What are EOL resistors and why should I have them?

An End-Of-Line resistor supervises the wiring between the alarm panel and the switch. Some panels have a limited number of detection zones and do not require EOL resistors. But if the panel can use EOL resistors to supervise the wiring, the resistors should be used. If more than one detection device is on a single detection circuit, the EOL should be at the last device. When feasible, every alarm detection device should have an EOL resistor at the device. If the EOL resistor is at the burglary contact on a door, the panel sees the wire open when the door is opened and sees the resistor when the door is closed. The thief who shorts wires together hides the EOL resistor from the panel, letting the panel know that something is going on. The thief who cuts the wire causes an open circuit, the same as if he had opened the door. Hold-up alarms and Fire alarms use an "open circuit" detection scheme. The panel normally sees the EOL resistor at the end of the detection circuit. A short on the wire causes an alarm and a break in the wire causes a trouble report. Since normally closed switches can weld due to high voltage like lightning, the normally closed switches used for burglary detection must be checked after severe weather to see that they still work properly. Since devices on a "open circuit" normally cannot weld open, lightning and power surges should not "weld" these switches into an inoperable state. Some security systems even use this high security mode for their burglary devices.

Where should EOL resistors be installed?

What makes EOL resistors so important is that they can supervise the wires only when they are at the end of the line. When properly placed in the circuit, EOL resistors prevent wire tampering on normal switches. EOL resistors in the control panel are usually ineffective. The usual net effect of putting the EOL resistors in the panel is that the panel acts as if it has no EOL resistors. Even open circuit devices like smoke detectors and hold-up buttons are not properly supervised when the EOL resistor is attached in the panel before the end of the line. Many companies install the EOL resistors in the panel or use panels that do not supervise the wiring. Some alarm manufacturers show the EOL connected at the panel. The proper placement of an EOL resistor is at the End-Of-Line, the last switch on the detection circuit. EOL resistors do not belong at the panel. Keep in mind when ordering, only one contact is required at the end of the line. In other words if you have a zone with five door contacts you would use for regular contacts (NO EOL) and one EOL contacts at the end.

PBF-100T
Push Button Switch

Manufactured by GRI

• Lifetime Warranty

• UL and ULC Approved

• Terminal Block

• PBF 3/4" x 1 1/2" Fits 3/4" Hole

• N.C Loop

• White or Brown

DS-01T
Short Roller Ball

Manufactured by GRI

• Lifetime Warranty

• UL and ULC Approved

• Terminal Block

• Hardware and Spacers Included, also Available Separately

• N/C Loop

• White or Brown

N20RS-T

Manufactured by GRI

• UL and ULC Approved

• Terminal Block

• 3/8” Press Fit Rare Earth Magnet With Terminals

• N/C Loop

• Rare earth magnets can be mounted with the mounting screws and/or double stick tape included

• White or Brown

110T
Miniature Surface Mount Switch

Manufactured by GRI

• Lifetime Warranty
• UL and ULC approved
• Adhesive and screw mount
• Standard gap up to 1”
•  N/C Loop
• White or Brown

1OOTC
Miniature Surface Mount Switch

Manufactured by GRI

• Lifetime Warranty
• UL and ULC approved
• Adhesive and screw mount
• Standard gap up to 1”
•  N/C Loop
• White or Brown

• Includes Cover

29A
Standard Surface Mount Switch

Manufactured by GRI

• Lifetime Warranty
• UL and ULC approved
• Standard gap up to 1", 5/8" on steel
• Side mounted screw terminals
• Spacers, covers and screws included
•  N/C Loop
• White or Brown

N/C (CLOSED CIRCUIT) WHEN MAGNET IS NEXT TO IT
N/O (OPEN CIRCUIT) WHEN MAGNET IS NEXT TO IT