I'm working in the United States. I'm looking into working with mains power, and I cannot for the life of me figure out where the ground line connects. For example, a simple power regulator from mains power: linear AC-DC power supply with transformer, rectifier, smoother and regulation Assuming that I'm using the standard mains, there are three pins. Two are "hot", and one is "ground". I understand that the two "hot" pins carry the AC and the ground pin is used to divert current in the case of a short circuit (I think?).

I've heard people say that "ground" should be attached to the case.

Where do I solder the ground line, with a metal and a non-metal case?


2 Answers 2


Two are "hot", and one is "ground".

This is incorrect. One is "hot". One is "neutral", and the third is safety ground.

My question is: if I'm building the circuit shown, where do I solder the ground line?

If the case is metallic, you should bond the ground wire to the case. This prevents the case becoming energized and electrocuting your users. (Because if the hot wire comes loose and contacts the case it will throw a breaker)

If your case is plastic and is "double-insulated" then no ground wire connection is required.

Since the ground wire is usually only connected to the case, it isn't normally shown in schematics for the circuits inside the case.

  • \$\begingroup\$ Note: In places that need more power than usual (like the utility room for washing machines and tumble driers ) it is possible to have 2 hot connections. (annsgarden.com/poles/JP0-PS-Schematic.jpg) the Power Company Single Phase Secondary is center tapped. The center tap goes to ground and is called Neutral. The upper and lower taps can go with a neutral to different parts of a building supplying 120V. But top and bottom taps can go to devices that needs the higher voltage / power. \$\endgroup\$
    – Spoon
    Jun 12, 2015 at 7:49
  • \$\begingroup\$ industrial-electronics.com/elecy3_18.html \$\endgroup\$
    – Spoon
    Jun 12, 2015 at 7:57
  • \$\begingroup\$ The difference between a hot and a neutral in a two wire feed is that the neutral is tied to ground somewhere somehow (resistor, capacitor and short are most common) \$\endgroup\$
    – hildred
    Jun 12, 2015 at 8:37

There is no connection to earth ground here, and one is not needed. In fact this is typically an advantage of using a transformer like this--it isolates your circuit from what may be a noisey ground.

You can connect the earth pin to the chassis if you want. That will help with noise pickup, if the chassis is metal. In the event the live wires come loose somehow and touch the case, it's also a nice safety feature: the voltage will be shunted to ground. If you're using plastic casing, you can just leave the earth wire disconnected.

  • \$\begingroup\$ With a plastic case, you could bolt the earth onto the body of the transformer. That would give you an extra layer of protection. \$\endgroup\$
    – F. Bloggs
    Jun 12, 2015 at 17:51

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