SPD meaning! !
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I think in the lightning protection of buildings, both direct lightning strikes and induced lightning can be protected through grounding and equipotential bonding; so what is the purpose of SPDs then? When faced with high currents, an SPD becomes a conductive path; however, the current from lightning is directed to the ground through the grounding system. Since the grounding resistance needs to be low in the first place, what’s the need to use an SPD for grounding? ? But I’ve seen that on the electrical diagrams of buildings, SPDs are shown connected to the incoming or returning circuits in the system diagram, and they are grounded through SPDs. However, the requirement is that the grounding resistance for lightning protection should be less than 4 ohms; yet SPDs have resistance – can they still be used? ? Why is this? ? Can someone give an example to explain? ?
A Surge Protective Device (SPD), also known as a surge protector, lightning protector, or surge arrester, is used to protect electrical equipment from damage caused by lightning electromagnetic pulses or operational overvoltages. It is divided into several types: 1. Switching type: Its working principle is that it presents high impedance when there is no instantaneous overvoltage, but once an instantaneous overvoltage caused by lightning occurs, its impedance drops sharply to a low value, allowing the lightning current to pass through. Devices used in such apparatus include discharge gaps, gas discharge tubes, thyristors, etc. 2. Voltage-limiting type: Its working principle is that it presents high impedance in the absence of instantaneous overvoltage, but its impedance decreases as the surge current and voltage increase; its current-voltage characteristics are highly nonlinear. Devices used in such apparatus include zinc oxide, varistors, surge suppressor diodes for three-phase power supplies, and avalanche diodes. 3. Shunt or choke type: Shunt type: It is connected in parallel with the equipment to be protected, presenting a low impedance to lightning pulses while exhibiting a high impedance at normal operating frequencies. Choke type: Connected in series with the equipment to be protected, it presents high impedance to lightning pulses and low impedance at normal operating frequencies. Devices used in such apparatus include: chokes, high-pass filters, low-pass filters, 1/4 wavelength short circuits, etc.
By application: 1. Power protectors: AC power protectors, DC power protectors, switching power supply protectors, etc. AC power surge protectors are suitable for protecting the power supply in systems such as distribution rooms, distribution cabinets, switchgear, and AC/DC power distribution panels ； Inside the building, there are distribution boxes with outdoor connections as well as distribution boxes on each floor of the building ； DC power surge protectors are suitable for various DC power systems, such as DC distribution panels ； Output terminals of DC power supply equipment, DC distribution boxes, electronic information system cabinets, and secondary power supply equipment. 2. Signal protectors: low-frequency signal protectors, high-frequency signal protectors, antenna feed protectors, etc. Application scope of network signal surge protectors: Used for protecting network devices such as 10/100Mbps SWITCHes, HUBs, and ROUTERS from overvoltages caused by lightning strikes and lightning-induced electromagnetic pulses ； Protection for network switches in network rooms, protection for servers in network rooms, protection for other devices with network interfaces in network rooms ；