Grounding and Cable Shielding for Electromechanical Sensors

 Avoiding Ground Loops

The most common problem encountered with grounding and shielding of any sensor circuit is current flow in the ground circuit called a ground loop. To avoid ground loops, a system must have only one common ground point. The preferred grounding and shielding practice is to not connect any sensor cable shield or drain wire to the sensor’s metal housing, including through a cable connector's housing, nor connect it to any separate or intermediate system ground point. Connect the other end of the sensor cable's shield only to the system's master ground point to merely act as a Faraday shield. For sensor support instrumentation, only a single wire should be connected to the system master ground from only one ground terminal on the equipment.

Cable Shielding

The question often arises as to which type of cable shielding is better, braided or foil-wrapped with a drain wire. Braided shielding has better mechanical properties and is adequate for low to medium frequency applications, but is somewhat harder to work with and is typically a bit more expensive. Foil shielding is typically less expensive than braided, is easier to work with, and is often better for high frequency applications, and for operation in locations with strong EMI fields.

A follow-on question about sensor cabling is whether to use cables with individually shielded conductors, shielded twisted pairs, a multiple conductor cable with overall shielding, or some combination of these. The answer to this question in not simple, as it depends on the sensor.

For many industrial applications of sensors with a short cable run to the system or its support electronics, an all-shielded cable is adequate. For DC in/DC out analog output sensors like an LVIT, DC-LVDT, potentiometers, or strain gages, it is satisfactory to utilize an all-shielded cable. For AC-in/AC-out sensors like an AC-LVDT or inductive half-bridge, the best practice is to use shielded twisted pair conductors, one pair for the primary excitation and another pair for the differentially-connected secondaries. In cases where all six LVDT leads must be connected to the LVDT's signal conditioner, one pair is used for each secondary and one pair for the primary. For serial bus digital output sensors, shielded twisted pair cabling is used, with power in one pair and the serial output in the other pair, but using the appropriate ground connections so the analog ground and the digital output ground are only connected at the system master ground point. Individually shielded conductors are very rarely required for common types of sensors.


Foil Shielded Cable With Drain

Cable with Braided Shielding

Individually Shielded Twisted Pairs


Another question about cabling and grounding deals with the size of the cable's conductors. For most industrial sensor applications, conductor sizes from 24 to 18 AWG are used, with number 22 being the most popular. Choice of conductor size is often driven by a cable's length, because longer cable runs usually require larger conductors to minimize resistive losses in the cabling.


But along with wire size, a major consideration is the capacitance of the cable, either conductor-to-conductor or conductor-to-shield. It is worth noting that cable capacitance depends on both conductor size and shielding scheme.  For short cable runs, up to 20 ft. (6 m), this is not usually a significant issue. But for longer cable runs, cable capacitance is an important consideration, especially for AC-operated sensors like AC-LVDTs and inductive half-bridges. In such cases, utilizing low capacitance cables, typically around 15 pF per foot, is recommended to prevent undesirable phase shifts between sensor input and output, and limitations on system frequency response. Such cables are often used for digital output sensors connected to high speed buses.

The subject of grounding and shielding in sensor systems has many facets and nuances. A popular and highly-regarded source for more detailed information on this subject is: "Grounding and Shielding Techniques in Instrumentation", 3rd Edition, written by Ralph Morrison and published by Wiley-Interscience, a division of John Wiley & Sons, Inc.


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