How Does SPD (Surge Protection Device) Work?

Surge protective devices (SPDs) are essential devices that protect electrical equipment from damage caused by voltage spikes or surges. These surges, which can be caused by lightning or power outages, can interfere with and even destroy sensitive electronic devices.

SPDs work by diverting the excess voltage that is generated during a surge away from the devices that are being protected. They do this by moving the excess energy through a path that leads to the ground, away from the equipment. SPDs can be installed at the point where electrical power enters a building or even on individual devices to protect them from the voltage spikes.

The two major components of SPDs are metal oxide varistors (MOVs) and gas discharge tubes (GDTs). MOVs are made of a metal oxide and are designed to conduct electricity when a voltage surge occurs. When the voltage exceeds a certain threshold, the MOV conducts electricity, shunting the excess voltage away from the equipment it’s protecting. GDTs work in a similar way, by breaking down and conducting excess voltage away from the device.

In addition to MOVs and GDTs, SPDs also usually include a fuse called a transient voltage suppressor (TVS). The TVS helps protect the SPD and the equipment it’s connected to from any short circuits that may occur.

In summary, SPDs are critical devices that help protect electronic equipment from potentially harmful voltage surges. Without them, devices could be easily destroyed by surges caused by lightning, power outages, or other voltage fluctuations. With the right use of SPDs in place, you can have peace of mind and protect your valuable electronic devices from unexpected surges and outages.

SPD Categories or Types

The two main types of SPDs are voltage limiting and voltage switching components. Voltage limiting components change in impedance as the voltages rise, resulting in clamping the transient voltage. Voltage switching components “turn on” once a threshold voltage is exceeded and immediately drop to low impedance. Most systems today incorporate both component types together to aggregate the strengths and limit the weaknesses of each individual part.

Examples of voltage limiting components are metal oxide varistors (MOVs) and transient voltage suppression (TVS) diodes. Voltage switching components include gas discharge tubes (GDTs) and spark gaps.

Working Principle

SPDs can vary in types but they have a common working principle: At normal voltage, it doesn’t affect the operation of the circuit. Once the high spike voltage is generated, the impedance of the surge protection device will decrease very quickly. Thus, the surge current will flow into the ground through the surge protector instead of entering the equipment to protect the circuits and other equipment connected to it.

Types of SPD

Types of SPD depend on the location, at which it is installed,

Type 1 SPDs – Main Distribution Panel

Type 2 SPDs – Sub-Distribution Panel

Type 3 SPDs – Load Protection Panel

By technology, there are two types of SPDs, Namely,

  • Gas Discharge Tube (GDT) – Uses a spark gap across electrodes placed in a sealed glass filled with noble gases which are designed to break down at a voltage higher than the operating voltage causing it to conduct. This is a voltage-switching system (acts slower than MOV).
  • Metal Oxide Varistor (MOV) – An MOV is a circular/rectangular-shaped body of sintered Zinc Oxide & other oxides of a single/multilayer sandwich of the same. When a surge occurs, the resistance across the MOV rapidly decreases causing a large current flow. MOVs are Voltage limiting system (acts quicker than GDT).
  • Avalanche Breakdown Diode (ABD) – A P-N Junction diode designed to experience an avalanche breakdown (an electrical phenomenon in a semiconductor that allows large amounts of current to flow) when a voltage is applied that is greater than the rated voltage.


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