Using a mA meter with an internal impedance that is too high to measure current over the transmitter’s test connection will result in erroneous measurement results!

By providing electrical isolation, you can break ground loops, increase the common-mode range of the data acquisition system, and level shift the signal ground reference to a single system ground. Safety isolation references standards that have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high-voltage and transient voltages to be transmitted across its boundary to other electrical systems with which the user may come in contact.

Incorporating isolation into a data acquisition system has three primary functions: preventing ground loops, rejecting common-mode voltage, and providing safety.

Isolation Types and Considerations when Taking a Measurement
http://www.ni.com/white-paper/3410/en/

If you are struggling with unstable pH values for your process measurement, there may be ground loop problems. A ground loop exists when an electric circuit is connected to earth ground at two or more points with different potentials. Different earth grounds are supposed to be at the same potential, but the potential of the earth varies from point to point.

In a typical process pH measurement, the electrode is connected through the process liquid and piping to earth ground. The pH transmitter is in most cases grounded with a grounding wire to the power outlet or safety ground and via the electrode to the process liquid. Those are two grounding points, and what will probably happen is that current will flow through the electrode wiring. The currents created by ground loops are often fluctuating a lot, and will therefore, produce an erratic and unpredictable pH measurement.

The 4-wire sensor transmitter is probably the least well-known of the current-loop sensor transmitter circuit types. These transmitters fit market needs for applications that require additional transmitter isolation options that aren’t possible with 2- and 3-wire transmitters.

Unlike the 2- and 3-wire transmitter representations shown in Figure 2, the 4-wire circuit has separate paths for the power current and signal current. Also, the 4-wire receiver does not share a common return (GND) with the power supply. This allows for several new isolation schemes, including fully isolated, power-isolated and output-isolated transmitters that expand on the input-isolated and non-isolated topologies we described for 2-wire and 3-wire transmitters.

While non-isolated and input-isolated systems exist for 2- and 3-wire sensor transmitters, these isolation schemes are not possible when designing with 4-wire transmitters. This is because non-isolated and input-isolated transmitters do not require isolation between the power supply and the output transmitter and receiver.

A: The classical “ground loop” is really a low-frequency phenomenon (<50 kHz, or so) and is usually typified by 60/120 Hz buzzing in audio systems through common-impedance coupling. However, I understand your question regarding shielding at one end or both ends. For systems with highly distributed signal references, where the potential difference between the main controller digital return and and various sensor returns can be quite different. The result would be noise currents flowing in the shield. In this case, it might be best to connect just the one end.

In the aerospace world, where we might construct an umbilical cable 300 feet long for a missile, NASA, ESA, and other like agencies specify all cables be constructed using the “Spacewire” standard (http://www.spacewire.esa.int/). This standard dictates that cable shields be connected at the source end only, but optionally may use the “hybrid” grounding scheme where a series capacitor is used to connect the non-source end of the shield. Refer to that standard (free download) for typical wiring diagrams.

For sensor technologies, it’s also common to use various means to “break” any noise currents in the shielded twisted pair by using opto-isolators, differential pairs, common-mode chokes, and the like.

Source: http://www.edn.com/electronics-blogs/the-emc-blog/4438390/EMC-questions-answered–part-6-?_mc=NL_EDN_EDT_EDN_today_20150121&cid=NL_EDN_EDT_EDN_today_20150121&elq=76d9d8ab506e46bdaddd00a63e5bc411&elqCampaignId=21259