Sports Helmet (5e Equipment)
From D&D Wiki
A sporting helmet of the sort used by people such as hockey players, skaters, snowboarders, and climbers. It provides a good deal of protection against falls and blunt impacts.
Partial armor only grants its AC bonus if you are wearing light or no armor. If you are wearing only one piece of partial armor, you are still considered to be wearing your base armor type (e.g. no armor or light armor). If you are wearing two or three pieces of partial armor, you are considered to be wearing the heaviest type of partial armor worn (e.g. medium or heavy armor). You cannot wear four or more pieces of partial armor.