When troubleshooting legacy alarm systems, be aware that protective looped circuits previous may have been built into the system at either panel or sensor by original installers.
Complex zone loop arrangements are used to report loop events at the panel – some of the most common types include open loops, single closed loops, open and closed loops, double closed loops, double closed loops grounded, double closed loops with supervised grounds, end-of-line terminator loops, break and cross loops and distributed resistance loops.
Each loop type will offer different functions and security levels, and each will react differently to different faults in a line. Making things more complex, some loops will be normally open and some normally closed – relying on intuition may lead you to erroneous conclusions.
While modern installers might use complex zone loops, these arrangements were more common with older systems, which required greater installer creativity when eliminating problems caused by gaps in controller, power supply and sensor reporting capability.
A well-installed system will have diagrams showing where and what loops exist and if such diagrams don’t exist, it’s good practise to map out what you discover and leave the patterns you roughed out taped in the panel lid for the benefit of the technicians coming behind you.
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