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Tech Note 124: Cable Grounding (Isolated and Non-Isolated Shields)
Last Updated: 12/11/2018

Cable Grounding (Isolated and Non-Isolated Shields)Cable Grounding: Shields used in cable assemblies reduce the coupling with nearby cables and minimizes the intrusion of external noise. This protects the strength and fidelity of the signal of interest. Grounding of shields, and the way in which they are grounded, has consequences on their effectiveness. To provide proper shielding and prevent ground loops, cable grounding schemes should be carefully considered and implemented.

Figure 1 (Above): STI's CMCP402L bulk sensor cable (used for CMCP602L(ST) connector cable assemblies)

Cable and shield: The cable shield serves to protect the signal from ESD (Electrostatic discharge) and EMI (Electromagnetic interference). To avoid ground loops, the shield should be grounded at only one common point, either to the instrument or to the sensor casing. Many instruments are designed to accommodate grounding of the sensor cable shield at the instrument (Figure 2 Below). STI's standard connector cable assemblies accommodates this grounding scheme. This is referred to as "Isolated" (as in open or not connected to the sensor casing).

Alternatively, it is also possible to ground at the sensor side. Most sensors are designed to allow for such a grounding scheme. Assuming the sensor is stud-mounted to the machine and the machine is properly grounded, the shield is thus grounded to the machine through the sensor, and the drain wire should not be connected to ground at the instrument end (Figure 3 Below). For this grounding scheme, connector cable assemblies with the extension -NI must be specified. This is referred to as "Non-­lsolated" (connected to the sensor casing).

There are many more considerations when it comes to grounding, this abstract only explains the different options in STl's new connector cable assemblies on all all CMCP600 series cables. The new connector assembly features a shorting pin which allows the sensor shield to make contact with the sensors housing. During assembly, the cables shielding is attached to the shorting pin on all Non-Isolated (-NI) assemblies otherwise the shield is left disconnected.

To summarize, all STl's connector cable assemblies are for grounding on instrument side, if not ordered as the -NI (Non-lsolated) version then grounding on the sensor end (machine side) is possible. Familiarity with your facilities grounding scheme is important to ensure all instruments are grounded at the same location. Ground loops are achieved when two sensors are installed on the same system but with different grounds. Preventing ground loops is important to ensure the strength and fidelity of the signal of interest.

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