Surge protective devices (SPDs) are designed to protect against electrical overvoltage surges. But how do you ensure they are correctly sized for the protection required by electrical devices?

In a Plant Engineering article, How to properly size surge protective devices, Emerson’s Mark Dziedzic shares how to specify an SPD and better understand the ratings associated with the application.

Surge protective devices are essential for protecting electrical equipment from voltage spikes that exceed normal peak voltage levels. These spikes can cause significant damage or failure to equipment if not adequately mitigated. SPDs are designed to absorb and redirect these surges, ensuring the safety and longevity of electrical systems.

When specifying an SPD, it is crucial to understand the various ratings associated with its application. The main performance values and ratings include:

• Maximum Continuous Operating Voltage (MCOV): This rating indicates the maximum voltage the SPD can handle continuously without degrading its performance.
• Voltage Protection Rating (VPR): This rating measures the clamping voltage level, the voltage level at which the SPD starts to conduct and limit the surge.
• Nominal Discharge Current (In): This rating represents the current level the SPD can handle during a surge event without being damaged.
• Short Circuit Current Rating (SCCR): This rating indicates the maximum short-circuit current the SPD can safely withstand.

The article emphasizes that a higher surge current (kilo-amperes, kA) rating does not necessarily mean better protection. Instead, the appropriate rating should be based on the specific requirements of the electrical system.

Mark explains that the:

…UL1449 standard was developed to take the ambiguity out of the marketplace and ensure proper protection with a level playing field. However, it has undergone many changes over the years and any SPDs (or filters) installed in your facility or equipment prior to 2009 should be examined for compliance.

It’s essential that:

…SPDs should be installed at all levels of the electrical distribution system. This is known in the electrical industry as cascading or layering. IEEE refers to it as “protection in depth.”

Recommendations from the IEEE Standard 1100 include:

…cascading levels of protection from the service entrance to distribution and branch panels, and even protection for individual critical loads. The closer to the service entrance, the more robust the device should be rated. This protection in-depth strategy protects the facility and critical loads. In recommending a kA per phase rating a general rule of thumb – “the 3-2-1 rule of thumb” applies: The service entrance should be 300 kA, distribution panels 200 kA, and finally branch panels can be 100 kA per phase.

This approach provides additional suppression from large transients and reduces let-through voltages, which are the voltages that appear on the equipment side of an SPD during a surge event. Cascading also helps suppress more frequently generated internal transients.

Mark explains that no proven equation or calculator for determining the exact surge current rating is needed. Instead, recommendations from manufacturers and industry standards should be considered.

Read the article for more on the common misconceptions, types 1 through 5 surge protection devices, and best locations where these SPDs should be applied.

Visit the Power Protection section on Emerson’s Appleton website for more information on the SolaHD surge protection and filtering devices designed to clamp high-voltage spikes off to a level acceptable by even the most sensitive electronic equipment.