Figure 8-1.Contacts are connected into the positive wire
only. Break positive wire only at door contacts.
connected to the two contact terminals, as shown in
figure 8-l. The neutral or common wire is not cut but
continues on in parallel with the positive or hot wire.
The pair is then run on to the next contact-a door,
window, or sensor-and again only the hot wire is
connected to the contacts. This procedure is repeated
until all contacts are wired in series, and then the pair of
wires is run from the last contact device on the system to
the protective-circuit terminals in the panel. Although
the markings will vary from manufacturer to
manufacturer, the terminals for the starting connections
will read something like LOOP POWER OUT, while
the terminating terminals will read IN, or a similar term.
Figure 8-2.Negative conductor is run with a positive
conductor to all contacts, even though the system would
operate with just a single-wire, positive-leg wire run from
contact to contact.
A simple circuit of the wiring connections just
described is shown in figure 8-2. Obviously, the
system would operate with just a single-wire,
positive-leg circuit run from contact to contact, with
the negative power-supply terminal connected
directly to the negative protective-circuit terminal
within the cabinet. However, manufacturers
discourage this practice since troubleshooting a
single-wire circuit can be extremely time consuming.
and the single wire is more vulnerable to defeat by an
intruder with no trouble symptoms occurring to warn
the user of the loss of protection.
An exit/entry delay relay is sometimes used on
security systems so that authorized personnel may
exit and enter (using their door keys) without
activating the alarm. However, a shunt switch is more
often preferred (fig. 8-3). The purpose of the shunt
lock is to enable an authorized person with a key to
shunt out the contacts on the door used for exit/entry,
allowing him or her to enter or leave the premises
without causing an alarm when the alarm system is
turned on. The shunt lock does extend outside the
protected premises; however: it is a potential weak
link in the system. Following the two procedures
suggested below makes defeat of the shunt lock much
1. Install the shunt lock at the door that is most
brightly illuminated and most readily visible to
2. Wire the shunt lock switch to the magnetic
contact terminals, as shown in figure 8-4. This
arrangement traps the lock so that any attempt to pull it
out to gain access to its terminals will break the
positive side of the protective circuit and cause an
alarm to sound.
Contacts used to signal the opening of doors,
windows, gates, drawers, and so forth are usually
mounted on the frame of the door or window, while
Figure 8-3.Typical shunt switch circuit.