Serious defects in alarm system design and installation commonly result from the incorrect termination of end-of-line resistors (EOLRs) within the control panel set on both normally open (NO) and normally closed (NC) circuits.
Surprisingly, some in the alarm industry erroneously defend this configuration, stating the manufacturer gives them the option to employ EOLRs on the loop or not. In fact, manufacturer specifications, as well as UL and industry standards, call for EOLRs to be placed at the end of each of the protection loops.
It is also sometimes argued that the installer does not have to heed the manufacturer’s instructions if the system is not UL certificated or listed. Indeed, this is an example of junk science.
Imagine a fire engulfing the home of a subscriber and, tragically, several deaths occur. Suppose during the investigation it is determined that the fire alarm system did not activate because the circuit wiring between the control panel set and the automatic fire alarm initiating devices was damaged.
Also suppose it is determined that the EOLR is found to be terminated within the control panel set, in parallel with the normally open fire protective loop circuit. In other words, even though the damage to the fire alarm protective loop circuit was accidental, there was no way the subscriber and remote station could have been alerted to this critical loss of functionality.
It is also important to understand the same issue holds true for burglar alarm protective loop circuits. If a loop is accidentally or intentionally shorted out—and the affected nonfunctional zone protects an entry point or interfaces with an interior motion detector—a criminal could enter the premises undetected.
Essentially, this type of methodology has now directly increased the integrator’s liability. If properly installed, the system would have otherwise detected this event, and alerted the subscriber and remote station when the loss of functionality occurred.
The EOLRs are included with the equipment to serve as an intelligent safeguard for each active zone of the alarm system. The customer paid for the equipment and there is not a single disadvantage in utilizing end of line resistors consistently on each and every installation.