Heat detectors are normally a two wire device, although some installations run 2 pair with the second going back to a local junction box to terminate the end of line resistor.
The gas detectors are normally 3 terminal devices so triad is best. With the Honeywell/Seiger type detection rack make sure your all field connections are tight. Any loose terminals in junction boxes will show up as a drift in reading
Gas Detectors / Any detectors termination is decided by the Supplier / Vendor, Main factor is Simple being POWER CONSUMPTION / POWER REQUIREMENT by the Detector for Functioning.
When and where the Sensor Electronics requires more power in order to Support the Functionality of it, The Device has 3 Terminals (+ , - , Signal) thus it becomes 3 Wire (Traid Cable).
I hope this would give a better Explanation of the Explanation.
You can't decide without seeing the mfg dwawings/specs, and knowing how you want to use them.
If using an analog signal, they will be at least shielded triads w/ drain wire if they are new equipment (+, COM, and analog signal out)
You might also be choosing ones with a communication protocol instead, like Hart, Fieldbus, Modbus, or a mfg's proprietary comm system.
You might also only want discrete signals instead, so you wouldn't be needing analog cable but might need more conductors.
You also have to decide how you will calibrate them or perform functional tests. For example, if your gas detectors are located 30 feet above the floor, you might need another conductor going from the floor level up to the sensor in order to "trigger" or initiate the cal sequence. Some flame sensors don't have a portable test lamp to use, so you have to perform a self-test initiated by placing a magnet next to the sensor or using a remote test button (another conductor).
BOTTOM LINE IS: WHAT MAKE AND MODEL ARE YOU INSTALLING, WHAT SIGNAL (DISCRETE OR ANALOG) DO YOU WANT, WHAT IS DISTANCE FROM CONTROLLER TO THE SENSOR, ETC?
WE have gas detector which is use triad shielded #18awg. this cable was damage by the rat. presently we don't have spare cable. my question is can I use 2 x 1 pair shielded cable to replace this damaged triad cable? I am in urgent situation because we could not buy triad within short time!
Go ahead and use 3 cores of the cable, but i suggest you use a over-all shielded 2 x 1 pair cable. So it is more or less like your triad with extra core.
