Dual-loop is a method of electrical circuit termination used in electronic security applications, particularly modern intruder alarms.
Its use became widespread in the early 21st century, replacing the basic closed-circuit system, mainly because of changes in international standards and practices.
[1] Dual-loop allows the burglar alarm control panel to read the values of end-of-line resistors for the purpose of telling a zone's status.
When the detector goes into an active state (i.e. a door contact being opened), the circuit path has been altered and it must now pass through a second resistor wired in series with the first.
If a resistance reading is not recognised by the system either due to short-circuit or open-circuit, an anti-tamper alarm will trigger.