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I have this grounding mat, inside the electrical socket as shown in the picture, there are a capacitor and 100k ohm resistor connected serially to the earth point, the grounding mat is supposed to get negative electrons from the earth to our body, to neutralise our body free radicals, I can understand the 100k resistor is the same as resistance as our dry human body, to limit the current, but I don't understand why they place the capacitor, as it is open circuit, it will not be able to get an electron from the earth. Any reason why there is a capacitor?

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    \$\begingroup\$ I'm not sure I follow what exactly you're trying to do with free radicals, but I'm pretty sure that's not a capacitor. \$\endgroup\$
    – Hearth
    Nov 8, 2018 at 4:26
  • \$\begingroup\$ the free radical in our body is positive charge, and it causes cell damages, that is why we take antioxidants, the electrons from ground is to neutralise them. (same function as antioxidant) \$\endgroup\$ Nov 8, 2018 at 4:40
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    \$\begingroup\$ can that be a fuse? \$\endgroup\$ Nov 8, 2018 at 4:42
  • \$\begingroup\$ thanks, i think i know what is that, en.wikipedia.org/wiki/Resettable_fuse \$\endgroup\$ Nov 8, 2018 at 4:45
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    \$\begingroup\$ "negative electrons" as opposed to positive electrons? \$\endgroup\$
    – winny
    Nov 8, 2018 at 10:52

1 Answer 1


The resistor's there for your safety

Static-dissipative grounding apparatus (such as wrist straps and mats) will always have a high-value resistor between you and the metallic connection to the earthing system. This is because most of the time, you will be connecting this to the mains earth in your building via a mains receptacle, and a direct connection could prove quite disastrous if you connected to an improperly wired receptacle (where the ground terminal on the receptacle was connected to something other than a ground wire).

The other component is indeed most likely a polymeric-PTC resettable fuse, by the way. (Many static-dissipative setups get along just fine with resistors alone. I can't tell from here if they added it due to concern about resistor power or voltage ratings, or for personnel-safety reasons, though.)

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    \$\begingroup\$ It will also limit the current through the body should you come in contact with mains voltage while standing on the mat. It may also limit the current in a genuine static discharge event and lessen the pain - although the resistor value looks a bit low for that. \$\endgroup\$
    – Transistor
    Nov 8, 2018 at 7:32

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