What Resistance Should Security System Contacts Have?

The resistance that a security system contact should have is dictated by the security system being used. Most of the hardwired security systems offered by Alarm Grid use 2K resistors. Some exceptions are the VISTA 128BPT and the 250BPT, which use 10K resistors on their expansion zones.

Wired security zones are supervised with the use of End of Line Resistors. When an EoL Resistor is used, a cut or a short on the wire between the last contact and the alarm panel will result in either a trouble, a fault or an alarm. The resistor needs to be wired in series or parallel depending on the type of loop the device operates with.

A security device that works with a normally closed loop will need its resistor to be wired in series. Most wired security devices, such as door and window contacts, use a normally closed layout. To wire a resistor in series, the resistor must wire in line with the security zone. This requires that the resistor splices into only one wiring leg of the security zone.

A security device that uses a normally open loop will need its resistor to be wired up in parallel. Life-safety devices, such as heat and smoke detectors, typically use a normally open wiring loop. To wire a resistor in parallel, each leg of the resistor needs to splice into each leg of the zone wiring.

With the resistor installed near or at the security device, it can provide supervision on the security zone as intended. If the wiring for the security zone is tampered with, the system will see it based on the change in resistance from the expected level. This won’t prevent tampering, but it will allow the user to know if a tamper has occurred. If the tampering happens while the system is armed, in most cases, an alarm will be reported.

For systems with no programming option to remove the End of Line Resistor, if you're not concerned with wire supervision, the resistor can be installed at the panel. To install a resistor in series at the panel, one leg of the resistor can be installed on the panel zone terminal, and the other end can splice to that leg of the zone wiring. To install a resistor in parallel from the panel, install each leg of the resistor under each of the zone terminals.