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 more difficult:
1. Install the shunt lock at the door that is most brightly illuminated and most readily visible to passersby.
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.Continue Reading