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Instrumentation Voltage Drop problem
Designer miscalculated long run instrumentation cable, and we are facing voltage drop.

Gentlemen, we are connecting field H2S sensor and detector to a control room located 700m (2296.6 ft) away from the instrument. The design cable selection is 18 AWG 16pr for digital signals and 18 AWG 4TR for analog signals.

Now we are facing voltage drop, and the field instrument is not functioning. How to solve this issue without replacing the cables with 14 AWG?

Sensor/detector module: General Monitors S4000TH and S5000.

You haven't mentioned power cable or sizing - so assume supply wiring to be 18 AWG.

The specification for one instrument is 24vDC nominal 20vDC minimum. If you are sure you have less than 20vDC at the instrument there are several choices:

1. Raise the 24vDC supply until you have 24vDC at the instrument.

2. Install a power supply local to instrument.

The S5000 is a 4 wire transmitter and depending on config, number and type of sensors and distance you will need a heavier 24VDC power cable. See Table 1 Pg 26 in the manual.
http://s7d9.scene7.com/is/content/minesafetyappliances/General%20Monitors_S5000_Manual-EN

You could try using 2 or 3 pair 18AWG to supply 24VDC. Just make sure you fuse each pair on the supply end. That would get you the equivalent of heavier gauge wire.

For power or control wire it is better practice to install 14AWG. Then you get no surprises in the future and you are not individually designing each pair of wires.

good luck

Explain what is not working and how you have identified the problem.