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Understanding Surge Protective Device Ratings

Jun 21, 2011 5:06 PM, By David Komm, Mersen

How to know what kA rating to use

Selecting the appropriate surge protective device (SPD) can seem like a daunting task, especially with all of the different types on the market today. The surge rating or kA rating of an SPD is one of the most misunderstood ratings. Customers commonly ask for an SPD to protect their 200A panel. There¡¯s also a tendency to think that the larger the panel, the larger the kA device rating needs to be for protection. As you will see in this article, this is a common misconception.

When a surge enters a panel, it does not care or know the size of the panel. So how do you know if you should use a 50kA, 100kA, or 200kA SPD? As discussed in the IEEE standard C62.41, a building¡¯s wiring adds impedance that will limit the surge current. The standard also states that 10kA devices have been adequately limiting surge currents at the service entrance for several years. Therefore, it¡¯s reasonable to say the largest surge that can enter a building¡¯s wiring system is 10kA; however, a direct lightning strike would produce a much larger surge. The extremely high voltage associated with a direct lightning strike would most likely flashover, thereby ¡°self-limiting¡± the surge. So why would you ever need an SPD rated for 200kA? Simply stated ¡ª for longevity.

You might think: If 200kA is good, then 600kA must be three times better, right? Not necessarily. At some point, the rating diminishes its return, only adding extra cost and no substantial benefit.

Because most SPDs on the market use a metal-oxide varistor (MOV) as the main limiting device, we can explore how/why higher kA ratings are achieved. If an MOV is rated for 10kA and sees a 10kA surge, it would use 100% of its capacity. This can be viewed somewhat like a gas tank, where the surge will degrade the MOV a little bit (no longer is it 100% full).

If the SPD has two 10kA MOVs in parallel, it would be rated for 20kA. Theoretically, the MOVs will evenly split the 10kA surge, so each would take 5kA. In this case, each MOV has only used 50% of its capacity, which degrades the MOV much less ¡ª leaving more left in the tank for future surges.

Does this translate into surge ¡°stopping power?¡± No. Just because an SPD has two or 20 MOVs in parallel doesn¡¯t mean it will limit the 10kA surge any better than a single SPD of the same rating. The main objective of having MOVs in parallel is to increase the longevity of the SPD. Again, keep in mind that this is subjective ¡ª at some point you are only adding cost by incorporating more MOVs and receiving little benefit.

As mentioned before, panel size does not really play a role in the selection of a kA rating. The location of the panel within the facility is much more important. IEEE C62.41.2 defines the categories of expected surges within a facility as (click here to see Figure):

• Category C: Service entrance, more severe environment: 10kV, 10kA surge.
• Category B: Downstream, ¡Ý30 ft from category C, less severe environment: 6kV, 3kA surge.
• Category A: Further downstream, ¡Ý60 ft from category C, least severe environment: 6kV, 0.5kA surge.

Category C devices can be used in Category B or A locations; however, a Category C device would be excessive for a Category B location. Some engineers may decide to specify Category C devices to have a conservative design, but this will also only add cost while adding little to no benefit.

Although UL 1449 third edition does not use the exact category terminology as IEEE C62.41.2, it does define three major types. Type 1 can be installed on the line side of the service entrance overcurrent device (no extra overcurrent device needed), which is similar to Category C. Type 2 is similar to Category B and can only be installed on the load side of the service entrance overcurrent device. Type 3 and Category A are point of utilization devices like a surge power strip that is plugged into a wall outlet. While UL types and IEEE categories are similar, they are not 100% interchangeable. UL Type 1 devices are often used in Type 2 locations. The benefit of doing this is that there is no extra overcurrent device needed.

How do you know what kA rating to use? The IEEE categories provide a good base for selecting kA ratings. There are many ¡°right¡± sizes for each category, but there must be a balance between redundancy and added cost. Qualified judgment should always be used when selecting the appropriate kA rating for an SPD.

Komm is the technical services supervisor for Mersen in Newburyport, Mass. He can be reached at david.komm@mersen.com.

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