Protection from a quality grants four specific shields, often remembered by the acronym DEBT: the creature can't be Damaged, Enchanted/Equipped, Blocked, or Targeted by sources of that quality. The 'E' explicitly covers Equipment, so a creature with protection from artifacts cannot have an artifact Equipment attached to it. (CR 702.16b)
If someone attempts to attach an artifact Equipment to the protected creature — whether by casting an equip ability, by a spell, or by any other effect — that attachment is simply illegal and does not occur. If the Equipment is somehow already attached (for example, protection was gained after attachment), the Equipment immediately falls off as a state-based action and goes to the battlefield unattached. (CR 702.16e, 704.5n)
Note that protection from artifacts applies to all artifact sources, not just Equipment. Damage from artifact sources is prevented, artifact spells can't target the creature, and artifact creatures can't block it. The Equipment rule is one part of this broader shield.
Concrete example: You control a Darksteel Colossus equipped with a Sword of Fire and Ice. Your opponent's creature gains protection from artifacts. The Sword immediately becomes unattached and sits on the battlefield unequipped as a state-based action, because the creature can no longer legally have an artifact Equipment attached to it.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.