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The 8-position RJ45 has shorting bars to complete the circuit even if there is no cord plugged into the jack.
This assures that if there is an alarm condition, the alarm panel can have exclusive access to the phone line by
cutting off the remaining inside jacks.
2. The alarm jack must be installed after any POTS splitter in the circuit, as seen here.
3. For POTS only service, the circuit bypasses the splitter (as shown by the red dotted line) and continues
directly to the alarm jack.
Figure 65: Circuit from POTS to Alarm Jack
Let’s take a more detailed look at the alarm jack wiring as shown below.
The alarm jack is the demarcation point between the telephone service and the alarm equipment.
It is should be placed before the star connection point.
In some cases, an alarm company may require it in another location; this is not recommended since it may
require inside wire to be run back to the star connection point. The greater the length of wire used, the
greater the potential for damage, rendering the alarm inoperative.
Alarm demarcation allows service to remain uninterrupted. Dial tone is on the pair from the protector. An
internal shorting bar loops the circuit to inside set runs. An alarm plug removes the short and loops the circuit
through an alarm dialer. When the dialer activates, it opens a circuit that removes the telephone sets from
the circuit, so that any burnt set runs or phones that are off the hook do not prevent the outgoing alarm call.
Since xDSL is often impaired by alarm equipment, the xDSL splitter is placed between the 356 protector and
the alarm jack, which means the set run to the xDSL modem does not go through the alarm jack.
Never plug a telephone jack or banjo into an alarm jack.
MDU 25 in multi-dwelling unit
It can be used as a single module or in multiples of 25 and is located in the MDU main terminal room, or MTR,
as seen here.
The MTR, is the cross-connecting point of incoming cables from the telecommunications external network
and the MDU cabling system. You will also hear it referred to as the main telephone room.
