What is the Purpose of a Surge Protection Device?

Surge Protection Devices (SPDs) are extremely common and important appliances that protect electronics from a phenomenon called a surge or transient overvoltage. These surges can either slowly deteriorate or immediately damage sensitive electronic devices, create a fire risk, and can cost businesses and individuals greatly. This blog will discuss the causes and impacts of surges, the classification system for SPDs, and how these devices protect against surges.

When one plugs an appliance into a wall socket to power it, the electrical energy used is called an alternating current (AC). In an alternating current, voltage is constantly changing direction from a peak positive to a peak negative. A transformer is used to either increase or decrease voltage to allow for a safe and appropriate amount to reach our appliances and devices. For example, in the United States, an average wall outlet will only produce 120v, which is the tolerance for most electrical devices. However, during a surge, the voltage can reach up to 6000v and last as little as microseconds. This sudden increase in voltage can generate heat and damage the small and sensitive circuitry in devices. Surges are commonly caused when restoring power to a grid, such as after a blackout, faulty wiring, or when lightning strikes an electrical system.

Surge protection devices are classified as either type one, two, or three, and their function and location in the system vary. Type one surge protection devices are permanent and can be installed anywhere between the transformer and the service entrance where the breaker or main disconnect panel are located. Type two devices are installed on the “load” side of the service entrance; that is to say, it is downstream of the breaker. Most reading this will be familiar with the third class of surge protection devices, referred to as point of utilization devices. These devices are power strips with multiple outlets and are meant to be receptacles for the electronic devices that are being protected (i.e., Computers). Rarely do experts refer to the fourth type of SPD, which is the protection component built into the end device.

The end goal of surge protection devices is to keep the voltage of a system within tolerable limits, even in the case of transient overvoltage. The method in which an SPD will do this is by allowing the excess voltage to ground. Keeping the volta  ge down to acceptable levels is called “clamping” and is most commonly done with the help of a small component called a metal oxide varistor (MOV). The resistance of a MOV is in inverse correlation with the voltage that flows across it. Under normal voltage conditions, the varistor has high resistance and is therefore not conductive. During a surge event, the varistor’s resistance will decrease, and the extra voltage is forced through it, which causes a short. This interruption in current protects downstream devices from voltage spikes. Another component that is commonly combined with MOVs is gas discharge tubes (GDTs). GDTs are small glass tubes containing inert gas that gain a charge in the event of a voltage spike. This ionized gas becomes conductive and, in a similar fashion to the MOV, causes a short. Because this ionization process can take longer to trigger, they are usually considered backups to the other modalities.

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