Principle of surge Protective device
The principle of operation of Surge Protective Devices (SPDs) is centered on safeguarding electrical systems from transient overvoltages, also known as surges. The main concept is to limit these voltage spikes by either diverting or limiting the surge current. Here’s how it works:
Metal Oxide Varistor (MOV): One of the most common components within an SPD is the metal oxide varistor. A varistor is an electronic component with a resistance that varies with the applied voltage, showcasing a nonlinear, non-ohmic current-voltage characteristic. When the voltage is normal, the MOV has a very high resistance, allowing normal operation of the electrical system. However, during a surge event, the MOV’s resistance drops dramatically, becoming very low. This change allows the MOV to “absorb” excess voltage and then act as a “shunt” by diverting the excess current away from the protected load and safely to the ground.
How SPDs Divert Current:
1.When a voltage spike occurs, the SPD quickly reacts, creating a low-impedance (low-resistance) path to the ground.
2.This diverts the impulse current away from the critical loads.
3.By diverting the current, the SPD also reduces the resulting voltage experienced by the connected equipment to a safer level.
Other Components of SPDs:
SPDs may also use Gas Discharge Tubes (GDTs), Silicon Avalanche Diodes (SADs), or Transient Voltage Suppressor (TVS) Diodes, depending on the specific design and protection requirements.
Result of SPD Action:
By functioning this way, SPDs protect sensitive electronic devices from voltage spikes caused by lightning strikes, power surges, and other types of electrical disturbances.
The goal is to prevent these transients from causing damage or operational issues, extending the lifespan of the equipment and ensuring its functionality.
Selection of SPD:
The proper selection of an SPD is based on several factors including the location within the electrical system, the types of surges expected, and the vulnerability of the connected equipment.
The selection criteria involve the SPD’s Maximum Continuous Operating Voltage (MCOV), Nominal Discharge Current (In), and Voltage Protection Rating (VPR), among other parameters.