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A door chain, security chain, or security door chain consists of a small chain attached to the door frame, which attaches to a track on the door for security purposes. It is a type of lock that is often used along with other types of locks to secure a door.
Door chains are mainly intended to allow a person inside a structure to open the door slightly for purposes of communicating with individuals outside or exchanging small objects through the door while still preventing the individuals outside from gaining unauthorized access into the structure.
Door chains also can be set in place when the door does not need to be opened. They can only be touched from inside when the door has been opened, and therefore, if the locks on the door are compromised, a door chain may cause some delay, draw more attention, or leave more evidence of force being used when gaining unauthorized access to the structure.
Door chains installed on hollow-core doors, or that are not anchored into solid materials (such as the wood door frame) do not provide protection. This is because the chain can easily be defeated by pushing on a door. The chains are helpful to sound an alert to the person inside that someone is breaking through a door. The noise may be heard by the occupants giving them time to call for help, escape via another exit, or get a weapon to protect themselves.
Door chains are fairly easy to install and use. Use of a door chain provides a little security when dealing with strangers. A properly installed door chain is resistant to tampering from the outside requiring use of physical force to open the door, which will leave obvious evidence.
Door chains normally come equipped with very short screws which easily rip out. Any amount of force capable of defeating a deadbolt will also defeat a door chain. The tracks are usually straight, and any number of simple tools will allow the chain to be separated from the track. Elderly individuals, uncoordinated people, and individuals with hand tremors may find it difficult to connect the chain to the track. The chain is often a thin metal. Door chains are often installed improperly, making them easily subjected to a number of attacks.
UPVC/PVCU front doors are generally unsuitable for retrofit security devices. Not only is the material not strong enough to support devices fitted with steel screws unless secured into the internal metal framework, but such changes to the original design may invalidate an existing warranty or possibly damage the integral locking assembly.