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End of Line Resistors (EOLRs)
In this article we will discuss End of Line Resistors and discover what they are, how they work and why they are used in Fire and Gas systems and other high integrity control circuits. We will also discuss alternative methods of supervising circuits to ensure the loop integrity has not been broken.
What are End of Line Resistors?
End of line resistors (EOLR) are resistors, of a known value, that are used to terminate protective loops or zones, such as those found in Fire and Gas (F&G) systems and some Emergency Shutdown (ESD) systems.
What do EOLRs Do?
The purpose of EOLR's is to allow the control system to supervise the field wiring for open or short circuit conditions. How the control system responds to each of these fault conditions depends on the system's configuration, but generally speaking, a control system views an open circuit as a fault or alarm condition, and a short circuit as an alarm condition (if armed). The EOLR's allow the control panel to differentiate between the two conditions by looking for a known resistance.
Some manufacturers allow the use of double EOLRs, often referenced as DEOLRs. In this case, a second resistor is added across the terminals of the sensor, so that it is in parallel with the circuit. The second resistor adds the capability of differentiating between an open sensor and an open, or broken, circuit. If the sensor opens, the current passes through both the series and parallel resistors. The panel senses the reduced voltage and knows that the sensor is open. With this configuration, zero voltage can mean only one thing: a broken circuit. This constitutes the maximum supervision of loop wiring.
The use of EOLR's is recommended when there is the potential for the field wiring to be subject to damage or compromise.
Where Should End of Line Resistors be Placed?
An EOL resistor should always be installed at the last device in the loop and never inside the control panel. Placing resistors anywhere other than the end of the line does nothing to supervise the wiring, which is the reason for using resistors.
Are there any reason not to use EOLRs?
A couple of reasons are often quoted:
First, is cost. Although the price of each resistor is very low, the time taken to install the EOLRs and the potential for increased commissioning and maintenance time need to be considered. Generally however the cost is not regarded as prohibitive.
Second is compatibility. Adding EOLRs to sensors may interfere with how they work or indeed may invalidate hazardous area certification of the device to which the resistor is added. Always check with the sensor manufacturer prior to installing EOLRs.
What value of EOL Resistor should I use?
There are no standards for EOLR values. Each manufacturer of protective systems e.g. fire alarm system specifies their own value. And for different types of input card in a system the value may be different. Always check with the system supplier on the size of resistor to be used in any given application.
The term "Line Monitoring" is often used to describe a circuit that uses end of line resistors, e.g. "We are using line monitoring on the input from the manual call point".
Why do Circuits Need Line Monitoring?
Fire zones usually use EOLRs. This is because fire sensors are "Normally Open" devices; they only close the circuit when they trip. Therefore, a fire zone in its normal state would appear the same to the F&G system as one where the wire has been cut: namely, open. To prevent this, a resistor is used to close the circuit (with reduced voltage), so that it can be monitored.
Alternative Ways to Line Monitor
Panel Mounted Resistors
By using a four conductor cable for a two wire sensor, the two free conductors can be used to extend the circuit from the sensor location to the control panel, through the resistor, and back to the sensor. The resistor would actually be at the end of the line, and supervision would be accomplished.
Some manufacturers allow the use of zone doubling where two zones, each with a different resistor value, are connected in parallel to the same two terminals. Since they are in parallel, the system sees the total of both reduced voltages when both zones are closed. If one opens, that voltage is removed, and because of the different resistors, the system knows which one is open and which remains closed. This effectively doubles the number of zones available on the main board, although it does not change the maximum capacity of the system.
The following pages on Control and Instrumentation.com give more detail on other electrical system design and related topics:
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