Utilize references to your answer.
A physical protection system is something that physically prevents access. A bullet-proof vest is a physical protection device. It is a real object that prevents bullet access to the body (hopefully). In the old days, body armor was the "tin-can" variety (armor)that strictly limited mobility, and weighed a ton. http://inventors.about.com/od/bstartinventions/a/Body_Armor.htm
Your skin is your body's physical barrier to disease organisms, like physical barriers prevent bad people from getting in a structure or a property. http://www.humanillnesses.com/General-Information-and-Infectious-Diseases-A-Co/Body-Defenses.html
Other physical protection devices include such things as physical barriers, similar to the bullet-proof vest example - in medieval times, castle walls and water hazards, called moats, that provided physical protection and made access to the protected structure difficult. So did locating your fortified castle near or on a water source so that you always had access to water, and on the top of a hill so you could see enemies approaching. Small windows kept most of the arrows out, especially the flaming ones. They were another physical protection device, ...
Discussion of historical and modern physical protective devices, with examples and reference URL Web sites for further learnign.