How to properly size surge protective devices

To specify an SPD, understand the ratings associated with its application.

By Mark Dziedzic May 18, 2021


Learning Objectives

  • Surge protection device (SPD) help is available in Cascading Surge Suppression IEEE Standard 1100.
  • SPDs should be in at every level.
  • Get recommendations on SPD ratings.

What are surge protective devices?

When an electrical surge occurs, voltage greatly exceeding accepted peak voltage levels could pass through building circuits to electrical equipment. Without proper protection, this equipment is susceptible to damage or failure. A surge protective device (SPD) can negate these spikes.

Specifying the SPD requires identifying and understanding the ratings associated with its application. Performance values and ratings associated with an SPD include maximum continuous operating voltage (MCOV), voltage protection rating (VPR), nominal discharge current (In), and short circuit current rating (SCCR). The most misunderstood rating is the surge current rating, typically quantified in kilo-Ampere (kA).

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.

Guidance on SPDs

Little published data or even recommendations exist on the correct level of surge current (kA) rating for different locations. The Institute of Electrical and Electronics Engineers (IEEE) provides input on what surge ratings are but does not publish recommendations. Unfortunately, no proven equation or calculator is available that allows inputting system requirements and receiving a solution. Any information a manufacturer provides, via calculators or other means, is merely a recommendation.

A tendency exists to assume that the larger the panel, the larger the kA device rating needed for protection. Another misconception is that if 200 kA is good, then 400 kA must be two times better. As you will see, this is not always the case. Based on experience in the electrical industry, Emerson has generated some guidance on how to apply surge current ratings.

Cascading surge protection, in-depth

To optimize suppression throughout a system, 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 “protection in depth.”

Cascaded surge protection provides additional suppression from large transients, that step their way through from the service entrance, by further reducing the let-through voltages. Also suppressed are more frequently generated internal transients.

Let-through voltage is the voltage appearing on the equipment side (load side) of an SPD when an impulse voltage/current of a defined wave-shape and amplitude is applied to the line side of an SPD. It can be used to compare different SPD’s abilities to lower the surge voltage to the equipment requiring protection.

IEEE Standard 1100 recommends 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.

Types of SPD locations

Panel size does not play a major role in the selection of a kA rating. What is much more important is the location of the panel within the facility. UL1449 defines the location types within a facility as:

Type 1 Surge Protection Devices

Type 1 Surge Protection Devices are intended for permanent application at the service entrance. It can even be before the main disconnect. A UL1449 Type 1 device can be installed on the primary of buildings or the 1st disconnect.

Type 2 Surge Protection Devices

Type 2 Surge Protection Devices are intended for installation on the load side of the main entrance panel.

Type 3 Surge Protection Devices

Type 3 Surge Protection Devices are for specific devices, referred to as “point of utilization” in the standard.

Type 4 Surge Protection Devices

Type 4 Surge Protection Devices would be a component device that is intended to be part of a larger assembly and is not approved for standalone use without additional safety evaluation. Be careful if you are offered a Type 4 device to be installed into a control panel. The panel builder would be responsible to submit for 3rd party safety approval otherwise it wouldn’t be covered in a catastrophic failure.

Type 5 Surge Protection Devices

Type 5 Surge Protection Devices, which are the basic component, such as a metal oxide varistor, silicon avalanche diode or gas discharge tube. These clearly can’t be directly installed into a facility.

Recommendations: SPD ratings

Choosing the appropriate surge rating for an SPD comes down to two things: 1) the location of the SPD within the electrical distribution and 2) the facility’s geographic location.

Location kA

Service panel 300kA/phase

Distribution panel 200kA/phase

Branch panels 100kA/phase

Emerson recommends the above surge current ratings based on SPD location within the electrical distribution using the general “3, 2, 1 Rule of Thumb” mentioned earlier.

Larger, more destructive surge currents are mostly found at the facility service entrance. On rare occasions, if, for example, the exposure level is “extreme” as in states like Florida, it might be wise to increase surge current ratings. In these cases, the SPD will be exposed to larger surge events more frequently.  With the correct surge current rating for your application, the SPD can then be exposed to a higher number of surge events before needing replacement. In addition, SPD event counters come standard on some models to monitor events for these types of locations.

Experience with SPD products shows that a device carrying a surge current rating between 240 and 250kA for a service panel or critical load provides many years of service in “high to medium” exposure locations over time.

Final thoughts on SPD selection

The purpose of a surge protective device is to shunt and suppress transient voltages being introduced into an electrical distribution system from either an external or internal source. Selecting the proper surge current-rated SPD throughout the electrical distribution system provides the best performance life for equipment. When selecting SPDs, keep these key points in mind:

Providing proper surge suppression to a facility and its equipment requires more than a single SPD located at the service entrance. We recommend cascaded SPDs with a proper surge current rating for each location. This provides superior suppression for a service panel or critical load. A single SPD, no matter how big or expensive, will not provide the same level of system protection.

Over-sizing an SPD for its application cannot hurt a system, but under-sizing the SPD can result in premature SPD failure.

For direct lightning strikes, SPDs alone are not a replacement for comprehensive lightning protection (refer to UL96A Master Lightning Certification).

Following these guidelines for sizing and placing SPDs throughout an electrical distribution system takes the guess work out of it and maximizes surge suppression at every point. Remember, bigger isn’t always better. Size appropriately for the load and protect critical panels and loads to ensure a maximum return on investment.


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Author Bio: Mark Dziedzic is senior product marketing manager, SolaHD power quality products, Appleton Group. SolaHD is part of the automation controls group at Emerson. The Appleton brand of electrical solutions is found in  industrial environments around the world.