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I am making a switch to control a desk light using this pilot relay. My module is plugged into a half-hot wall outlet that's controlled by a wall switch.

The module uses a momentary-on switch to energize the relay, which then holds itself closed until a momentary-off switch breaks the connection. Additionally, removing power from the module (turning off the wall switch) resets the relay so I can turn my lamp on or off from either the wall or the desk.

My question is: what is an appropriate safety feature for such a module, since the switches will be conducting 120 VAC, and my shock-intolerant fingers will be using the switches?

The switches are in an insulated housing, and there is a 3-conductor 28 AWG insulated cable connecting the switch housing to my module. Assume I am paranoid about liquid spill onto the switches or damage to the connecting cable.

Fusing the entire module wouldn't work, since it's driving up to a 10 A lighting load. I could conceivably fuse all three wires in the cable.

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  • \$\begingroup\$ What is the relay coil voltage, or what voltage are the switches actually switching - I hope your #28 wires are not carrying 120 VAC. \$\endgroup\$
    – Peter Bennett
    Feb 23, 2020 at 6:09
  • \$\begingroup\$ Yes, everything operates at 120 VAC (no transformers or regulators). The wire/switches do carry that voltage, but the current is about 30 mA, so I don't think ampacity is a problem. Is there another consideration? \$\endgroup\$
    – abatea
    Feb 23, 2020 at 15:56
  • \$\begingroup\$ If the cable is rated for 120 VAC use there should be no problem. But if you are recycing an old mouse cable, or something like that, is would not be rated for 120 VAC, and the insulation may break down. If the switches are rated for 120 VAC use, there should be no problem from them. \$\endgroup\$
    – Peter Bennett
    Feb 23, 2020 at 16:47
  • \$\begingroup\$ Okay, thanks. The switches, wire, and enclosures are all OTS parts and properly rated. Do you think I am being overly cautious by wanting some additional safety feature? Fuse? GFCI? I'm interested in how to prevent shocks in some simple failure cases. \$\endgroup\$
    – abatea
    Feb 23, 2020 at 19:15

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If you are worried about shocks, then you need a GFCI protecting the circuit. A fuse is no guarantee of protection as 30mA can kill a human, and you won't get a fuse to blow at that current.

But the main thing is to design the enclosure properly, and use appropriately rated switches for the voltage. If the switches aren't fully insulated, then ground the metal bits.

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