Protection from a quality means the protected permanent cannot be the target of spells or abilities with that quality, cannot be enchanted or equipped by permanents with that quality, cannot be blocked by creatures with that quality, and all damage from sources with that quality is prevented. This is summarized by the acronym DEBT (Damage, Enchant/Equip, Block, Target), defined in CR 702.16b.
Equipment cards are artifacts by definition (CR 301.5 states all Equipment are artifact cards). Because the creature has protection from artifacts, the "E" in DEBT applies — it cannot be equipped (attached to) by any artifact Equipment. If you attempt to activate an Equipment's equip ability targeting that creature, the ability is illegal to activate because the creature is an invalid target (CR 702.16b, 508.1c).
Furthermore, if a creature somehow gains protection from artifacts while an Equipment is already attached to it, the Equipment immediately falls off (becomes unattached and put on the battlefield) as a state-based action, per CR 704.5m, which checks that Auras and Equipment are legally attached.
Example: Your opponent controls a Sword of Fire and Ice (an artifact Equipment) and tries to equip it to your creature that has protection from artifacts. The equip ability cannot legally target that creature, so you can simply point out the creature is an illegal target — the ability cannot even be activated.
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.