4-20mA and grounding of the instrument
Thread Content
For instruments, they are generally designed with 4-20mA inputs or (and) outputs. So, how is 4-20mA connected to the instrument’s ground? (Note: Includes 2-wire, 3-wire, and 4-wire systems; power supplies include 24VDC and 220VAC)
For grounding requirements, refer to HG/T20513-2014 \"Code for Grounding Design of Instrumentation Systems\".
To avoid electromagnetic interference, 4-20mA signals are transmitted using shielded cables with proper shielding. As for the 4-20mA signal you mentioned, it cannot be grounded; if it is connected to ground, how can the instrument receive this signal?
What was said upstairs is correct – the 4-20mA signal must not be grounded; if it is grounded, the signal won’t be able to reach the control room
This post was last edited by xxkhc on 2015-11-30 at 13:40. In practice, the negative wire of a 2-wire 4-20mA signal is generally not allowed to be left floating (except in multi-point grounding systems with isolated power distribution). Therefore, the connection between the 4-20mA signal and the instrument is actually grounded. It’s just that this grounding point is at the signal common point, so it’s not very intuitive. We know that in a detection and control system, the point with the lowest potential should be chosen as the common signal point, and this common point must be grounded. In a 4-20mA signal system using a 2-wire configuration, the negative terminal of the 24V power supply has the lowest potential, and it serves as the common point for the system signals.
The staff on the 5th floor at Haiyou has a fairly comprehensive understanding of 4-20mA; thank you
The Haiyou staff on the 5th floor has a correct understanding of the 4-20mA signal system
As a supplementary note, the shielding layer of analog signal cables should be grounded on the power supply side, while digital signal cables can be grounded at both ends or at multiple points. It should be noted, however, that grounding must be integrated into the plant’s unified grounding grid; separate grounding measures should not be implemented.
There used to be card boards with single-point isolation, and it can be assumed that the signal ground was floating; it is said that floating the ground is done to prevent interference (which seems reasonable). It’s just that the cost is too high; entering the bid is definitely not worthwhile. PLCs also have a floating system, which requires that all 0V levels of direct current be set to floating, including the 0V of a 24V DC supply. So it can’t be generalized. @jiaguoyun
4-20MA is a current signal; safe grounding is used to direct the current into the ground, while logical grounding refers to a common grounding point that serves as a voltage reference to prevent voltage differences. 4-20ma is a standard signal, and grounding is a standard practice; it’s not clear what meaning there is in comparing the two.
4-20ma is the signal; grounding includes the grounding of the shield layer in DCS systems and the grounding of the shields of field instruments. In theory, only one end can be grounded. It actually depends on the actual conditions on site.