I'm currently designing a circuit that will use a microcontroller and several relays to switch multiple loads connected to AC mains, and the connections to mains will be inside the enclosure. I'm considering using a plastic enclosure for the circuit. Each load has a protective earth conductor.

Is it good practice to wire together the protective earth conductors of each load inside the enclosure using a terminal block?

  • \$\begingroup\$ There may be impulse EMI from load switching and coil switching unless you choose appropriate snubbers and isolation to uC circuits for gnd conducted and loop radiated noise. Reverse diode for DC coils and RC rated snubber for AC loads. EMI is complex unless well isolated and needs much more info. \$\endgroup\$ Nov 30, 2017 at 21:45
  • 1
    \$\begingroup\$ What is your alternative? \$\endgroup\$
    – Trevor_G
    Nov 30, 2017 at 22:05
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    \$\begingroup\$ What else would you wire them to? \$\endgroup\$
    – user253751
    Dec 1, 2017 at 1:27
  • \$\begingroup\$ Is this just for yourself, or are you designing a product for mass production? \$\endgroup\$
    – user57037
    Dec 1, 2017 at 4:08

1 Answer 1


Using a plastic enclosure is problematic for mains grounding and safety, so I say use a metal enclosure. Ground the metal enclosure. Use standard knockouts for your entry and exit of AC mains cable or conduit, with correct cable clamps. Everything done with mains must be to Electrical Code rules, which are 90% about installation methods and packaging.

Some enclosures even have DIN rail built right in.

You can get appropriate metal enclosures at an electrical supply house. If they have a warehouse full of junction boxes, conduit, and spools of wire, that's the place.

If their enclosure doesn't have knockouts in all the right places, they can sell you a drill or punch that'll make standard sized knockout holes.

For small stuff, a 4" or 120mm square box is very handy.

Lastly you don't need to have one grand big box that has all the mains switching, it's a lot easier to run a thermostat wire and bring the low voltage to the mains than the other way 'round. So feel free to have several smaller boxes in convenient locations if that makes sense.


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