Describe the principle behind surge protection devices (SPDs) and how they safeguard electrical equipment.

Surge Protection Devices (SPDs) safeguard electrical equipment by diverting transient voltage surges to ground, preventing them from damaging sensitive components. The principle behind SPDs relies on their ability to quickly change from a high-impedance (resistance) state to a low-impedance state when a voltage surge occurs. A voltage surge is a temporary and unwanted increase in voltage that can be caused by lightning strikes, switching of electrical loads, or equipment malfunctions. SPDs typically contain voltage-sensitive components, such as metal oxide varistors (MOVs), silicon avalanche diodes (SADs), or gas discharge tubes (GDTs). Under normal operating conditions, these components exhibit high impedance, effectively isolating the protected equipment from the ground. When a voltage surge exceeds a predetermined threshold, these components rapidly switch to a low-impedance state, providing a path for the surge current to flow to ground. This diverts the excess voltage away from the protected equipment, limiting the voltage across the equipment to a safe level. Once the surge has subsided, the SPD automatically returns to its high-impedance state, allowing normal operation to resume. SPDs are typically installed in parallel with the electrical equipment they are protecting. For example, an SPD installed at the service entrance of a building can protect all the electrical equipment within the building from lightning surges. Similarly, SPDs can be installed at individual pieces of equipment to provide localized protection. The effectiveness of an SPD depends on its voltage rating, surge current capacity, and response time. Selecting the appropriate SPD for a given application is crucial for ensuring adequate protection.