Surge protection in industrial control cabinets
Under normal conditions, MOVs remain in a high-resistance state. When a surge occurs and the voltage crosses a threshold, their resistance drops, enabling them to shunt the surge current to ground. After the event, they automatically return to their high-resistance state. Design challenges with SPDs While SPDs are designed to absorb high-energy transients, repeated exposure to smaller overvoltages on the power distribution system can lead to premature aging and device failure. This occurs when the maximum continuous operating voltage (MCOV) rating is too close to the system’s nominal operating voltage, making the SPD vulnerable to routine voltage fluctuations. To resolve this issue, it is recommended that the MCOV of the device be at least 115% of the nominal system voltage, ensuring the device is unaffected by normal voltage variations in the power distribution system. In some cases, the current magnitude exceeds several hundred thousand amperes. To evaluate the performance of an SPD, the primary benchmark is the nominal discharge current rating, which demonstrates the device's ability to withstand repetitive current surges without damage or degradation.
■ Type 3 devices: Placed close to the load, offering localized protection for sensitive electronics. Littelfuse offers the SPDN-A series of SPDs that are intended for installation at the sub-distribution board downstream from the main panels to protect the branch circuits and connected equipment. For example, the SPDN-A480- 3D (Figure 2), has a nominal voltage of 480 V and is capable of continuously withstanding up to 550 V without degradation. The device features line-to-neutral, line-to-ground, and neutral-to- ground protection, along with the ability to suppress electromagnetic and radio frequency interference. This SPD is designed using multiple
Examples of industrial surge protection devices Surge protection in an industrial setting involves a strategic approach with different types of SPDs installed at various points in the electrical distribution system to reduce surge energy. According to UL 1449, SPDs are classified by installation location: ■ Type 1 devices: Installed very close to the service disconnect. They are designed for the line side of the main overcurrent protective device but can also be used on the load side. ■ Type 2 devices: Located downstream of the service disconnect, protecting against residual surges from external events.
MOVs in a layered structure, enabling a compact design.
Figure 2. Littelfuse’s SPDN-A480-3D surge protection device with an MCOV rating of 550 V in a 3-phase delta configuration (Image source: Littelfuse).
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