Yes, it can be done, especially with an adequate wire gauge. The real
problem is interference.
1 - make absolutely certain the XT end is ungrounded.
2 - make certain you have a surge arrestor.
3 - Make absolutely certain you are using twisted, shielded pairs. This may
be difficult with the heavier gauges.
4 - Do NOT run parallel to any power lines. Fortunately this is an inverse
cube relationship so even small increases yield large improvements.
Repeaters are neither necessary nor helpful. They will not help against
interference problems. Understand that there is no signal loss with a 4-20
mA loop. That is why we use them.
Just out of curiosity, is it worthwhile to use digital communications
vs. current loop, or even fiber? You obviously know how to make this
work, but is it the recommended approach for new installations?
It depends on a lot of things. I just answered the question as asked.
When I'm faced with a situation of an analog measurement located 2 km from
the control room I first ask how many more of these there are before I start
laying wire for one only. These days there seems to be a lot more fibre.
On the thread concerning control network security I mentioned a current
situation where internet was the way to send four signals only 1 km.
(Different plants, different owners, different control systems.)
Other considerations like how many channels, update speed, type of data,
route and route cost for a few need to be considered. At some point
something like fiberoptics, MW, RF, or laser maybe more practical with less
interference, even though converter maybe needed at both ends.
WiFi with directional hi gain antenna's should also work and longer range
standard out before too long.
Lots of things we need to know to consider, for a better answer.
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