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Rich's Tech Tip

What Happens When an SPD Fails?

The principal components used in surge protective devices (SPD's) to limit the amount of high voltage are:

  • Metal Oxide Varistor (MOV)
  • Silicon Avalanche Diode (SAD)
  • Gas Discharge Tube (GDT)

The Metal Oxide Varistor is made up of, typically, zinc oxide that conducts when it is exposed to an overvoltage that exceeds its rating. Movs will degrade over time and service, and can short creating an end of life scenario after many small transients. This condition will cause a circuit breaker to trip, or in the case of Thermally Protected Varistors (TMOV), a fused link to open. Larger transients will cause the component to open quickly, thus bringing about an end to the component's service life. This condition will result in the circuit no longer providing protection. Lights or other diagnostics monitoring the health of the SPD will indicate the SPD's need for replacement. This component is typically used to suppress transients found in AC power circuits.

The Silicon Avalanche Diode is made up of silicon or other semiconductor material. These components provide the best clamping or limiting of voltage than other components, but they have lower current handling capabilities. When component ratings are exceeded, the device will short creating a ground fault, indicating the need for replacement. This component is typically used to suppress transients found in low voltage communication and data circuits.

The Gas Discharge Tube uses a sealed tube with an inert gas trapped inside. This gas becomes ionized and conducts when energized. These components can typically conduct more current than other surge components. This component also degrades over time and service and will create a short circuit while in conduction mode. When the vessel containing the inert gas is breached or other physical damage occurs to the electrodes or conductive plane, this component will reach end of life. It will no longer conduct and is then invisible to the circuit. This component is typically used in conjunction with other surge protection components as a hybrid protection circuit to take the leading edge or large energy hit. The most common use for this component is telecommunications equipment.

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