Low Voltage Surge Protection Devices

Low Voltage Surge Protection Devices

Created by: Glen Zhu | Updated Date: April 16th, 2024

Low Voltage SPD

What is low voltage system

Low voltage is defined as ranging from 0 to 1000V AC or 0 to 1500V DC by the International Electrotechnical Commission (IEC) Standard. The definition of low voltage varies by context, but typically, less than 1000 V is considered the normal standard.

A low voltage system typically refers to an electrical power system that operates for domestic, light industrial and commercial use. In general, low voltage systems have voltages below 1,000 volts.

Low voltage systems provide a range of benefits including enhanced safety, energy efficiency, flexibility, and compatibility with modern electronics. These advantages make them a preferred choice for various residential, commercial, and industrial applications.

Low voltage systems can be found in:

  • Telecommunication systems
  • Security systems
  • Home automation systems
  • Lighting
  • Solar PV system

Impact of power surges on low voltage devices and equipment

Low voltage devices and equipment are sensitive to power surges due to the sudden increase in electrical current flowing through the system.

Low voltage devices typically use smaller electronic components that have lower voltage tolerances and are more susceptible to damage from sudden increase in voltage. Tighter voltage margins add the likelihood of surge impact even small voltage fluctuations or spikes.

Low voltage systems often involve closer integration of components and circuits, which can amplify the effects of power surges. A surge that affects one component or circuit in a low voltage system is more likely to spread to other connected components, leading to widespread damage.

Power surges can have detrimental effects on low voltage devices and equipment, leading to various consequences that can impact their lifespan and performance.

To protect low voltage appliances from these impacts, utilizing surge protection devices and maintaining stable voltage supply are essential measures to safeguard against potential damage and ensure the longevity and optimal performance of sensitive electronic devices.

Figure 1 – Overview of low voltage surge protection applications

What is low voltage surge protection?

Low voltage surge protection involves the use of low voltage surge protection devices to limit transient voltages by diverting or limiting surge current, safeguarding low voltage systems from damage caused by transient voltages and surges.

These devices are cost-effective solutions that prevent downtime, improve system and data reliability, and eliminate equipment damage caused by power surges for power and signal lines operating at 1,000 volts and below.

Low voltage surge protection devices are installed at the critical points in the electrical system to divert excess voltage away from sensitive equipment.

The primary function of surge protection devices is to limit the voltage supplied to electrical devices to a certain threshold by short-circuiting current to ground or absorbing the spike when a transient occurs. This action prevents damage to appliances and devices connected to the surge protector, ensuring the longevity and performance of electronic equipment.

Surge protection devices come in different types such as service entrance surge protectors that protect the entire premises from external power surges and point-of-use surge protectors that provide localized protection for specific electronic equipment within a building.

How Surge Protection Work in Low Voltage System

Work principle of low voltage surge protector

Low voltage surge protective devices act as a barrier between the electrical system and sensitive electronic equipment, diverting and dissipating excess voltage to prevent damage.

When a voltage surge occurs in the electrical system, the surge protective device detects the increase in voltage above a certain threshold. This threshold is typically set slightly above the normal operating voltage of the system.

Once the surge protective device detects a surge, it rapidly diverts the excess voltage away from the connected equipment. This is usually achieved by shunting the excess voltage to the ground or another low impedance path, effectively bypassing the equipment.

The internal components such as metal oxide varistors, gas discharge tubes, or silicon avalanche diodes are designed to absorb and dissipate the excess energy generated by the surge.

Once the surge has subsided and the voltage returns to normal levels, the surge protective device resumes its normal operation, ready to detect and suppress any future surges.

The key component of a surge protective device

Metal Oxide Varistors

Metal Oxide Varistors (MOVs) are semiconductor devices utilized in power supply circuits directly connected to and powered by the AC mains. They are a prevalent technology employed in surge protection devices, notably in widely available surge protection power strips designed to safeguard consumer devices connected to outlets.

MOVs function by adjusting their resistance in response to the voltage passing through them at any given moment. During periods of high current, an MOV’s resistance decreases, redirecting potentially harmful energy to the earth ground. This process protects vulnerable circuit components and prevents damage to the system.

While this surge protection technology is effective for short-duration surges commonly encountered in consumer applications, it does have limitations, especially in commercial settings. MOVs are not suitable for sustained overvoltage situations, and their efficacy diminishes over time.

Despite being robust initially, MOVs degrade with use and eventually become ineffective, necessitating replacement to maintain surge protection capabilities.

Silicon Avalanche Diodes

Silicon Avalanche Diodes (SADs) represent a prevalent surge protection technology widely utilized in high-speed data transmission, low-voltage DC applications, and networked devices. Unlike Metal Oxide Varistors (MOVs), SADs offer a faster response time, making them particularly suited for applications where swift protection against surges is critical.

SADs function by undergoing avalanche breakdown, a process wherein an electric current multiplication leads to a rapid surge in current. Despite the potential for catastrophic failure in standard diodes experiencing avalanche breakdown, SADs are specifically engineered to withstand and control this phenomenon, remaining undamaged in the process.

One notable advantage of SADs over MOVs is their minimal impact on circuit capacitance. This characteristic ensures that data transmission across networks remains unhindered, allowing for the free flow of data while still providing robust surge protection. This is crucial for networked devices as it helps prevent packet loss and throughput issues.

However, while SADs offer faster surge protection response times compared to MOVs, they are generally less robust. SADs require a lower surge threshold to self-sacrifice and may necessitate replacement more frequently as a result. Nonetheless, their rapid-reacting nature makes them indispensable for applications where immediate surge protection is paramount.

Gas Discharge Tubes

Gas Discharge Tubes (GDTs) stand out as one of the most resilient surge protection components available. These devices establish a connection between the power line and a ground line, utilizing an inert gas as the conductor bridging the two lines. Under normal conditions, when the line voltage remains below a certain threshold, the gas does not conduct electricity.

However, in the event of a power surge or spike, the gas molecules undergo ionization, breaking into positive and negative ions. This ionized gas then transforms into an exceptionally efficient conductor, directing the current towards the ground line and diverting the surge away from the protected device. Following the surge, the ions recombine to revert to gas molecules.

GDTs offer several advantages over alternative surge protection devices. Their robust shielding capabilities make them well-suited for defending against exceptionally large surges, rendering them ideal for safeguarding externally mounted devices and installations prone to lightning or other significant power events.

Additionally, their compact size facilitates easier installation, particularly on devices with limited space. However, GDTs do exhibit slower reaction times compared to other surge protection components, rendering them less suitable for swiftly reacting to fast-traveling, sudden surges.

Types of Low Voltage Surge Protection Devices

Types of low voltage surge protection devices could be vary based on different classifications and criteria. They could be classified based on type or class, application, design topology and mounting.

Type/Class Classification:

Type 1 SPDs: Designed for installation between the secondary of the service transformer and the line, protecting against direct lightning strikes.

Type 2 SPDs: Intended for installation on the load side of the service equipment overcurrent device, safeguarding against indirect lightning strikes.

Type 3 SPDs: Installed near sensitive equipment to protect against transient over-voltages.

Type 1+2 SPDs: Provide comprehensive protection against both direct and indirect lightning strikes.

AC/DC Classification:

AC SPDs: Commonly used for protecting AC electrical systems.

DC/PV SPDs: Specifically designed for DC circuits, such as those in photovoltaic (PV) installations.

Mounting/Appearance Classification:

DIN-rail Mounted SPDs: Installed on DIN rails in electrical panels.

Panel Mounted SPDs: Mounted directly on panels or enclosures.

These classifications help in selecting the appropriate surge protective device based on the specific requirements of the electrical system, the level of protection needed, and the application environment.

Different types of low voltage surge protection devices

Type 1 vs. Type 2 vs. Type 3

Type 1, 2, 3 is the most common word we mention in the field of low voltage surge protective devices, the big difference between them is waveform, specific configurations and applications.

Type 1 SPDs:  These surge protectors are installed at the main electrical service entrance or primary distribution panels. Type 1 SPDs are designed to protect against high-energy surges originating from lightning strikes or power grid switching.

They provide the first line of defense by diverting large surges away from the building’s electrical system. Type 1 SPDs are ideal for installations in areas prone to frequent lightning activity or where the risk of direct lightning strikes is high.

Type 2 SPDs: Type 2 surge protectors are installed at the sub-distribution or branch circuit level within a building. They offer protection against moderate to high-energy surges generated internally or externally, including those caused by indirect lightning strikes or large electrical equipment operation.

Type 2 SPDs safeguard sensitive equipment and electrical systems within the building and are commonly used as a secondary layer of protection after Type 1 devices.

Type 3 SPDs: Also known as plug-in surge protectors or point-of-use devices, Type 3 SPDs are installed directly at electrical outlets or individual equipment. They provide localized protection against low to moderate-energy surges generated internally or from nearby equipment.

Type 3 SPDs are commonly used to protect electronic devices such as computers, TVs, and appliances from voltage spikes and transient surges. They serve as a tertiary layer of protection in the overall surge protection scheme.

Surge Protection Device Types



Maximum Discharge Current (Imax)

Voltage Protection Level (Up) Rating

Location of Installation

Application & Coverage

Type 1



50 kA

≤2.5 kV

Main service entrance or source of power supply

For large facilities and high-threat locations

Type 2



40 kA

≤1.5 kV

Sub-distribution panel or electrical panel

For facilities of medium size

Type 3


Combination of voltage waves (1.2/50μs) & current waves (8/20 μs)

10 kA

≤1.0 kV

Outlets or near the specific terminal equipment.

For certain devices and circuits

The Selection of Low Voltage Surge Protection Devices: MOV

The MOV, or Metal Oxide Varistor, serves as the core component of surge protection devices. Varistors with zinc oxide as the main material are predominantly used for surge protection in low voltage electrical appliances available in the market.

The Metal Oxide Varistor (MOV) exhibits a symmetrical volt-ampere characteristic curve when applied, typically connected in parallel within circuits. During normal circuit operation, the MOV remains in a high-impedance state, allowing the circuit to function without interference.

However, when abnormal transient overvoltage occurs and reaches the varistor voltage threshold, the MOV transitions from a high-impedance state to a low-resistance state.

In this state, it discharges the instantaneous overcurrent caused by the abnormal transient overvoltage to the ground, effectively clamping the overvoltage to a safe level. This process safeguards the downstream circuit from potential damage caused by abnormal transient overvoltage events.

Figure 2 – V-I curve of metal oxide varistors

How to Select the Correct MOV for Low Voltage Surge Protection Devices?

To select the correct metal oxide varistors for low voltage surge protection devices, first step is to find the voltage rating of MOVs, and then determine which MOV disc size to use.

Many countries have embraced a unified standard for AC power grid voltage, typically set at approximately 230V with a tolerance of +10% and -6%. This translates to an acceptable voltage range fluctuating between 207V and 253V.

As a result, varistors rated for 270V or 275V could be appropriate for safeguarding against voltage spikes in most regions.

Here is a step-by-step guide to choose specific MOV for SPDs.

MOV vs. Spark gap vs. GDT

Metal Oxide Varistors (MOVs), Spark Gaps, and Gas Discharge Tubes (GDTs) are three components used in surge protection devices, each with its own characteristics and applications. Here is a comparison of MOVs, Spark Gaps, and GDTs based on the information provided in the sources:

Operating Principle:

  • MOV: Operates based on the nonlinear voltage-current characteristic of the metal oxide material. It has high resistance under normal conditions but rapidly decreases resistance when a voltage surge occurs, absorbing and dissipating surge energy.
  • Spark Gap: Operates by creating a spark or arc across a gap between two electrodes when a certain voltage threshold is exceeded, providing a low-resistance path for excess voltage diversion.
  • GDT: Uizes gas discharge technology where gas between two electrodes is confined in a container. When breakdown occurs, it creates a conductive path for surge energy dissipation.

Voltage Handling:

  • MOV: Capable of handling high-voltage surges and clamps at a specific voltage level.
  • Spark Gap: Designed to handle high-voltage surges effectively by providing a rapid path for surge conduction.
  • GDT: Offers protection against over-voltages by creating a conductive path through gas discharge.

Response Time:

  • MOV: Typically has a faster response time compared to some other surge protection devices but may be slower than spark gaps.
  • Spark Gap: Offers very fast response time as sparking occurs almost instantaneously when the threshold voltage is reached.
  • GDT: Provides rapid response to overvoltage events due to gas discharge technology.


  • MOV: Designed to recover after surge events, returning to its high-resistance state for future protection.
  • Spark Gap: In many cases, spark gaps are one-time-use devices and may not return to their original state after conducting.
  • GDT: Capable of multiple operations and recovery after surge events.

Gas Discharge Tube


Encapsulated spark gapZinc oxide varistorGDT and varistor in seriesEncapsulated spark gap and varistor in parallel
Operating modeVoltage switchingVoltage switchingVoltage limitingVoltage – switching and – limiting in seriesVoltage – switching and – limiting in parallel
Operating curves    

■Telecom network

■LV network

(associated with varistor)

LV networkLV networkLV networkLV network
TypeType 2Type 1Type 1 or Type 2Type 1+ Type 2Type 1+ Type 2

What Is the Difference Between a Surge Protector and a Power Strip?

The primary difference between a power strip and a surge protector lies in their functionality: a power strip provides additional outlet space, while a surge protector defends against voltage spikes that could harm electronic devices.

This difference is often discernible through the presence of a joules rating on surge protectors, indicating their ability to absorb energy and protect connected equipment. Joules represent the duration of protection against power surges, with a higher rating offering greater protection.

A power strip is simply a device that plugs into a wall outlet, then provides multiple electrical sockets for you to plug your devices into. They are often used in areas where there are not enough outlets or as a way to centralize power cords.

On the other hand, a surge protector is a device that not only provides multiple electrical sockets but also protects your devices from power spikes. A power spike or surge is a sudden increase in voltage that can potentially damage electronic equipment in an instant.

While there is surge protection built into some power strips, their primary attributes are equally distributing voltage across a multiplicity of electronic devices and their lower average cost.

Surge protectors, on the other hand, come in at a bit pricier cost but are specifically designed to protect against power surges.

Both power strips and surge protectors are useful, but surge protectors integrated with MOVs offer a long-term protection against damaging voltage spikes and surges at only a slightly higher cost. For normal protection, you just need to choose what’s the best for you.

When it comes to expensive property or the intricate equipment, it recommended to no compromise, specific surge protectors are your first choice with quick response to cut off the surge impact.

Why Choose a Surge Protector?

Surge protectors, like power strips, are beneficial when you have multiple electronics in one area. They offer multiple outlets to accommodate devices like phones, computers, and TVs, allowing them to charge while safeguarding them from voltage surges, which is measured in joules.

These surge protectors are cost-effective, making them a great option to protect electronics such as televisions, computers, and home entertainment systems. They are typically priced under $20, with more expensive options providing higher joule protection levels.

For larger operations with significant appliances or sensitive equipment like refrigerators, AC units, computers, or servers, commercial surge protectors may be necessary unless connected to an uninterrupted power supply (UPS).

While commercial surge protectors can be more expensive, they are a cost-effective solution compared to replacing damaged equipment in the event of a power surge. Some surge protectors even come with warranties.

Investing in a surge protector is particularly wise for those living in areas prone to lightning strikes or voltage spikes or for individuals with valuable electronics to shield their devices from potential electrical damage.

Wiring and Installation for Low Voltage Surge Protection Devices

To make sure your surge protection devices are fully made use of, incorrect wiring and installations can result in a range of consequences including electrical hazards, fire hazards, equipment damage, system malfunction, and safety risks for individuals.

It is crucial to ensure proper wiring and installations are carried out by qualified professionals to prevent these potential issues and maintain a safe electrical environment.

How to install ac spd for home

How to install a surge protection device?

A surge protection device is correctly wired when:

  • The protected equipment is equipotentially bonded to the same earth to which the SPD is connected.
  • The SPD and its associated backup protection are connected.
  • The total length of the surge protection devices circuit does not exceed 0.5m.
  • Cross-section of the SPD connection conductors should be in accordance with IEC standard.

Installation guidelines

  • It is better to separate protected wires from those which are not protected.
  • Measures should be taken to avoid cross coupling of transients between power and signalling cables.
  • Identify the wires which cause an additional voltage drop on the equipment terminals.

Low Voltage Surge Protection Devices: Performance Requirements and Tests

SPD design and basic functions

IEC standard has described the function of surge protection devices in low voltage power system as below.

In the absence of surges: the SPD shall not have a significant influence on the operational characteristics of the system to which it is applied.

During the occurrence of surges: the SPD responds to surges by lowering its impedance and thus diverting surge current through it to limit the voltage (in most cases much lower than its protective level Up).

The surges could initiate a power follow current through the SPD depending on the SPD design (SPD with power follow current).

After the occurrence of surges: the SPD recovers to a high-impedance state after the surges and extinguishes any possible power follow current.

SPDs age with each lightning strike, necessitating protection against short-circuiting and overload currents to prevent device failure.

Low Voltage Surge Protection Devices: Performance Requirements and Tests

Electrical requirements

For electrical aspects, all SPDs with a terminal for the protective conductor, the residual current shall be measured. The limiting voltage of the SPDs should not exceed the voltage protection level.

The Surge Protective Device (SPD) must endure designated discharge currents when subjected to the maximum continuous operating voltage Uc without experiencing any detrimental alterations to its characteristics.

The SPDs shall be protected against overheating due to degradation or overstress.

Mechanical requirements

Surge protection devices should be provided with appropriate mounting methods, they will not work loose if the clamping screws or lock nuts are tightened or loosened. The connection of cables has a minimum and maximum cross-sectional area.

Class I, II and III tests

The Class I test is intended to simulate partial conducted lightning current impulses. SPDs subjected to Class I test methods are generally recommended for locations at points of high exposure.

SPDs tested to Class II or III test methods are subjected to impulses of shorter duration.

Type tests requirements for SPD refer to IEC/EN 61643-11.

Surge Protection for Low Voltage Systems

Low voltage equipment comprises a big part of our daily use. This includes power supply systems, telecommunication, security systems, PV systems and etc. Due to the frequently use of these equipment, low voltage surge protection devices are designed to handle the sudden high voltage exposure.

Low voltage surge protective devices are now employed in various applications that are closely related to our daily lives.

Surge protection for power supply system

The power supply to family houses is a critical aspect of modern living, providing the essential energy needed to operate appliances, lighting, heating, cooling, and various electronic devices. A stable and reliable power supply is of paramount importance for the functionality and comfort of households.

Implementing Surge Protective Devices (SPDs) for low-voltage power distribution is the initial measure to safeguard our premium electronic equipment from potential power surges.

Surge protection for CCTV

Security systems, including cameras, alarms, access control panels, and DVRs, are susceptible to damage from power surges caused by lightning strikes. To safeguard against failures in cameras and display systems, components must be shielded from atmospheric discharges and surge voltages.

Surge protectors play a vital role in diverting excessive voltage away from equipment, ensuring continuous functionality and preventing downtime.

Surge protection for telecom

Telecommunication encompasses the transmission of information through diverse technologies across wire, radio, optical, or electromagnetic systems. It involves encoding, transmission, and decoding, enabling message and data exchange globally.

Telecom infrastructure is vulnerable to transient overvoltage surges, primarily from lightning strikes, due to its extensive network and communication mediums.

The susceptibility arises from reliance on electronic devices, cables, antennas, and towers, which inadvertently capture electromagnetic energy from lightning. Communication system breakdowns can lead to significant loss of life, property, and other damages.

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