Securitron DPA_Series - Manual (Page 4)

PN# 500-15700
Page 4 Rev. C, 04/11
5.1 FORBIDDEN CONDITION DETECTION
If you have both a door status switch and lock status switch available,
you can employ the DPA in a higher security mode that includes a
unique type of tamper detection. To understand this feature, you must
first think about how a lock status switch and door status switch work
together. We know a door can be closed but not necessarily locked. So
it's possible for a door status switch to be reporting secure while a lock
status switch is reporting insecure. The opposite, however, is not
possible. A door cannot be reporting locked from its lock status sensor
and yet reporting physically open from its door status sensor. We call
this the "forbidden condition". The only way that such an event could
happen (assuming the status switches have not failed) is if someone
tampered with the lock status sensor to make it read secure regardless
of the true state of the lock.
This exact problem is a very real security risk. Consider the case of a monitoring strike which
detects the presence of a panic bar latch in the strike. This is a type of lock status detector. An
individual can easily stuff material into the strike which will jam the switch down so that the lock
status sensor "thinks" the latch is secure and in place when its not. He then is free to use the
door later without creating an alarm. More sophisticated lock status sensors such as are
available as options on electric locks can also be fooled, although with more difficulty. One way
is to gain access to the wiring and short it. Another way with magnetic locks (for instance) is to
manage to cut the strike plate's mounting to the door so that the magnet is holding the strike
plate but the door is still free to open.
When you connect a door status switch to terminal IN and a lock status switch to terminal LS,
the DPA automatically monitors any occurrence of this forbidden condition. Be sure to remove
the factory jumper from "+" to IN. The lock status line will have been tampered so as to
falsely report secure but when the door opens and this is reported to the DPA via the door status
sensor, the board will go into instant alarm as follows:
Relay #1 will energize so long as the door is open. This will sound the local alarm
Relay #2 will "flash" twice per second
Relay #3 will change state
This dramatic alarm signal is a latching condition. To recover from it, the door must first be reclosed
and
then the unit given a reset input. Security personnel should then investigate to determine if
tampering with the lock sensor has occurred. The fastest way to do this is simply to open the
door a second time. It can then be checked if the problem is tampering or failure of the status
sensing components.
6. RESET
We have talked about "resetting" the DPA several times
earlier in this manual. Reset recovers from two latching
alarm conditions: the standard fourth stage condition at
the end of time range two and the special forbidden
condition alarm signal (see Section 4.1). The board will
receive a reset signal when +V is input to terminal RS.
There are two different approaches to the reset function.
The first is with a separate switch. This is often a
keyswitch at the door with N.O. contacts connected
between "+" and RS. The advantage of this is that once
the unit has gone into latching alarm, an authorized
person with a key is required to go to the door and personally verify that no
security problem is occurring.
If such a person is not available, the DPA can be wired for a type of automatic reset. Simply
connect terminals RS and LS together. Since LS has power on it when the door is secure, RS
will too. When the door becomes insecure, the board will begin to go through its timing stages
and RS will have no power on it. If the fourth latching alarm stage is reached, recovery is by
+
LS
BP
FE
IN
RS
DC
DOOR STATUS
SWITCH CONNECTS
TO "IN"; LOCK
STATUS TO "LS".
BOTH ARE
CLOSED
WHEN
SECURE.
+
LS
BP
FE
IN
RS
DC
USE N.O.
SWITCH
FOR
MANUAL
RESET
+
LS
BP
FE
IN
RS
DC
CONNECT
LS TO RS
FOR AUTO
RESET