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I am looking for commercially available SPDs and came across two different options:

Option - 1: Return path is earth and there is an MOV on all the lines. Schematic would look like this:

mov spd

Option - 2: Return path is neutral. There are MOVs between line and neutral and there is a GDT between neutral and earth. Schematic as shown below:

gdt spd

Both of these products are manufactured by the same brand called Elmex (Catalog here).

I could find products from other brands falling into both categories. Which one is a better wiring option for a residential environment?

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  • \$\begingroup\$ What type of over-voltage threat are you trying to protect against and what is the thing (the potential victim) you are trying to protect? \$\endgroup\$
    – Andy aka
    Sep 13, 2022 at 11:21
  • \$\begingroup\$ @andyaka - Its a general protection for regular house hold electronic equipment (consider lights, tv, refrigerator, music system etc) against surges that come mostly during lightning strikes during rainy season. These surge voltages range from 1 kV spikes to 4-5 kV typically. It can be worse but I am yet to experience personally. The SPDs mentioned in the question are able to save the equipment against 8/20 uS waveform of 6 kV (i didn't test higher). So seems like adequate for the job. I am just confused about the right wiring since there are two prominent connection diagrams out there. \$\endgroup\$
    – Whiskeyjack
    Sep 13, 2022 at 11:29
  • \$\begingroup\$ As far as I know, if you are protecting against indirect lightning, you would want to protect between live and earth. \$\endgroup\$
    – Andy aka
    Sep 13, 2022 at 11:51
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    \$\begingroup\$ I’m voting to close this question because it should be migrated to DIY.SE \$\endgroup\$
    – Elliot Alderson
    Sep 13, 2022 at 12:53
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    \$\begingroup\$ This assumes that you do have switchboard Gnd-Neutral links in place.. The following may not be much use, but: I've not met this question before BUT I'd feel that to protect against line to line faults the neutral return one would be better BUT for lightning which may well peak on P & N simultaneously that the Ground return one would be better. || However as N and Gnd are usually bonded at the switchboard there may not end up being much difference. || If I HAD to choose one without further research I'd probably choose the Ground return one. . \$\endgroup\$
    – Russell McMahon ♦
    Sep 20, 2022 at 11:33

1 Answer 1

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From page 3 of this catalog, it looks like the choice of configuration is based on whether or not there's a RCD/GFCI device. The first configuration (earth return) is for placement upstream of a RCD. The second configuration (neutral return) is for placement downstream of a RCD to maintain galvanic separation between neutral and earth.

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  • \$\begingroup\$ Vir, thank you so much for your response. I think config-1 is forbidden downstream of RCD/GFCI because MOV between earth and other lines will have some continuous leakage current going to earth line, and will cause nuisance tripping, especially after MOVs have taken a hit and it got partially damaged. Config-2 has GDT and will ensure there is no leakage current unless there is a surge event. Config-2 also gives a flexibility to be used upstream or downstream of RCD/GFCI, I believe. Thanks for linking the catalog. \$\endgroup\$
    – Whiskeyjack
    Sep 14, 2022 at 5:05

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