Keri Systems SB-293 - Manual (Page 35)

Keri Systems, Inc. Technical Reference Manual – PXL-250 and SB-293
Revision 5.5 P/N: 01836-004 Page 35
8.2 Output Relays
In many respects, a Form C output relay performs the opposite task of an input. An
input detects a state change generated by a device outside of the PXL-250 or SB-293.
An output relay, however, receives a signal from the controller that energizes the
output relay, switching its state. This state change typically prompts an action outside
of the controller. In most cases, inputs create state change signals that drive output
relays.
An example of this process is when a secure door is forced open. As the door is
opened, the door status switch opens. The door status switch input detects the switch's
state change. Programmed instructions within the controller send a signal instructing
the alarm relay to energize. The alarm relay switches its state to activate an audio
alarm notifying everyone in the immediate area that the door has been forced open. A
variety of devices may be activated by an output relay such as an electric door strike, a
magnetic lock, an alarm, a light, or a video camera.
A Form C relay has both normally closed and normally open circuits. When the relay
is not energized, the normally closed circuit is closed (allowing current flow) and the
normally open circuit is open. When the relay is energized the circuits switch roles; the
normally open circuit is closed (allowing current flow) and the normally closed circuit
is open. This dual nature of Form C relays (having both normally closed and normally
open circuits) allows for two types of applications outside the controller. A device may
be attached to the normally closed circuit so that it is always on until the relay
energizes to open the circuit and turn it off. Or, a device may be attached to the
normally open circuit so that it is always off until the relay energizes to turn it on.
8.2.1 Lock Relay
Unlocking a door is controlled by the Form C Lock Relay. When installing a door lock
there are two things to consider: safety versus security, or should the door be "fail-
safe" or "fail-secure."
8.2.1.1 Fail-Safe 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 and exit.
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. Refer to Figure 9-10 on page 52 (for the PXL-
250) and Figure 10-3 on page 73 (for the SB-293) for possible fail safe Lock Relay
wiring diagram.