damage the equipment and pose a safety risk to users.

In the emerging market of Electric Vehicle (EV) charging applications, SPDs play a critical role in ensuring the safety and reliability of EV charging systems.

These SPDs protect the charging station from voltage spikes and surges that can damage the equipment and pose a safety risk to users.

SPD for solar photovoltaic applications

Photovoltaic applications also require SPDs to protect against lightning strikes and other electrical disturbances that can damage or degrade the performance of solar panels and other components in the system.

SPDs are installed between the solar panels and inverters and between the inverters and the grid.

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Reliability in surge protection!

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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