How Does Surge Protection Work

How Does Surge Protection Work

Created by: Glen Zhu | Updated Date: Aug 27, 2022

What is surge protective device (SPD)?

A surge protection device (SPD) or simply a surge protector is a device used to protect electronic devices from power surges or transient voltage.

This device is connected in parallel to the power supply circuit of the loads that it has to protect. It can also be used at all levels of the power supply network.

What is the function of surge protection device?

A surge protective device is a protective device for limiting transient voltages by diverting or limiting surge current.

The surge protective device are used to protect sensitive electronic equipment connected to the installation, such as computers, televisions, washing machines and safety circuits, such as fire detection systems and emergency lighting. Equipment with sensitive electronic circuitry can be vulnerable to damage by transient overvoltages.

The SPD is a component of the electrical installation protection system.

Does Surge Protection Work?

A surge protective device (SPD) is designed to protect electrical systems and equipment from surge events by limiting transient voltages and diverting surge currents.

Surges can originate externally, most intensely by lightning, or internally by the switching of electrical loads. The sources of these internal surges, which account for 65% of all transients, can include loads turning on and off, relays and/or breakers operating, heating systems, motors, and office equipment.

Without the appropriate SPD, transient events can harm electronic equipment and cause costly downtime. The importance of these devices in electrical protection is undeniable, but how do these devices actually work? And what components and factors are central to their performance?

How SPD Works?

In the most basic sense, when a transient voltage occurs on the protected circuit, an SPD limits the transient voltage and diverts the current back to its source or ground.

To work, there must be at least one non-linear component of the SPD, which under different conditions transitions between a high and low impedance state.

At normal operating voltages, the SPDs are in a high-impedance state and do not affect the system. When a transient voltage occurs on the circuit, the SPD moves into a state of conduction (or low impedance) and diverts the surge current back to its source or ground. This limits or clamps the voltage to a safer level. After the transient is diverted, the SPD automatically resets back to its high-impedance state.

Surge Protection Device Working Principle

Surge protection devices contain at least one non-linear component (a varistor or spark gap), its electrical resistance varying in the function of the voltage which is applied to it. Their function is to divert the discharge or impulse current and to limit the overvoltage at the downstream equipment.

The operating principle of a surge protection device is as follows:

  • During normal operation (e.g., in the absence of surges), the surge protection device has no influence on the system where it’s installed. It acts as an open circuit and maintains the isolation between the active conductors and earth.
  • When a voltage surge occurs, the surge protection device reduces its impedance within a few nanoseconds and diverts the impulse current. The surge protection device behaves like a closed circuit, the overvoltage is short-circuited and limited to an acceptable value for the electrical equipment connected downstream.
  • Once the impulse surge has stopped, the surge protection device will return to its original impedance and return to the open circuit condition.
Figure 1 Principle of protection system in parallel

How Surge Protective Device (SPD) work?

The ability of an SPD to limit overvoltages on the electrical distribution network by diverting surge currents is a function of the surge-protective components, the mechanical structure of the SPD, and the connection to the electrical distribution network. An SPD is intended to limit transient overvoltages and divert surge current, or both. It contains at least one nonlinear component. In the simplest terms, SPDs are intended to limit transient overvoltages with a goal of preventing equipment damage and downtime due to transient voltage surges reaching the devices they protect.

For example, consider a water mill protected by a pressure relief valve. The pressure relief valve does nothing until an over-pressure pulse occurs in the water supply. When that happens, the valve opens and shunts the extra pressure aside, so that it won’t reach the water wheel.

If the relief valve was not present, excessive pressure could damage the water wheel, or perhaps the linkage for the saw. Even though the relief valve is in place and working properly, some remnant of the pressure pulse will still reach the wheel. But pressure will have been reduced enough not to damage the water wheel or disrupt its operation. This describes the action of SPDs. They reduce transients to levels that will not damage or disrupt the operation of sensitive electronic equipment.

Technologies Used

What technologies are used in SPDs?

From IEEE Std. C62.72: A few common surge-protective components used in manufacturing SPDs are metal oxide varistors (MOVs), avalanche breakdown diodes (ABDs – formerly known as silicon avalanche diodes or SADs), and gas discharge tubes (GDTs). MOVs are the most commonly used technology for the protection of AC power circuits. The surge current rating of an MOV is related to the cross-sectional area and its composition. In general, the larger the cross-sectional area, the higher the surge current rating of the device. MOVs generally are of round or rectangular geometry but come in a plethora of standard dimensions ranging from 7 mm (0.28 inch) to 80 mm (3.15 inch). The surge current ratings of these surge protective components vary widely and are dependent on the manufacturer. As discussed earlier in this clause, by connecting the MOVs in a parallel array, a surge current value could be calculated by simply adding the surge current ratings of the individual MOVs together to obtain the surge current rating of the array. In doing so, consideration should be given to coordination of the operating characteristics of the MOVs selected.

There are many hypotheses on what component, what topology, and the deployment of specific technology produces the best SPD for diverting surge current. Instead of presenting all of the options, it is best that the discussion of surge current rating, Nominal Discharge Current Rating, or surge current capabilities revolve around performance test data. Regardless of the components used in the design, or the specific mechanical structure deployed, what matters is that the SPD has a surge current rating or Nominal Discharge Current Rating that is suitable for the application.

A more extensive description of these components follows. The components used in SPDs vary considerably. Here is a sampling of those components:

1. Metal oxide varistor (MOV)

Typically, MOVs consist of a round or rectangular shaped body of sintered zinc oxide with suitable additives. Other types in use include tubular shapes and multilayer structures. Varistors have metal particle electrodes consisting of a silver alloy or other metal. The electrodes may have been applied to the body by screening and sintering or by other processes depending on the metal used. Varistors also often have wire or tab leads or some other type of termination that may have been soldered to the electrode.

The basic conduction mechanism of MOVs results from semiconductor junctions at the boundary of the zinc oxide grains formed during a sintering process. The varistor may be considered a multi-junction device with many grains acting in series-parallel combination between the terminals. A schematic cross-sectional view of a typical varistor is shown in Figure 1.

Varistors have the property of maintaining a relatively small voltage change across their terminals while the surge current flowing through them varies over several decades of magnitude. This nonlinear action allows them to divert the current of a surge when connected in shunt across the line and limit the voltage across the line to values that protect the equipment connected to that line.

2. Avalanche Breakdown Diode (ADB)

These devices are also known as silicon avalanche diode (SAD) or transient voltage suppressor (TVS). The P-N junction breakdown diode, in its basic form, is a single P-N junction consisting of an anode (P) and a cathode (N). See Figure 2a. In DC circuit applications, the protector is reverse biased such that a positive potential is applied to the cathode (N) side of the device. See Figure 2b.

The avalanche diode has three operating regions, 1) forward bias (low impedance), 2) off state (high impedance), and 3) reverse bias breakdown (relatively low impedance).

Figure 2 Basic form of avalanche diode

These regions can be seen in Figure 3. In the forward bias mode with a positive voltage on the P region, the diode has very low impedance once the voltage exceeds the forward bias diode voltage, VFS. VFS is usually less than 1 V and is defined below. The off state extends from 0 V to just below a positive VBR on the N region. In this region, the only currents that flow are temperature dependent leakage currents and Zener tunneling currents for low breakdown voltage diodes. The reverse bias breakdown region begins with a positive VBR on the N region. At VBR electrons crossing the junction are accelerated enough by the high field in the junction region that electron collisions result in a cascade, or avalanche, of electrons and holes being created. The result is a sharp drop in the resistance of the diode. Both the forward bias and reverse bias breakdown regions can be used for protection.

The electrical characteristics of an avalanche diode are intrinsically asymmetric. Symmetric avalanche diode protection products consisting of back to back junctions are also manufactured.

3. Gas discharge tube (GDT)

Gas discharge tubes consist of two or more metal electrodes separated by a small gap and held by a ceramic or glass cylinder. The cylinder is filled with a noble gas mixture, which sparks over into a glow discharge and finally an arc condition when sufficient voltage is applied to the electrodes.

When a slowly rising voltage across the gap reaches a value determined primarily by the electrode spacing, gas pressure and gas mixture, the turn-on process initiates at the spark-over (breakdown) voltage. Once spark-over occurs, various operating states are possible, depending upon the external circuitry. These states are shown in Figure 4. At currents less than the glow-to-arc transition current, a glow region exists. At low currents in the glow region, the voltage is nearly constant; at high glow currents, some types of gas tubes may enter an abnormal glow region in which the voltage increases. Beyond this abnormal glow region the gas discharge tube impedance decreases in the transition region into the low-voltage arc condition. The arc-to-glow transition current may be lower than the glow-to-arc transition. The GDT electrical characteristic, in conjunction with the external circuitry, determines the ability of the GDT to extinguish after passage of a surge, and also determines the energy dissipated in the arrester during the surge.

If the applied voltage (e.g. transient) rises rapidly, the time taken for the ionization/arc formation process may allow the transient voltage to exceed the value required for breakdown in the previous paragraph.

Gas Discharge Tube GDT 5kA 10kA

This voltage is defined as the impulse breakdown voltage and is generally a positive function of the rate-of-rise of the applied voltage (transient).

A single chamber three-electrode GDT has two cavities separated by a center ring electrode. The hole in the center electrode allows gas plasma from a conducting cavity to initiate conduction in the other cavity, even though the other cavity voltage may be below the spark-over voltage.

Because of their switching action and rugged construction, GDTs can exceed other SPD components in current-carrying capability. Many telecommunications GDTs can easily carry surge currents as high as 10 kA (8/20 µs waveform). Further, depending on design and size of the GDT, surge currents of >100 kA can be achieved.

The construction of gas discharge tubes is such that they have very low capacitance – generally less than 2 pF. This allows their use in many high-frequency circuit applications.

When GDTs operate, they may generate high-frequency radiation, which can influence sensitive electronics. It is therefore wise to place GDT circuits at a certain distance from the electronics. The distance depends on the sensitivity of the electronics and how well the electronics are shielded. Another method to avoid the effect is to place the GDT in a shielded enclosure.

Definitions for GDT

A gap, or several gaps with two or three metal electrodes hermetically sealed so that gas mixture and pressure are under control, designed to protect apparatus or personnel , or both, from high transient voltages.


A gap or gaps in an enclosed discharge medium, other than air at atmospheric pressure, designed to protect apparatus or personnel , or both, from high transient voltages.

  • LCR filters

These components vary in their:

  • energy capability
  • availability
  • reliability
  • cost
  • effectiveness

From IEEE Std C62.72: The ability of an SPD to limit overvoltages on the electrical distribution network by diverting surge currents is a function of the surge-protective components, the mechanical structure of the SPD, and the connection to the electrical distribution network. A few common surge-protective components used in manufacturing SPDs are MOVs, SASDs, and gas discharge tubes, with MOVs having the largest usage. The surge current rating of an MOV is related to the cross-sectional area and its composition. In general, the larger the cross-sectional area is, the higher the surge current rating of the device. MOVs generally are of round or rectangular geometry but come in a plethora of standard dimensions ranging from 7 mm (0.28 in) to 80 mm (3.15 in). The surge current ratings of these surge protective components vary widely and are dependent on the manufacturer. By connecting the MOVs in a parallel array, a theoretical surge current rating could be calculated by simply adding the current ratings of the individual MOVs together to obtain the surge current rating of the array.

There are many hypotheses on what component, what topology, and the deployment of specific technology produces the best SPD for diverting surge current. Instead of presenting all of these arguments and letting the reader decipher these topics, it is best that the discussion of surge current rating, Nominal Discharge Current Rating, or surge current capabilities revolve around performance test data. Regardless of the components used in the design, or the specific mechanical structure deployed, what matters is that the SPD has a surge current rating or Nominal Discharge Current Rating that is suitable for the application and, probably most importantly, that the SPD limits the transient overvoltages to levels that prevent damage to the equipment being protected given the expected surge environment.

Basic Operating Modes

In each mode, current flows through the SPD. What may not be understood, however, is that a different type of current can exist in each mode.

The Awaiting Mode

Under normal power situations when “clean power” is supplied within an electrical distribution system, the SPD performs minimal function. In the awaiting mode, the SPD is waiting for an overvoltage to occur and is consuming little or no ac power; primarily that used by the monitoring circuits.

The Diverting Mode

Upon sensing a transient overvoltage event, the SPD changes into the Diverting Mode. The purpose of an SPD is to divert the damaging impulse current away from critical loads, while simultaneously reducing its resulting voltage magnitude to a low, harmless level.

As defined by ANSI/IEEE C62.41.1-2002, a typical current transient lasts only a fraction of a cycle (microseconds), a fragment of time when compared with the continuous flow of a 60Hz, sinusoidal signal.

The magnitude of the surge current is dependent on its source. Lightning strikes, for example, that can in rare occurrences contain current magnitudes exceeding several hundred thousand amps. Within a facility, though, internally generated transient events will produce lower current magnitudes (less than a few thousand or hundred amps).

Since most SPDs are designed to handle large surge currents, one performance benchmark is the product’s tested Nominal Discharge Current Rating (In). Often confused with fault current, but unrelated, this large current magnitude is an indication of the product’s tested repeated withstand capacity.

From IEEE Std. C62.72: The Nominal Discharge Current Rating exercises an SPD’s ability to be subjected to repetitive current surges (15 total surges) of a selected value without damage, degradation or a change in measured limiting voltage performance of an SPD. The Nominal Discharge Current test includes the entire SPD including all surge protective components and internal or external SPD disconnectors. During the test, no component or disconnector is permitted to fail, open the circuit, be damaged or degrade. In order to achieve a particular rating, the measured limiting voltage performance level of the SPD must be maintained between the pre-test and post-test comparison. The purpose of these tests is to demonstrate the capability and performance of an SPD in response to surges that in some cases are severe but might be expected at the service equipment, within a facility or at the installation location.

For example, an SPD with a nominal discharge current capacity of 10,000 or 20,000 amps per mode means the product should be able to safely withstand a transient current magnitude of 10,000 or 20,000 amps a minimum of 15 times, in each of the modes of protection.

End Of Life Scenarios

From IEEE Std C62.72: The greatest threat to the long-term reliability of SPDs might not be surges, but the repeated momentary or temporary overvoltages (TOVs or “swells”) that can occur on the PDS. SPDs with an MCOV – that are precariously close to the nominal system voltage are more susceptible to such overvoltages which can lead to premature SPD aging or premature end-of-life. A rule of thumb that is often used is to determine if the MCOV of the SPD is at least 115% of the nominal system voltage for each specific mode of protection. This will allow the SPD to be unaffected by the normal voltage variations of the PDS.

However, aside from sustained overvoltage events, SPDs can age, or degrade, or reach their end-of-service condition over time due to surges that exceed the SPDs ratings for surge current, the rate of occurrence of surge events, duration of the surge, or the combination of these events. Repetitive surge events of significant amplitude over a period of time can overheat the SPD components and cause the surge protective components to age. Further, repetitive surges can cause SPD disconnectors that are thermally activated to operate prematurely due to the heating of the surge protective components. The characteristics of an SPD can change as it reaches its end-of-service condition – for example, the measured limiting voltages can increase or decrease.

In an effort to avoid degradation due to surges, many SPD manufacturers design SPDs with high surge current capabilities either by using physically larger components or by connecting multiple components in parallel. This is done to avoid the likelihood that the ratings of the SPD as an assembly are exceeded except in very rare and exceptional instances. The success of this method is supported by the long service life and history of existing SPDs installed that have been designed in this fashion.

With regard to SPD coordination and, as stated with regard to surge current ratings, it is logical to have an SPD with higher surge current ratings located at the service equipment where the PDS is most exposed to surges to aid in the prevention of premature aging; meanwhile, SPDs further down-line from the service equipment that are not exposed to external sources of surges might have lesser ratings. With good surge protective system design and coordination, premature SPD aging can be avoided.

Other causes of SPD failure include:

  • Installation errors
  • Misapplication of a product for its voltage rating
  • Sustained over-voltage events

When a suppression component fails, it most often does so as a short, causing current to begin flowing through the failed component. The amount of current available to flow through this failed component is a function of the available fault current and is driven by the power system. For more information on Fault Currents go to SPD Safety Related Information.

Example of SPD operation

Without a surge protection device, the surge reaches the electrical equipment. If the surge exceeds the electrical equipment’s impulse withstand voltage, the isolation is reduced and the impulse current flows freely through the device, damaging it.


With the use of a surge protection device between the active conductors and earth (TT network), the overvoltage is limited and the discharge current is safely diverted, establishing an equipotential connection between phase and earth.

SPD Categories

The two main types of SPDs are voltage limiting and voltage switching components. Voltage limiting components change in impedance as the voltages rise, resulting in clamping the transient voltage. Voltage switching components “turn on” once a threshold voltage is exceeded and immediately drop to a low impedance.

Most systems today incorporate both component types together to aggregate the strengths and limit the weaknesses of each individual part.

Examples of voltage limiting components are metal oxide varistors (MOVs) and transient voltage suppression (TVS) diodes. Voltage switching components include gas discharge tubes (GDTs) and spark gaps.

Three Types of SPD

There are three different types of Surge Protective Devices:

  • Type 1 SPD installed at the origin, e.g. main distribution board.
  • Type 2 SPD installed at sub-distribution boards (Combined Type 1 & 2 SPDs are available and are usually installed in consumer units).
  • Type 3 SPD is installed close to the protected load. They must only be installed as a supplement to Type 2 SPD.

Type 1 SPD

The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings, protected by a lightning protection system or a meshed cage.

It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.

Type 1 SPD is characterized by a 10/350 µs current wave.

Type 2 SPD

The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of overvoltages in the electrical installations and protects the loads.

Type 2 SPD is characterized by an 8/20 µs current wave.

Type 3 SPD

These SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.

Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).

Surge Protection Device (SPD) normative definition


Direct lightning stroke

Indirect lightning stroke

Indirect lightning stroke

IEC 61643-1:2005

Class I test

Class II test

Class III test

IEC 61643-11:2011

Type 1

Type 2

Type 3

EN 61643-11:2012 + A11:2018

Type 1

Type 2

Type 3

VDE 0675-6-11

Class B

Class C

Class D

Type of test wave

10/350 μs

8/20 μs

1.2/50 μs + 8/20 μs


Iimp – Impulse discharge current (kA)

In – Norminal discharge current (kA)

Imax – Maximum discharge current (kA)

Uoc – Open-circuit voltage (kV)

Note 1: There exists Type 1+2 Surge Protection Device SPD combining the protection of loads against direct and indirect lightning strokes.

Note 2: Some Type 2 Surge Protective Devices can also be declared as Type 3 SPD.

Surge protection devices based on spark gaps

They are called switching surge protection devices. The spark gap is a component composed of two electrodes in close proximity which isolate one part of the circuit from the other up to a certain voltage level.

These electrodes can be in the air or encapsulated with a gas. During normal operation of the system (at rated voltage), the spark gap does not conduct current between the two electrodes.

In the presence of a voltage surge, the impedance of the spark gap rapidly decreases to 0.1-1 Ω with the formation of an electric arc between the electrodes, typically in 100 ns. The electric arc is extinguished when the surge finishes, restoring the isolation.

Varistor surge protection devices

Varistors are components that have their impedance controlled by the voltage, with a characteristic continuous but not linear “U in the function of I”.

surge protection devices based on varistors, also known as voltage limiting, are characterized by a high impedance when there is no surge present (normally above 1 MΩ).

When a surge occurs, the varistor’s impedance falls rapidly below 1 Ω within a few nanoseconds, allowing the current to flow. The varistor regains its isolation properties after disc

A peculiarity of varistors is that a negligible current is always flowing through them, known as residual current, IPE (100 to 200 µA).

Comparison between spark gaps and varistors

The main characteristic of spark gaps is their capacity to manage large quantities of energy from direct lightning strikes, while varistors have a very low level of protection (therefore high-performance) and are fast-acting. We will now examine the difference between the two technologies.



Spark Gap

Isolation properties

A varistor, although it presents a very high impedance at rest, always has a minimum continuous current, IC, flowing through it (e.g., 0.5 NA).


This current tends to increase as the varistor wears until it reaches high levels. For this reason, Varistor SPDs must always be protected against short circuits and cannot be used for N-PE connection upstream of the RCDs.


+ include internal protection that guarantees a safe end of life

A spark gap is a true open circuit when at rest, ensuring that there is no current flow at all either in normal operating conditions or when it reaches the end of its life;


for this reason, an SPD may be installed upstream of an RCD (therefore protecting it from the flow of impulse or discharge current) only if the connection between the active conductors and earth provides for a spark element.

Resistance when conducting

Even in the discharge phase, the resistance remains appreciably greater than zero, limiting the possibility to reduce the surge overvoltage to 3-4 times the rated mains voltage.

When the arc is ignited, the resistance becomes negligible.

Response time

Very rapid, a few nanoseconds

Generally slow, but accelerated by the electronic device.

Ignition / limiting voltage

Low, thanks to the fast response time

Generally high, thanks to the excellent insulating properties of the air, but reduced with the aid of the electronic device.

Extinction of the short-circuit

Varistors are not characterized by a follow-through short circuit current, as their impedance returns to very high values as soon as the surge ceases.

SPDs with spark gap technology must necessarily be designed in a way that enables the interruption of the following current (such as an arc extinguishing chamber)


A varistor progressively loses its isolating performance; at the end of its life, it can therefore become a low impedance short-circuit.

A spark gap is no longer able to ignite the arc at the end of its life, due to the wear of its electrodes or because the electronic ignition circuit has faded. It, therefore, becomes a permanently open circuit.

Need for backup protection

Back-up protection is necessary in order to ensure short circuit end-of-life safety. In case of short circuit end of life of the varistor, the thermal disconnector is generally not able to open the circuit.

Back-up protection is to be provided in all cases to ensure safety in the case of a fault with the SPD and to interrupt the electrical arc if the short-circuit current in the installation point is greater than the SPD’s performance for interrupting the short-circuit follow-through current (ISC>If).

Main Technical Parameters of Surge Protective Device (SPD)

1. Nominal voltage Un: The rated voltage of the protected system matches. This parameter indicates the type of protector that should be selected. It marks the effective value of AC or DC voltage.

2. Rated voltage Uc: The maximum effective value of voltage can be applied to the designated end of the protector for a long time without causing changes in the protector characteristics and activation of the protection element.

3. Rated discharge current In: The maximum impulse current peak value that the protector can withstand when a standard lightning wave with a waveform of 8/20μs is applied to the protector for 10 times.

4. Maximum discharge current Imax: When a standard lightning wave with a waveform of 8/20μs is applied to the protector for one impact, the maximum impulse current peak value that the protector can withstand.

5. Response time tA: It mainly reflects the action sensitivity and breakdown time of the special protection element in the protector. The change in a certain period of time depends on the slope of du/dt or di/dt.

6. Insertion loss Ae: The ratio of the voltage before and after the protector is inserted at a given frequency.

7. Return loss Ar: It indicates the proportion of front wave reflected at the protection device (reflection point), which is a direct measure of whether the protection device is compatible with the system impedance.

8. Voltage protection level Up: the maximum value of the protector in the following tests: 1KV/μs slope of flashover voltage; residual voltage of rated discharge current.

9. Data transmission rate Vs: It indicates how many bits are transmitted in one second, unit: bps. It is the reference value for the correct selection of the lightning protection device in the data transmission system, and the data transmission rate of the lightning protection device depends on the transmission method of the system.

10. Maximum longitudinal discharge current: It refers to the peak value of the maximum impulse current that the protector can withstand when a standard lightning wave with a waveform of 8/20μs is applied to the ground for one time.

11. Maximum lateral discharge current: It refers to the peak value of the maximum impulse current that the protector can withstand when a standard lightning wave with a waveform of 8/20μs is applied between the line and the line.

12. Online impedance: It refers to the sum of the impedance and inductance of the circuit flowing through the protector under the nominal voltage Un. Usually called “system impedance”.

13. Peak discharge current: There are two types: rated discharge current Isn and maximum discharge current Imax.

14. Leakage current: It refers to the direct current flowing through the protector under the nominal voltage Un of 75 or 80.

Primary Components

1. Spark gap

It is generally composed of two metal rods exposed to the air separated by a certain gap. One of the metal rods is connected to the power phase line L1 or the neutral line (N) of the required protective equipment. The other metal rod is connected to the grounding line (PE) phase connection.

When the instantaneous overvoltage strikes, the gap is broken down, and a part of the overvoltage charge is introduced into the ground, avoiding the voltage increase on the protected equipment.

The distance between the two metal rods in the spark gap can be adjusted as required, and the structure is relatively simple, but the disadvantage is that the arc extinguishing performance is poor.

The improved spark gap is an angular gap. Its arc extinguishing function is better than the former. It relies on the electric power F of the circuit and the rising effect of the hot air flow to extinguish the arc.

2. Gas discharge tube (GDT)

A gas discharge tube (GDT) is composed of a pair of cold negative plates separated from each other and encapsulated in a glass tube or ceramic tube filled with a certain inert gas (Ar).

In order to improve the triggering probability of the discharge tube, there is an auxiliary triggering agent in the discharge tube. This gas-filled discharge tube has a two-pole type and a three-pole type.

The technical parameters of gas discharge tubes mainly include DC discharge voltage Udc; impulse discharge voltage Up (usually Up≈(2~3) Udc; power frequency withstand current In; impulse withstand current Ip; insulation resistance R (>109Ω)); Interelectrode capacitance (1-5PF).

The gas discharge tube can be used under DC and AC. The selected DC discharge voltage Udc is as follows:

Under DC: Udc≥1.8U0 (U0 is the DC voltage for normal operation of the line)

Under AC: Udc≥1.44Un (Un is the effective value of the AC voltage for normal operation of the line)

3. Metal oxide varistor (MOV)

It is a metal oxide semiconductor with non-linear resistance with ZnO as the main component. When the voltage applied to both ends reaches a certain value, the resistance is very sensitive to voltage.

Its working principle is equivalent to the series-parallel connection of multiple semiconductor P-Ns. Varistors are characterized by good non-linear characteristics (I=non-linear coefficient α in CUα), large current capacity (~2KA/cm2), low normal leakage current (10-7 ~ 10-6A), and low residual voltage, the response time to instantaneous overvoltage are fast (~10-8s), and there is no freewheeling.

4. Transient voltage suppression (TVS) diode

A TVS diode has the function of clamping and limiting voltage. It works in the reverse breakdown zone. Because of its low clamping voltage and fast action response, it is particularly suitable for use as the last few levels of protection components in multi-level protection circuits.

The volt-ampere characteristics of the suppression diode in the breakdown zone can be expressed by the following formula: I=CUα, where α is the nonlinear coefficient, for the Zener diode α=7~9, in the avalanche diode α=5~7

5. Choke coil

The choke coil is a common mode interference suppression device with ferrite as the core. It consists of two coils of the same size and the same number of turns symmetrically wound on the same ferrite toroidal core, which forms a four-terminal device.

It has to suppress the large inductance of the common-mode signal, while the small leakage inductance for the differential mode signal has almost no effect. The use of choke coils in balanced lines can effectively suppress common-mode interference signals (such as lightning interference) without affecting the normal transmission of differential mode signals on the line.

A cost-effective solution - Surge Protection Device (SPD)

Surge protection is a cost-effective solution to prevent downtime, improve system and data reliability, and elimination of equipment damage due to transients and surges for both power and signal lines. It is suitable for any facility or load (1000 volts and below).

Typical SPD applications within heavy-duty industrial systems including power distribution panels, communications systems, and process control systems.

  • Power distribution, control cabinets, programmable logic controllers, electronic motor controllers, equipment monitoring, lighting circuits, metering, medical equipment, critical loads, back-up power, UPS, HVAC equipment;
  • Communication circuits, telephone or facsimile lines, cable TV feeds, security systems, alarm signaling circuits, entertainment center or stereo equipment, kitchen or household appliances.

Learn More About Surge Protective Devices

Surge protection is a critical aspect of coordinated electrical protection for any facility. To learn how to keep sensitive equipment safe and limit downtime (among other benefits), download our Surge Protective Device Guidelines.

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