We all know that it is not so easy to choose a proper surge protection device. The parameter of a surge protective device is not like the parameter of a smartphone which is obvious and easy to understand for most people. There are a lot a misunderstandings when selecting a SPD.

One of the common misunderstanding is that the bigger surge current capacity (measured in kA per phase ), the better the SPD. But first of all, let’s introduce what do we mean by surge current capacity. Surge current per phase is the maximum amount of surge current that can be shunted (through each phase of the device) without failure and is based on the IEEE standard 8×20 microsecond test waveform. For example, when we talk about a 100kA SPD or 200kA SPD. We are referring to its surge current capacity.

Surge current capacity is one of the most important parameters for a SPD. It offers a standard to comprar different surge protection device. And SPD manufacturers are required to list the surge current capacity of their SPDs. And for customer, they also understand that a SPD installed at service entrance should have a higher surge current capacity comparing the SPD installed at branch panels.

So here comes the problem, many people believe that a 200kA SPD is better than a 100kA SPD. What’s wrong with this opinion?

First, it doesn’t take the cost into account. If the 200kA SPD costs the same as the 100kA SPD and other parameters are all the same, you should indeed buy a 200kA SPD. Yet the fact is, a 200kA SPD cost higher than 100kA model so we have to calculate whether the extra protection it provide worth the extra money.

Second, a 200kA SPD is not necessary to to have a lower voltage protection rating (VPR) than a 100kA SPD. VPR is the residual voltage that will impose to the downstream electrical equipment.

So are you saying that lower surge current capacity SPD is sufficient and the SPD with larger kA is only a waste of money.

No. How many kA should you choose is mainly depended on the application. Whether the protected asset is located in high, medium or low exposure location affect the size of the SPD you select.

IEEE C62.41.2 defines the categories of expected surges within a facility.

• Category C: Service entrance, more severe environment: 10kV, 10kA surge.
• Category B: Downstream, greater than or equal to 30 ft from category C, less severe environment: 6kV, 3kA surge.
• Category A: Further downstream, greater than or equal to 60 ft from category C, least severe environment: 6kV, 0.5kA surge.

So if you have assets in high exposure area, it is always better to choose a SPD with larger surge current capacity because the surge at this location is greater. So can I choose a less kA SPD at high exposure location. Technically, you can. But the problem is that the less kA SPD will soon come to end of life and then you have to purchase and re-install a new one. The maintenance cost may be higher than the SPD itself.

So it bring up another reason to use larger kA SPD. A larger kA SPD has a longer lifespan and thus save the time and cost for maintenance. For example, some of the telecom stations are located in remote area or even in mountain tops. A SPD protecting such facility should have a very long lifespan, better to be lifetime maintenance free.

Summary

In this article, we discuss the issue of the surge current capacity when selecting an SPD. Larger surge current capacity SPD does not offer a better voltage protection rating (VPR) and is sometimes not necessary when you take the extra cost into account.

Yet if your assets are located in high exposure area or the maintenance work is difficult or costly to conduct, then a higher kA SPD is desirable.