Regarding the shield grounding and protective grounding of the on-site junction box
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As shown in the figure, the signal lines and shield wires of the main cable have been connected to the terminals. Question: 1) Where is the main shield of the main cable connected? 2) What kind of wire is the yellow-green wire? Is it a protective ground wire or a shielding ground wire? 3) Does the shielding of the branch cable need to be connected to a terminal? 4) Besides the enclosure, does the instrument box need to be connected to ground internally as well? I’d like to ask the experts, thank you!
1) If equipotential bonding cannot be achieved, such as in large plant areas, single-ended shielded grounding should be used in accordance with the specifications. The ground side is on the power supply side, usually the DCS side. 2) The yellow-green wire is the protective ground wire, PE wire. 3) The shield wire of the branch cable is connected to the terminal block in order to lead it back to the ground side, maintaining the continuity of the shield. 4) Generally, only protective grounding of the enclosure is required
1. The shield of the main cable is connected to the terminal, and the shields of the other branch cables are also connected to the terminal of the main cable’s shield; the outer side of the branch cable shields remains unconnected, while the shield of the main cable on the DCS side is connected to ground. 2. The yellow-green ground wire is used for protective grounding; it is necessary to install it on all metal enclosures that are not normally energized
It seems there are too many yellow and green lines in the diagram, and then there’s that terminal; after all, there’s also a protective grounding terminal outside the junction box.
It is recommended to refer to Figure 5.1.5 in HG/T20513-2014
1) Where is the main shield of the main cable connected? R: The main cable shield is connected at the DCS. 2) What is the function of the yellow-green wire – is it for protective grounding or shielding grounding? R: This is actually the same question as above; since IE is grounded at the DCS, what’s left is PE. 3) Does the shielding of the branch cable need to be connected to a terminal? R: Obviously, the junction box in the diagram has only one grounding bar, and it is a PE grounding bar; therefore, the junction box must have shielding terminals provided. The branch cables are connected to these terminals, and from there they are connected via thick wires to the DCS ground. 4) Besides the enclosure, does the instrument box also need to be grounded internally? R: Instrument junction boxes are generally equipped with grounding bolts. Typically, there is also a grounding bolt on the door cover of the junction box, and this grounding bolt needs to be connected to the junction box’s grounding bolt using a grounding wire.
This shielding wire is connected in a really clever way; even if I designed it this way, the incompetent workers on site wouldn’t carry out such work, so segmented grounding is generally used.
1. The main shield can be connected to the last terminal on the terminal block. 2. The yellow-green wire below the terminal block is the protective ground, as it is clearly connected to the enclosure. 3. It needs to be connected to the terminals, with a one-to-one correspondence in shielding between it and the main cable. 4. The exterior and interior are connected. If the enclosure is grounded, there is no need to ground the interior.
First of all, that grounding bar is not intended for protective grounding; it does not need to be connected to the enclosure, so remove that yellow-green wire. The shielding wire of the main cable has been connected to the terminal; the main shield of this cable can be connected to that grounding bar ； The shielding wires of each branch cable can be connected to terminals (corresponding to those of the main cable), while the main shielding of each branch cable can be connected to that grounding bar. In this way, all the shield wires and shielding layers are connected inside the junction box, and then gathered on the same grounding bar in the control room.
1) Where is the main shield of the main cable connected? It hangs suspended on one side of the junction box; on the side of the DCS cabinet it is connected together with the separate shielding and attached to the ground, eventually leading to the main ground ； 2) What kind of wire is the yellow-green wire? Is it a protective ground wire or a shielding ground wire? The yellow-green line represents the protected area; it is connected to the enclosure and, through grounding on the outside of the junction box, is ultimately connected to the ground ； 3) Does the shielding of the branch cable need to be connected to a terminal? The shielding of the branch cables must be connected to terminals; field instruments should be kept ungrounded, thereby enabling connection from the instrument branch shielding to the main cable branches, as well as from the main shielding to the DCS working ground. 4) In addition to the enclosure, does the instrument box also need to have its interior connected to the protective ground? For signal cables, it is not necessary. Only field instruments with a voltage higher than 36V require protective grounding.
The new SHT3081-2019 standard will come into effect on January 1 next year. On-site instruments, whether powered by 24V or higher voltage, must have protective grounding, including for flammable and toxic gas detectors.