Do you dare to avoid grounding just by installing safety barriers? These two are never Single choice question questions!

“Can Control Cables in the Cabinet Room Remain Ungrounded with Safety Barriers? Some believe that "with safety barriers for surge protection and interference isolation, the grounding of control cable shielding layers can be done casually." However, relevant standards clearly specify the requirements and methods for grounding the shielding layers of control cables in the cabinet room. Is the grounding of the shielding layer of control cables on the indoor side of the cabinet room related to the presence of safety barriers? Today, we clarify everything about grounding the shielding layers of control cables in the cabinet room.”

The grounding of the shielding layer of control cables in the cabinet room primarily serves to resist interference, with the following specific functions and purposes:

1. Preventing external electromagnetic interference: Once grounded, the shielding layer acts like a "barrier," directing currents induced by external electromagnetic fields into the earth, thereby preventing interference signals from entering the cable and affecting the accuracy of transmitted control signals.

2. Preventing internal signals from interfering with external devices: Signals transmitted within the cable may generate electromagnetic fields. A grounded shielding layer can suppress these fields from spreading outward, avoiding interference with surrounding equipment.

3. Protecting equipment and personnel safety: If the shielding layer is not grounded, static buildup or abnormal voltages may cause equipment malfunctions. Grounding diverts excess charges into the earth, reducing safety risks.

Key standard requirements and practices for grounding the shielding layers of control cables in the cabinet room:

1. GB 50093-2013 "Code for Construction and Quality Acceptance of Automation Instrumentation Engineering

10.2.10 The shielding layer of instrument cables and wires shall be grounded on the control room panel/cabinet side. The shielding layer of the same circuit shall have reliable electrical continuity and shall not be left floating or grounded repeatedly.

10.2.11 The armor of armored cables shall be grounded for protection at both ends.

10.2.12 In intermediate junction boxes, the primary cable's subdivided shielding layers shall be connected to the corresponding secondary cable shielding layers using terminals. Different shielding layers shall be connected separately without mixing and shall be insulated.

10.2.13 When anti-interference requirements exist, spare cores in multi-core cables shall be grounded at a single point. For shielded cables, the spare cores and the shielding layer shall be grounded on the same side.

2. SH/T 3081-2019 "Design Code for Instrument Grounding in Petroleum and Chemical Industries"

APPENDIX A

(Normative Appendix)

Shielded cable grounding map

The shielding layer of the multi-core main cable in the overall screen division is continuously connected to the cabinet room as shown in Figure A-1. The shielding layer of the multi-core main cable in the overall screen division is segmented and connected to the junction box and cabinet room as shown in Figure A-2. The shielding layer of the multi-core main cable in the overall screen division is continuously connected to the cabinet room as shown in Figure A-3. The shielding layer of the multi-core main cable in the overall screen division is segmented and connected to the junction box and cabinet room as shown in Figure A-4.

The wiring instructions for the shielding layer in Figures A-1 to A-4 are shown in Table A. The branch cable from the field instrument to the junction box is a single shielded cable or a single armored shielded cable. Armored cables entering the junction box use cable joints with grounding connections for easy grounding connections. The shielding layers of each branch cable in Appendix Figures A-2 to A-4 should be connected together in the junction box using terminals or busbars.

The shielding layers of branch cables and main cables can be connected and grounded at the control room cabinet, or separately at the junction box and control room cabinet grounding the section.

The shielding layer of the branch cable in Figure A-1 is connected to the inner shielding layer of the main cable, and the shielding layer of the branch cable in Figure A-3 is connected to the shielding layer of the main cable in the junction box. The fully connected shielding layer is grounded at the control room cabinet at one end.

The shielding layer of the branch cables in Figures A-2 and A-4 is grounded at a single end in the junction box, while the inner shielding layer of the main cable in Figure A-2 and the shielding layer of the main cable in Figure A-4 are grounded at a single end in the control room cabinet.

The armor layer of the main cable is grounded at both ends according to Article 4.4.1, and the metal cable tray is grounded at both ends according to Article 4.1.8, with repeated grounding every 30m. 

Table A   Wiring Method for Shielding Layer

Engineering practical application and on-site real-life pictures:

The grounding of the shielding layer on the inner side of the control cable in the cabinet room is not related to whether safety barriers (Isolation Barriers, surge protectors, etc.) are configured. Unless there are special requirements in the design documents, they must be strictly implemented in accordance with the specifications.