How does a surge protection device work?

Surge protection devices (SPDs) are designed to minimise the destructive effects of surges on electrical and electronic equipment. There are different types of SPDs depending on the technology they include and what characterises their operation against transient overvoltages.

Transient overvoltages are voltage surges, of very short duration, measured between two conductors, or between conductor and earth. Transient overvoltages can cause significant damage to electrical and electronic equipment and installations.

Adequate internal surge protection is capable of minimising their destructive effects. Surge protection is based on limiting the amplitude of each surge and divert the surge current through specific protection components.

An ideal surge protection device (SPD) should, at voltages higher than the rated voltage of the device, conduct the current, keeping the voltage stable for the duration of the surge, but stopping the conduction as soon as the voltage returns to the rated value.

In this article, we will briefly discuss some of the technologies used in SPDs and how they work to protect against transient overvoltages.

Transient overvoltage protectors: short-circuitors and voltage trimmers

According to the standards, a surge protector contains at least one non-linear component and its function is to limit transient overvoltages. In most SPDs, the non-linear electronic components divert the extra energy from the overvoltage by employing two technologies: short-circuiting (“crowbar-type” surge protectorzs) and/or voltage cutting (“clamping type”).

Short-circuiting surge protectors have a high impedance when there are no transients, but undergo a sudden change in the value of this impedance in response to overvoltages. That is, they short-circuit the high voltage to the ground until the current level decreases. Crowbar-type SPDs include spark gaps, spark gaps, explosors, gas dischargers, thyristors (silicon rectifiers), and triacs.

Voltage cutting SPDs also have a high impedance when there are no transients, but they reduce their value continuously as the voltage and current transient increases. Therefore, when the voltage exceeds a certain threshold, the resistance is reduced to divert the overvoltage energy. This impedance varies non-linearly as a function of the current flowing through the device or the voltage at its terminals. These devices include selenium rectifiers, avalanche (Zener) diodes, and varistors made of different materials such as silicon carbide, zinc oxide, etc. In addition, SPDs consisting of transient suppression diodes are often used for a last level of protection in data lines.

In addition, there are also combined SPDs that include cut-off and step-down components and can act as short-circuit breakers, trimmers, or both, depending on the voltage present.

The first SPDs that were developed were simply spark gaps that act as equipotential bonding by keeping two terminals apart unless an overvoltage occurs. Spark gaps are mostly used in earthing systems. As protectors of supply and telecommunication lines they have certain limitations as they require more complex technology or topology to de-earth and thus extinguish the currents after the spark has occurred.

Gas dischargers

Gas dischargers were an innovation concerning spark gaps because, instead of using air between the electrodes, they use a noble gas that favours spark jumping. This inert gas ionises and conducts the current during the overvoltage. The gas requires a certain time to ionise, so the protectors may need several microseconds to act. They have a small divert capacitance, so they do not limit the bandwidth of high-frequency circuits as much as other non-linear components.

These SPDs can also experience what is known as subsequent current. That is, even though the overvoltage has already disappeared and the arcing arc is extinguished, the electrodes are still hot and the gas is ionised, it may ignite in the next AC half-cycle. Depending on the power source, the subsequent current may be sufficient to damage the electrodes.

Gas dischargers are commonly used in data and telephone lines, where the subsequent current is not as important an issue as in the case of electrical circuits. These elements work very well at high frequencies.

Varistors and surge suppressor diodes

Varistors are semiconductor ceramic devices that function as non-linear impedances. They are generally composed of sintered metal oxides, the most common being zinc oxide, and suitable additives. Their structure consists of a conductive matrix of metal oxide grains, so that each intergranular boundary presents a specific voltage barrier. When this voltage is exceeded, the grains become current-carrying, forming a low-resistance path that absorbs the surge energy.

Surge suppressor diodes are similar to regulator-type Zener diodes (heavily doped silicon diodes used to regulate relatively stable voltages), but are specifically designed to limit surge pulses with silicon rectifier technology. Their main advantage is that their behaviour is close to that of an ideal SPD in terms of voltage limiting. These diodes suppress all voltage above their rated voltage, with a faster response than other protection components such as varistors and gas dischargers (in the order of picoseconds). However, they are not suitable for lightning surge protection because of their limited energy dissipation capability.

There are advantages and limitations to the various technologies discussed. However, what is important for proper surge protection are the technical specifications of each SPD such as maximum pole current (Imax), coordinated impulse current (Iimp), protection level (Up), maximum operating voltage (Uc), etc.

In addition, a single transient overvoltage protective device is usually not sufficient because it does not meet all the required discharge current and residual voltage characteristics. Therefore, several SPDs are required which are well-coordinated, acting staggered in several sequential protection stages. In this way, they can withstand the surge currents while leaving a residual voltage that is not harmful to the connected equipment.

Aplicaciones Tecnológicas S.A. has a wide range of surge protection devices for different fields, including industrial and domestic environments. All surge protectors have been tested in official and independent laboratories that certify their characteristics according to the applicable standards.

If you would like more information about the types of surge protection devices and which ones are suitable for your particular case, you can contact via this link.

We also offer free webinars from our experts on surge protection, which you can access via this link.


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Kularatna, N., Ross, A. S., Fernando, J. & James, S. Components Used in Surge Protection Circuits. in Design of Transient Protection Systems 29–42 (2019). doi:10.1016/b978-0-12-811664-7.00003-3.

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