Douglas Krantz - Technical Writer - Describing How It Works

Where Should An End-Of-Line Resistor go In A Security System?

It seems to make sense that the EOL Resistor (End of Line) would go at the end of the circuit. The installation book usually even shows that's where it goes. However, being a security system, it's often found at the beginning of the line.

Where Should An End-Of-Line Resistor go In A Security System?

Mr. Krantz

Fascinating Q&A's on your web site. Thanks for such a wealth of useful and practical information.

Our community is in the process of changing from one alarm-monitoring provider to another. They will be sending out technicians to swap out the control board in the alarm panel, and it got me curious enough to look around in the box before they arrive next week.

What concerns me, as a layman (but knowledgeable enough about electricity, electronics, and circuitry in general to have built a few dozen Heathkits as a kid) has to do with how the original alarm system was wired. Perhaps this is common practice, for some reason that I'm missing -- but it appears that each detection circuit (for a 2000 square foot single-story house) has the EOL resistor (1k, on an Ademco 4110XM) simply connected directly across the terminals at the box.

Please educate me as to why this isn't wrong. If the wiring loop to the sensors (for example, magnetic-reed switches on window- and door-frames) goes "open-circuit" due to a failure, and the EOL resistor is wired straight across the terminals at the box, won't the system be unaware of the failed condition?

Shouldn't the EOL resistor be physically, actually, for-real be located at the END of the loop, at the farthest point? Otherwise, isn't having the EOL resistor right there at the panel just "faking out" the system?

What am I missing?

Thanks for sharing your knowledge and experience. It helps lots of folk climb the learning curve.

Cheers, C S

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.

Douglas Krantz
Life Safety
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