From the research I have done on SPDs, it appears that it is recommended to layer or cascade the different types (types 1 through 4). For example, here is a video from Schneider Electric which explains the different types of SPDs. This seems to suggest that it is best to use all four types to protect against surges.

If I understand correctly, type 1 do not require an overcurrent protective device (breaker) whereas type 2 do require a breaker. In fact, the requirement of a breaker appears to be the only difference between type 1 and type 2 SPDs. This allows you to install a type 1 SPD on the line side of the home's load center, whereas type 2 SPDs need to be installed on a breaker inside the load center.

From what I understand, SPDs installed on the line side of the load center (type 1) protect from surges on the incoming power, whereas SPDs installed on the load side (type 2) protect from surges generated from inside the home (e.g., from an AC compressor).

But why wouldn't an SPD installed on the line side also protect from internal surges, and why wouldn't an SPD installed on the load side also protect from external surges?

Do type 1 and type 2 SPDs provide different kinds of protection? So if I install a type 1 on the line side of the home's load center (e.g., inside an outdoor rated meter / disconnect combo panel), is any additional protection offered by installing a second SPD (type 2) inside the home's load center on a 20A breaker (which is located on the other side of the exterior wall, just feet away from the meter)?

Specifically, I already own a Square D HEPD80 (type 1) and also a Siemens FS140 (type 2). If I install the type 1 at the meter / disconnect panel (i.e., to protect from incoming power surges), does installing the type 2 inside the main load center add a different type of protection (i.e., internal surges from appliances)? It seems to me that cascading SPDs in this way just adds more of the same protection from both sources of surges.

1 Answer 1


In short, Type 1 Protects from high voltage surges (it brings the voltage down to around 1 000 Volts), usually implemented as a spark gap. This will protect your house power grid from, say, lightning strike surge from air-hanging supply wires.

"But Mr. Stranger on the Internet", you'll ask, "1 Thousand volt will still kill my appliances!".

You are correct, and that is why Type 2 surge protector is installed (usually using varistors). They bring the voltage permitted to around 400V-280V-140V (depending what your regular supply is).
The stages need to have some wire between them (around 10m recommended), so the second stage doesn't get overwhelmed before first one kicks in to reduce the surge. The wire acts as coil (inductor) with those high voltage spikes, slowing them down.

Second stage usually doesn't need breakers before it until some very high current values (check manufacturer manual), but you can always install them if you have space. They prevent the scenario where wind hangs a phase wire over neutral, doubling your apparent house voltage - enough to have the surge protection on stage 2 to kick in - without breaker to stop that, it will burn out.

You should also use the Type 3 protection (aka "pc power strips") that further protect the sensitive devices. Plug your computers, TVs, phones into them.
If you have internet antenna, get a strip with filter for that as well.

  • Installation of a Type 1 requires getting the power company approval, and a temporary interruption of service. A worker has to go up in a bucket lift and disconnect your service drop, right? In my book, not a DIY job. A Type 2 is a DIY for someone who can install breakers. How much protection does a Type 2 give without a Type 1 also installed? Commented Aug 6, 2021 at 9:17
  • @Jim_Stewart It all depends how the service connection looks like. If there is enough distance between the service disconnect/meter and the main distribution panel, the Type 1 can be installed along the meter and Type 2 at panel. If meter and panel are in one box, a combined Type 1+2 surge protectors exist. They are not as good, but they exist. Neither of those are something I'd recommend a person to install themselves - its a job for electrician. They have proper tools and knowledge to verify ground impedance.
    – Thomas
    Commented Aug 6, 2021 at 9:37
  • Should a ground impedance measurement be made even for a Type 2, installed without a Type 1? Commented Aug 6, 2021 at 11:56
  • Generally, yes. You want electrician to check that Ground is below 10 Ohms, whether there even is ground (could be disconnected!) or its a shared Neutral-Ground, and connect it appropriately. Surge protector will work even without grounding rod if necessary - its job is to prevent voltage difference between Hot and Neutral.
    – Thomas
    Commented Aug 6, 2021 at 12:40
  • 10 Ohms between what points? 10 Ohms between gnd and neutral at a receptacle near the panel? 10 Ohms between something and one of my two ground rods? 10 Ohms between the end of my ground rod and the end of the power company's ground rod 40 ft away? Commented Aug 6, 2021 at 15:11

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