Monitor lead breakage and detect short circuits with IS barriers
Discrete input signals send an "on" or "off" signal from devices such as a pushbutton, switch, or contact. When the application requires an intrinsically safe circuit, the typical circuit consists of a field device, an intrinsic safety barrier, and a controller (DCS or PLC).
So, what happens if the wiring connecting the field device to the IS barrier is broken or inadvertently shorted? In many cases, when the circuit is checked for functionality, we discover that the sensor and the controller are working properly, but the correct I/O information is not being detected by the controller. We frequently find that there is a fault in the sensor field wiring. Fault detection and fault analysis in this manner by the customer/user is typically not efficient and may actually cause functionality problems within the application. The problem with this fault detection approach is that operators (usually) realize there is something wrong only after the problem arises and must quickly react to an emergency situation.
The first two drawings in figure 1 show a normal operation circuit. The third drawing shows a faulted circuit where a fault in the wiring has been introduced. You might imagine a fourth drawing that illustrates an inadvertent short to the field wiring. These types of faults might be caused by improper wiring, frayed conductors, or inadequate cable protection.
Intrinsic safety barriers are available with line fault detection (LFD). This feature is enabled with a simple DIP switch. We highly recommended always keeping the LFD in the "on" position whenever possible. When you enable the LFD in the IS barrier and a fault occurs in the wiring (lead breakage or short) the barrier will indicate this error with a flashing red LED.
Now that the error has been detected, the problem should be easy to troubleshoot and repair.
In order to properly detect the fault in the field wiring, additional resistance is required at the switch in the field. Figure 2 below shows the addition of a 1kΩ resistor in series and 10kΩ resistor in parallel to the switch. This resistance allows a small amount of current to flow during normal operation, allowing the IS barrier to differentiate between an open or short of the field wiring and the normal operation of the switch.
In figure 2, the resistance arrangement is used to monitor lead breakage and short circuit detection with mechanical contacts using the line fault detection feature in the intrinsic safety barrier. Note, an electronic NAMUR proximity sensor (inductive, capacitive or photoelectric) has this additional resistance included so external resistors are not required.