Relation to UL
The UL 1449 standard, in conjunction with NFPA 79, stipulates that an SPD is required when safety components are installed in the control cabinet. The SPD can be installed either on-site or inside the control cabinet.
What is a surge protection device?
A surge protection device (SPD) is a component designed to limit damage to electrical equipment by diverting power surges.
What is a power surge, and how does it occur?
In electrical engineering, a surge is defined as a voltage that exceeds the rated voltage. The rated voltage, including certain tolerance values, is defined by the device manufacturer and refers to the electrical voltage that occurs during normal operation. Exceeding the rated voltage can cause serious damage to the device and result in significant secondary damage (e.g., due to fire).
Power surges can be caused by a wide range of events. These range from phenomena of cosmic proportions, such as a solar storm striking Earth, to a switching error in a device. A very well-known cause is a lightning strike, but an electrostatic discharge, an electromagnetic pulse, or a disturbance in the power grid can also lead to a power surge.
Surge Protection or Lightning Protection?
Although a surge arrester also protects against current pulses caused by a nearby lightning strike, it should not be confused with a lightning protection system for buildings (“lightning conductor”). A lightning protection system serves a completely different function and is constructed entirely differently from the various types of surge protection devices.
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Types of Surge Protection Devices
There are various types of surge protection devices, which can be classified according to different criteria. In the case of surge arresters, a distinction is often made between coarse-protection and fine-protection elements. Coarse protection elements can also divert strong surges, but they have a significantly longer response time than fine protection elements. Coarse protection elements include gas discharge tubes and (historically) spark gaps, while fine protection elements include suppressor diodes and varistors.
Gas vent
The gas discharge tube(GTD) is based on the operating principle of gas discharge tubes. It consists of a glass tube filled with gas, with a cathode and an anode at each end. When a certain overvoltage is reached, it is automatically dissipated by a gas discharge that creates an electric arc—causing the resistance to drop dramatically.
Gas-discharge arresters are characterized by having the lowest intrinsic capacitance of all surge arresters; they can dissipate high currents (up to 20,000 amperes) and are inexpensive, robust, and durable. However, during a gas discharge, the voltage across the gas arrester drops well below the rated voltage, which has the same effect on the affected power system as a short circuit. Furthermore, gas arresters are not suitable for sustained high current flow, which is why they must be combined with a fuse.
Suppressor Diodes
Suppressor diodes (TVS, Transient Voltage Suppressor) are a type of fine-protection device. They protect electrical circuits from overvoltages by diverting the surge away from the component being protected. This is achieved through a parallel connection that is activated when a certain voltage threshold is exceeded.
Suppressor diodes are primarily used to protect semiconductor devices and assemblies on printed circuit boards. During normal operation, they behave almost neutrally. However, their intrinsic capacitance is higher than that of gas discharge tubes, which can cause interference in the high-frequency range. Like gas discharge tubes, suppressor diodes are only suitable for dissipating short voltage spikes, which is why they, too, must be combined with additional protective measures in a surge protection device.
Varistors
Depending on the type, a varistor (variableresistor) is a tablet made of compressed and sintered silicon carbide, zinc oxide, or other metal oxides. It therefore consists of many small grains of the material used, at the points of contact between which barrier layers form. The higher the voltage applied to the varistor, the more of these barrier layers are broken down, and the lower the resistance becomes. When an overvoltage occurs, the resistance drops sharply, and the voltage is dissipated.
Varistors are characterized by a very short response time of less than one nanosecond. They are also more robust than suppressor diodes; however, their intrinsic capacitance is significantly higher, which is why they can have a strong attenuating effect on high-frequency signals. Furthermore, varistors are not resistant to aging. Unlike other surge protection devices, varistors are not overloaded by sustained overvoltages, which is why they are frequently used in the design of current bridges, such as in string lights.
Surge Protection Devices and UL 1449
As we have seen, surge arresters are important components for protecting electrical equipment and ensuring user safety. It is not surprising, then, that the requirements for them are described in a separate UL standard. UL 1449 distinguishes between five types of surge protection devices based on their respective design characteristics. UL 1449 also covers the requirements for surge protection devices in specific application areas, such as telecommunications equipment.
Zusammenfassung
A surge protection device is always required when safety components are installed in a control panel. This helps prevent damage caused by overvoltage. Overvoltage is defined as any voltage that exceeds the rated voltage of a device as specified by the manufacturer. The requirements for surge protection devices are specified in the NEC and the UL 1449 standard, which distinguishes a total of five types of surge arresters. Another common distinction is between coarse protection devices—which include so-called gas discharge tubes—and fine protection devices, which include suppressor diodes and varistors.
