To prevent ground loops.
With one end connected, there is no current in the shield. (except for field interception, which is the purpose of the shield).
If a shielded signal line is grounded on both ends, a ground loop noise problem may be generated. You know a voltage difference will usually be found between various ground points in a plant.
Slight correction. It is not JUST a noise problem. You do not want the shield of you transmitter cable to be the wiring conduit between plant section grounding grids that may be at different potential. If you connect the shield to ground at
both ends, you will form this wiring path that may conduct hundreds to thousands of amps of current between the grids. You will surely burn out this line, perhaps start a fire, and maybe electrocute the installation technician. It's a Safety Hazard!
Dick Caro
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Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720
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The Consumer's Guide to Fieldbus Network Equipment for Process Control
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But we did have a situation where the level x-mitter would go to 0 once we started the pump.
We checked every possible piece of wire and everything was good. In a desperation we grounded x-mitter at both ends. Now it's reading correctly.
To prevent ground loops which in turn cause noise on the signal. It only takes small changes in ground potential across the plant to cause a current flow in the shield which then couples to the signal wires.
A good example of this would be a sterio system, if you ground the shields at the turntable you will get a very large 60Hz hum
