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Surge Protection Devices, (SPD's)

Surge Protection Devices (SPDs) are used on power, communications and data lines that are potentially at risk from a surge. These are different from the “arrester” that is typical installed on the utility side of the service entrance and merely connects between power and earth, excluding a neutral.  These are generally inexpensive, low surge current capacity devices that include gas-tube technology.

Many people have incorrect ideas of what SPDs do and how they function. This information is designed to clarify some of the misunderstandings and myths.

SPDs have two functions and are bi-directional in operation.

  • The first function is to “shunt” surge currents to earth/ground or bypass the device being protected.
  • The second is to equalize the potentials on all connected lines including the earth potential.

To achieve these functions, the SPD must have some trigger in order to activate. Voltage is the trigger, therefore, the SPD is simply a “voltage sensitive switch”.

A surge is a combination of current and voltage. The voltage breaks down the dielectric, or insulation,  and the current does the damage. An SPD is simply a voltage sensitive switch. When the SPD detects a voltage in excess of the device service voltage, or MCOV, it will activate and create a low impedance path between all the connected lines and earth. In doing so, it will equalize the voltage or potentials on all lines preventing “flash-overs” and shunt the current to earth or bypass the equipment being protected.  If the earth has an elevated potential due to a nearby lightning strike, the SPD will work in reverse equalizing all potentials and shunting the current off site to a distant lower potential. This can be the earth connection at the AC transformer or communications switch.  This concept and process is the same for any SPD irrespective of the application.  It is important to note that the bulk of lightning risk is from a direct of nearby lightning strike causing an elevated earth potential.

The differences lie in the SPD design engineers selection of technologies to use in the SPD as well as the voltage switch point of the components used. This is known as the “maximum continuous operating voltage" (MCOV). The technology of the components determines the speed of the solid state switch that occurs.

For instance, typical technologies are Silicon Avalanche Diodes (SAD), Metal oxide varistors (MOV) and Gas discharge tubes or spark gaps. An SPD may consist of one or more technologies to achieve the design goal.  This "speed" is referred to as the "response time".  The faster the response time typically results in a lower "let-thru-voltage"  (LTV) and better protection.

  • SADs are extremely fast but low surge current capacity and often expensive.
  • MOVs are moderately fast, high surge current capacity and relatively inexpensive.
  • Gas Discharge or Spark Gaps are very slow, high current capacity and extremely low cost.

Selection of an SPD for a particular application is relatively simple. We will  use AC power as an example.

  • Determine the application such as 120 volt AC power.
  • Will the SPD be a “primary” to be hard wired at the power panel or a “point of use” device (typically a plug in power strip with SPD capabilities)?

Then look at the performance of the SPD. For AC power, it should conform to national standards such as IEC or UL 1449 3rd edition implying that it conforms to national specifications and has been tested by the standards aurthority.

Then look to surge current capacity. This is the amount of surge current the device can handle before failure. A primary SPD should have at least 50 kA of capacity and a power strip of at least 5 kA.

Communications and data devices follow the same principles. We trust this explanation assists you, however, if you have any questions or need any additional information, please review our FAQ section or contact us by email.

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