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Choosing and Applying AC SPD

With the continuous development of modern power systems, AC surge protection devices (SPD) are playing an increasingly important role in power systems. AC SPD can effectively protect electrical systems and equipment from transient voltages and surges, providing safety assurance for industrial, commercial, and residential applications. This article will explore the selection and application of AC SPD to help users better understand and use this critical device.


What is AC SPD?

AC surge protection device (SPD) is a device connected to the AC (AC) network, designed to protect electrical systems and equipment by limiting transient voltages and diverting surge currents. Transient voltages and power surges pose significant risks to individuals and equipment, hence surge protection plays an important role in safety.

According to IEC standards, low-voltage power supply systems can be divided into TT,  TN , and IT systems. The TN system is further divided into TN-C, TN-S, and TN-CSsystems. The appropriate SPD should be chosen based on the grounding method of the low-voltage power supply system. For example, in a TN AC power supply system, the distribution lines drawn from the main distribution box of the building must adopt the TN-S grounding method.

AC SPD Types

According to IEC 61643-11:2011 and EN 61643-11:2012+A11:2018 , AC SPD can be divided into three types:

Type 1/I/B

Suitable for the main distribution equipment of buildings, capable of withstanding direct lightning current.

Type 2/II/C

Used to protect distribution equipment within the building.

Type 3/III/D

Used to protect end equipment, usually installed near the equipment.

When selecting an SPD for AC power systems, the following basic considerations must be taken into account:

  • Determine whether the power supply is single-phase or three-phase.

  • Determine the voltage protection level of the SPD. The impulse withstand voltage of the protected electrical equipment should be greater than the voltage protection level UP of the SPD.

  • Ensure to select the correct number of ports or sockets.

  • Check if the SPD is a transient voltage surge protector and if it has the TUV mark.

  • It is crucial to choose SPD of the appropriate specification. If the SPD is chosen too large, the system will not be harmed but the cost will increase; if chosen too small, the system may fail faster.

Certified SPD typically includes fault indicators (such as indicator lights or signs), making it easy for maintenance personnel to determine whether the SPD is functioning properly. Regular inspection is necessary, and SPD devices with non-function indicators should be replaced as soon as possible.

Industrial Sites

The equipment used on industrial sites is very sensitive and prone to being affected by lightning and switching surges. Professional lightning and surge protection methods will ensure the long-term stable operation of industrial site equipment. By installing AC SPD on key equipment and distribution systems, equipment failures caused by surge voltages can be effectively reduced, thereby enhancing the safety and reliability of industrial production.


AC SPD plays an important role in protecting electrical systems and equipment from surge voltage damage. Through the reasonable selection and correct application of AC SPD, the safety and reliability of power systems can be greatly improved. If you have any questions or needs regarding the selection and application of AC SPD, feel free to contact Techwin.

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