Rule 16 of 40 · Chapter III — Grounding, Bonding, and Protection
Never defeat the third pin
Why this rule exists
The grounding pin on a plug connects an appliance's metal body to the bonding system. If an internal fault energizes that body, the ground path carries the current and trips the breaker instead of waiting for you to become the path. Cut off the pin, use a cheater adapter without bonding it, or break the ground wire, and the first thing to complete the circuit when the case goes live may be your hand. The third pin is not optional hardware; it is the difference between a tripped breaker and an electrocution.
In practice
Use grounded three-prong plugs in grounded receptacles, and never remove or bend back the grounding pin. Do not use three-to-two adapters unless the tab is actually bonded to a grounded box, and verify it. If an appliance's cord ground is broken, repair it before use. When replacing receptacles, confirm the ground is present and connected with a receptacle tester, and fix bootleg grounds rather than hide them. A double-insulated tool with two prongs by design is fine; do not force a ground onto it.
When it doesn't apply
Double-insulated tools and appliances are listed to operate safely without an equipment ground, using two layers of insulation instead; their two-prong plugs are correct. Ungrounded two-wire systems in older homes may legitimately have two-prong receptacles; the fix is GFCI protection and proper labeling, not a bootleg ground. When in doubt about whether a ground exists, test, do not assume.