Three-prong outlet

The round slot connects to the building's grounding path, so a fault in an appliance can flow to ground and trip the breaker. OSHA says receptacles with grounding contacts shall have those contacts effectively grounded. It also says grounding-type receptacles are used where a grounding means exists in the box.

A three-prong face on an old outlet does not prove a ground is present. A tester or a licensed electrician must confirm it. See ungrounded outlet for older homes. The ground wire is the equipment grounding conductor. See the home electrical safety checklist.