The following three main categories of instruments require signal grounding (functional grounding) to be performed on-site, in accordance with the "single-point grounding of signal loops" principle, to prevent the formation of ground loops that could introduce interference.
Instruments that require signal grounding at the installation site
|
Type |
Reasons Why Field Signals Must Be Grounded |
Main basis |
|
Ground-type thermocouple |
Its measuring end (hot end) is connected directly to the process equipment (which is typically grounded) via a metal protective sleeve, and the signal source itself serves as a grounding point. If an additional ground connection is made on the control room side, a two-point grounding loop will be formed. |
HG/T 20513-2014, Clause 5.1.3 (Explanatory Notes): "Instruments such as grounded thermocouples, pH meters, and electromagnetic flowmeters shall only be grounded on-site." |
|
pH meter (acidity meter) |
The reference electrode is connected to ground via the electrolyte solution being tested, and the signal source (electrode) itself is already grounded. |
HG/T 20513-2014, Clause 5.1.3 (Explanatory Notes): "Instruments such as grounded thermocouples, pH meters, and electromagnetic flowmeters shall only be grounded on-site." |
|
Electromagnetic flowmeter |
The measurement principle requires that the sensor, the conductive medium being measured, and the process piping remain at the same potential, thereby establishing a stable, clean reference zero point for the weak induced electromotive force (in the millivolt range). This equipotential bonding must be completed at the sensor installation site (in the field). |
HG/T 20513-2014, Clause 5.1.3 (Explanatory Notes): "Instruments such as grounded thermocouples, pH meters, and electromagnetic flowmeters shall only be grounded on-site." |
Explanation
Single-Point Grounding Principle. For the three types of instruments mentioned above, the grounding point of the signal loop is fixed at the field location. If both the signal source (field instrument) and the receiving end (control room system) are grounded, a potential difference may exist between the two locations, thereby creating a "ground loop" in the shielding layer or signal line. This current will superimpose on the useful measurement signal, causing severe interference and resulting in unstable, fluctuating, or inaccurate measurement values. For the weak signals of electromagnetic flowmeters, ranging from a few microvolts to a few millivolts, such interference is catastrophic. Special Note: The shielding layer of the signal cables for these types of instruments should be grounded on the control room side (working ground); however, since the signal circuit itself (such as the negative terminal of a thermocouple or the signal line of a flowmeter) is already grounded at the field site, the control room side must remain insulated, and additional grounding is strictly prohibited.
The principle of "only one grounding point per signal loop": 1) HG/T 20513-2014, Clause 5.1.2, Explanatory Notes: Each instrument signal loop shall have only one grounding point. 2) GB 50093-2013, Clause 10.2.8: Each instrumentation circuit shall have only one signal circuit grounding point. Explanatory Notes: This clause is intended to prevent interference signals caused by potential differences. 3) SH/T 3551-2024, Clause 10.4.2: Grounding of signal circuits shall adopt a single-point grounding method.
GB 50093-2013 Code for Construction and Quality Acceptance of Automation Instrumentation Engineering, Clause 10.2.9: When using grounded thermocouples and instruments with already grounded sensing elements, no additional grounding shall be provided on the display instrument side.
Electromagnetic flowmeters must be grounded to ensure that the measuring tube, the fluid, and the electrodes are at the same potential, thereby providing a stable signal reference and reducing common-mode interference. Since electromagnetic flowmeters are velocity-type instruments based on electromagnetic induction, their electrodes detect the induced electromotive force generated when the fluid cuts through magnetic field lines. If the fluid potential fluctuates, the detection signal will be unstable. Therefore, this equipotential point must be forcibly established on-site using a grounding loop, grounding electrode, or connecting pipe. Proper grounding of the electromagnetic flowmeter is a necessary prerequisite for accurate measurement.
If the system design requires the signal to be grounded at the field location while the receiving instrument must be grounded in the control room (e.g., due to certain system architecture requirements), simply grounding both ends is not sufficient. An "electrical isolator" (such as an isolating safety barrier or signal isolator) must be used.
