Lightning, circuit switching, and other events can produce large overvoltages in electrical circuits for a short time. This kind of transient voltage disturbance is called a surge.

When a surge enters through power or signal lines, it can cause insulation breakdown, malfunction, or degradation of electronic components.

A surge absorber is a term used for components and devices that limit transient overvoltages to protect electrical and electronic equipment.

Among these, a device that contains at least one nonlinear component and limits surge voltages while diverting surge currents is called an SPD (Surge Protective Device) in IEC and JIS standards.

1.1 Role of Surge Absorbers

A surge absorber reduces the effect of a surge on a circuit by keeping the voltage applied to the protected circuit within a certain range or by diverting the surge current to another path.

At normal operating voltage, it has little effect on the circuit; it operates when an overvoltage exceeding a specified level is applied.

Surges can be divided into two modes: differential mode, which occurs between lines, and common mode, which occurs between a line and ground. For this reason, the connection points of protective devices in a protection circuit must be determined according to the path through which the surge enters.

1.2 Types of Surge Protection Devices

Typical devices used for surge protection include metal oxide varistors, gas discharge tubes, and TVS diodes.

They differ in operating voltage, response characteristics, the surge energy they can handle, capacitance, and other properties.

Table 1-1 shows the characteristics of typical surge protection devices.

Table 1-1 Typical Surge Protection Devices

Type	Abbreviation	Main Characteristics
Metal oxide varistor	MOV	Limits overvoltage using a nonlinear voltage–current characteristic
Gas discharge tube	GDT	Conducts through gas discharge between electrodes and diverts large surge currents to another path
Transient voltage suppressor diode	TVS diode	Clamps overvoltage using the breakdown region

Metal Oxide Varistors

A metal oxide varistor (MOV) is a nonlinear resistive element made mainly of zinc oxide or similar materials.

It shows high resistance at normal operating voltage, but its current increases sharply as the voltage rises. This characteristic limits the overvoltage applied to the protected equipment.

MOVs are widely used in surge protection circuits; however, when subjected to repeated surges, their electrical characteristics can change, reducing their protective performance and reliability.

Gas Discharge Tubes

A gas discharge tube (GDT) consists of electrodes and a sealed-in gas.

When the electric field between the electrodes exceeds the dielectric strength of the gas, a discharge occurs and the GDT changes from an insulating state to a conducting state. This allows a large surge current to flow and reduces the overvoltage applied to equipment connected in parallel.

GDTs are suited to applications in which large surge currents must be diverted, but follow current—power-frequency current that continues to flow after the GDT operates—must also be considered.

TVS Diodes

A TVS diode (Transient Voltage Suppressor Diode) is a semiconductor device that conducts current in its breakdown region when a specified voltage is exceeded, clamping transient overvoltages.

Some TVS diodes have a lower clamping voltage than MOVs and can be used to protect semiconductor circuits with low withstand voltages. They are also used for ESD protection and for protecting high-speed I/O.

1.3 Why Multiple Protective Devices Are Combined

Protective devices differ in the surge energy they can handle and in their response characteristics. For this reason, rather than relying on a single device, multiple devices may be combined in stages.

For example, in one configuration a GDT in the first stage diverts the large surge current, and a TVS diode in the second stage quickly suppresses the remaining overvoltage.

When MOVs, GDTs, TVS diodes, and other devices are combined, energy coordination between the devices is important.

1.4 Items to Check When Selecting

Surge protection components are selected after checking characteristics such as response, current handling capability, clamping voltage, and service life.

The main items to check are as follows.

• Maximum voltage applied during normal operation
• Turn-on voltage and breakdown voltage
• Limiting voltage or clamping voltage
• Surge current rating
• Energy that can be absorbed or handled
• Response characteristics
• Capacitance
• Leakage current
• Durability against repeated surges
• Operating temperature and ambient environment

In power circuits, the normal voltage and the expected surge energy must be checked. In communication lines and high-speed signal lines, the effect of capacitance on the signal is also a selection criterion.

In addition, the operating voltage of a protective device must be set within a range in which the device does not operate at normal operating voltage but limits the surge before the withstand voltage of the protected equipment is exceeded.

1.5 Degradation of Surge Absorbers

Surge absorbers can degrade as they repeatedly handle surge energy.

In MOVs, repeated overvoltages change the voltage–current characteristic and the reference voltage. As degradation progresses, leakage current and heat generation increase, and protective performance may decline.

Therefore, surge protection involves more than installing devices. It is important to also consider the expected magnitude and number of surges, the operating environment, behavior at failure, and monitoring of degradation.

Summary

A surge absorber is a component or device that limits transient overvoltages to protect electrical and electronic equipment.

Typical protective devices include MOVs, GDTs, and TVS diodes, each with different operating principles and characteristics.

Appropriate surge protection requires checking the normal voltage, surge current, limiting voltage, response characteristics, capacitance, durability, and other factors, and then selecting or combining protective devices according to the application.

About This Article

References

• “Laplace Transform-Based Modelling, Surge Energy Distribution, and Experimental Validation of a Supercapacitor Transient Suppressor,” Sadeeshvara Silva Thotabaddadurage
• “Modelling of Low-Voltage Varistors’ Responses under Slow-Front Overvoltages,” Lutendo Muremi, Pitshou N. Bokoro, Wesley Doorsamy
• “Experimental Study on The Performance of External Open-Circuit Failure Gas Discharge Tubes under Power-Frequency Follow Currents,” Hao Lu et al.
• “Reliability Analysis of Transient Voltage Suppression Diodes Under Direct Current Switching Surge Stress,” Daniel van Niekerk, Johan Venter
• “A Bibliometric and Comprehensive Review on Condition Monitoring of Metal Oxide Surge Arresters,” Tiago Gonçalves Zacarias, Wilson Cesar Sant’Ana
• “Hardware System and Preliminary Testing of Frequency Division Multiplexing Electrical Resistivity Tomography (FDM-ERT) Instrument,” Donghai Yu et al.

Sources

• IEC 61643-11:2025, Low-voltage surge protective devices—Part 11: Surge protective devices connected to AC low-voltage power systems—Requirements and test methods.
• JIS C 5381-11:2014, Low-voltage surge protective devices—Part 11: Surge protective devices connected to low-voltage power systems—Requirements and test methods (in Japanese)

(Last accessed: October 13, 2026)


*This article was compiled based on the author’s understanding, with reference to the above literature.