Keri Systems EntraGuard Silver Quick Start - Manual (Page 26)

EntraGuard
®
Silver Telephone Entry Control
Quick Start Guide
Page 26 of 27 P/N: 01960-001 Rev. D
8.0 General Information on Inputs
A controller input detects a state change generated by a device outside the controller that may prompt a response from the
controller. Input devices that generate a state change may be normally closed or normally open. This section provides a
brief description of normally closed versus normally open inputs.
8.1 Normally-Closed
A normally closed input device continually keeps a circuit active or complete. A state change is generated when the
normally closed input device is forced open, breaking the circuit. In an access control system, a door switch is a typical
example of a normally closed device. While the door remains closed, the switch remains closed. When someone opens the
door, the door switch is opened, breaking the circuit and generating a state change. The controller then responds to the
state change and generates an output (such as sounding an alarm if the door is a secure door).
8.2 Normally-Open
A normally open input device continually leaves a circuit open, or incomplete. A state change is generated when the
normally open input device is forced closed, completing the circuit. In an access control system, a request-to-exit (RTE)
button is a typical example of a normally open device. In an access control installation, an RTE button is located on the
secure side of a door. While there is no one there pressing the button, the switch remains open. When someone desires to
exit through a secure door, they press the RTE button, closing the circuit and generating a state change. The controller
then responds to this state change and generates an output (such as unlocking the door to allow egress).
9.0 General Information on Safety versus Security with Door Locks
When installing a door lock there are two things to consider: safety versus security, or should the door be “fail-safe” or
“fail-secure”.
9.1 Fail-Safe Door Lock
Fail-safe means that if the power should fail at a door (perhaps due to a power outage or equipment failure), the door will
automatically unlock allowing entrance or egress. Power is required to keep the door locked. A fail-safe door ensures
people will be able to enter and exit a secured area through that door in the case of an emergency. A typical fail-safe
application may use a magnetic lock. In this application, the controller energizes the lock relay, causing the lock relay to
change its state. In its new state the normally closed circuit is opened breaking the power to the magnetic lock and
allowing the door to be opened.
9.2 Fail-Secure Door Lock
Fail-secure means that if the power should fail at a door (perhaps due to a power outage or equipment failure), the door
will automatically lock and not allow entrance but will continue to allow egress. Power is required to unlock the door. A
fail-secure door ensures a secured area remains secure regardless of the situation. A typical fail-secure application may
use a door strike. In this application, the controller energizes the lock relay, causing the lock relay to change its state. In its
new state the normally open circuit is closed activating the release mechanism for the door strike on the door to be
opened.