Well, for starters, the EOL Resistor (End of Line Resistor) is supposed to go at the end of the loop, as the last device for both security alarm systems and fire alarm systems. However, even though the manufacturer's installation instructions show the EOL Resistor at the end of the circuit, away from the panel, installers of security systems don't always follow this practice.
Security devices are switches - either they are normally held closed so they continue the electrical path during non-alarm conditions, or they are normally held open so they don't short out the loop during non-alarm conditions.
Zone inputs to security panels have three conditions.
- Zone Open - Alarm - No electrical current passes through the zone.
- Zone Normal - Not in Alarm - Some electrical current passes through the zone. All current also passes through the end of line resistor. The end of line resistor limits this current to prevent a shorted zone condition.
- Zone Shorted - Alarm - All current bypasses the end of line resistor.
The ones that are normally held closed, like most door and window contacts, are in a series string with the end of line resistor included in that string to keep the zone from having a Zone Shored condition.
The ones that are normally held open are in parallel across the loop, and the end of line resistor is also in parallel across the zone to keep the zone from having a Zone Open condition.
End of Line Resistor at the Beginning of the Line
Installers have a problem following the manufacturer's instructions.
Often, with door and window contacts, if they place the end of line resistor at the end of the line, the resistor will be in an inaccessible location so it can't be maintained. So it can be maintained, the resistor would be wired in series with the rest of the loop and located in the panel.
Then again, in alarm, motion detectors and glass break sensors are often wired to short out the zone. Usually, however, especially in a residence, there isn't room inside the sensor housing to easily install the end of line resistor. In this case, even though it's incorrect, the resistor would be located across the zone at the zone contacts of the panel.
The big thing is to test to see if each device actually works as advertised.
To test, the first thing to do is call the monitoring company and tell them to disregard the signals while you test.
Then arm the system. One at a time, try each contact and sensor. Check to see if the panel goes into alarm, and then check with the monitoring company to make sure they received the alarm.