Electrical – How is a a two phase, three wire, grounding plug such as a NEMA 6-20P safe to use

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I'm looking to use a piece of commercial equipment in my home, and I've been reading about AC current and circuits. I currently have access to an old style non grounding 10-30 style plug, which I understand is unsafe to adapt this device to plug into, because it has no ground.

I've been reading that during normal operation, current is supposed to use the neutral as a return path, and in the event of a neutral fault, ground is supposed to become the return path, which trips the breaker. My question is, if the 6-20 plug doesn't have a neutral, and only has two hots and a ground, doesn't that mean that the ground wire is used as the return path, and the stainless enclosure of this appliance is ungrounded?

How is this safe?

EDIT: The outlet I currently have is for an older dryer, and is a NEMA 10-30. The plug for the new device is a NEMA 6-20p, and it's for an induction cook top.

Best Answer

I've been reading that during normal operation, current is supposed to use the neutral as a return path

While that statement is correct, it is also incomplete. Think about it this way.

A US standard 120 volt circuit is like a guy cutting down a tree using a hack saw. The guy is the live wire and he delivers power to the device, the saw. The neutral wire is the empty space on the other side of the tree where the saw sticks out; if the empty space didn't exist then the saw couldn't move.

A US standard 240 volt circuit is like 2 guys cutting down a tree with a saw that has handles on either end. Both guys are live wires pulling the saw, but taking turns. Their movements are out of phase. No neutral is needed because when one guy is pulling, the other guy is pushing.

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and in the event of a neutral fault, ground is supposed to become the return path, which trips the breaker

No. This is wrong. It sounds like you've incorrectly summarized what can cause a breaker to trip, and the purpose of a ground wire.

  • Overcurrent: If too much current flows out of a breaker, it will trip.
  • Ground fault: If the current flowing out of the breaker on the live wire does not match the current flowing back in through the neutral, then it must be going somewhere else, like through you. That will trip a GFCI breaker or device.
  • Arc fault: When a wire breaks, it can cause a spark. If the wires remain close enough, a continuous spark (or arc) can be maintained. Arcs send out radio frequency noise that can be detected at the breaker, which will then trip.

The purpose of the ground wire is not to cause a trip, but to instead provide a better path for the electricity to flow if, for example, the case of a metal appliance become electrified because of a broken wire. Since your body presents a higher resistance to electricity than a copper wire, the ground wire will take the majority of the current, thereby sparing you the brunt of the shock.

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