The sturdy Protectron model of robot was created before the great war, and served as a security automaton in both the private and business sectors.”— Fallout 3 loading screen
Created by the RobCo corporation, protectron robots were designed (as their name suggests) for protection, although they can also be seen being used in a manual labor role as well. Typically used in office-type settings, their secondary programming is modular, making them useful as greeters, ticket collectors, bartenders, celebrity or historical figure impersonators, even sexual partners. In terms of combat, protectrons are usually weak and easy to defeat, because of their weak weapons and slow movement speed.
Some attributes of the protectron's looks were derived from 1950's comic books, which keeps with Fallout's overall retrofuturistic theme.
In Fallout 3, as well as in Fallout: New Vegas, protectrons are the second weakest robot variant encountered, with only the Eyebot having less firepower. They do not pose much of a threat, due to their weaker weaponry and slow speeds. They are equipped with dual hand-lasers as well as a head-laser, so that in the event that one weapon is disabled, the protectron can still function in battle. They are not as common in Fallout: New Vegas as they are in Fallout 3; they are in every metro station in the latter, whereas in Fallout: New Vegas, they are mostly found patrolling a few office buildings and the Securitron vault.
Factory protectrons are protectrons designed for use as manual labor inside of factories. Computer entries inside Sunset Sarsaparilla Headquarters in Fallout: New Vegas mention robots being shipped to the factory to replace the human workers. This is probably the case in Fallout 3 as well, and a few examples of these protectrons can be found inside The Mill in the Fallout 3 expansion The Pitt, although they are already deactivated.
Metro protectrons are a model of protectron found in Washington, D.C.'s metro stations that will ask for the player's metro ticket when activated, and the player's metro ticket will be removed from their inventory. If they do not happen to have a ticket in their inventory, the protectron will attack.
Nuka-Cola security are protectrons found in the Nuka-Cola plant. They have the Nuka-Cola Corporation logo on them as well as red coloring around their head and hands. A deactivated one is found in Fallout: New Vegas, in the Lucky 38 penthouse.
Outcast protectrons are protectrons reprogrammed by the Brotherhood Outcasts. They are found patrolling with Outcast patrols or outside of their bases.
These factory protectrons are found in the Pitt steelyard and were being used as a replacement for human labor within the steel mills. They are heavy duty industrial robots, featuring a reinforced "head" part, a protective cage on the back and distinctive yellow coloring.
Shipping workers are protectons found in the Sunset Sarsaparilla Headquarters. There are only two of them, and they can be found on the bottom floor of the factory with the bottle cap press machine.
Sunset Sarsaparilla security are a slightly tougher variant of the protectron, of which three are still functioning, and can be found patrolling inside the Sunset Sarsaparilla Headquarters. Their function was to protect the factory, possibly against industrial espionage. Several disabled models can also be found within the factory and scavenged for parts.
The protectron Mark series was an attempt by scientists in Big MT to improve on the base model of protectron built by RobCo. These protectrons come in six versions, Marks I-VI. Mark I is on par with the protectrons found in the Sunset Sarsaparilla Headquarters, while the other marks are scaled on increasing durability based on the players level. However, the firepower on each Mark series is the same.