Instrument tray for chemical plants
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How should instrument cable trays in Class A chemical production workshops be properly grounded? The instrument grounding is connected back to the tray through the instrument explosion-proof tube; is it necessary to use flat bars for grounding within the instrument tray?
I don’t understand; isn’t the instrument grounded at the wiring cabinet where the instruments are located? Is it a galvanized cable tray? If the connection itself is fine, then as long as the grounding is done according to the distance requirements, there should be no problem.
For the grounding of cable trays, it depends on the material of the tray; if it is a hot-dip galvanized steel tray, it is sufficient to use flat steel strips every 50 meters to lead to the grounding device； In the case of fiberglass cable trays, flat steel bars need to be placed inside the tray grooves; the various sections are connected to each other using conductive wires, which in turn lead to the grounding device. The grounding of instruments is not done through the instrument’s explosion-proof enclosure; instead, the grounding terminal of the instrument is connected to the cable conduit. The grounding of such cable conduits is carried out by connecting them to the grounding grid, or to other metal components that are already grounded, such as metal frames, platforms, fences, and equipment. Grounding is repeated at regular intervals.
Generally, trough-type cable trays are used for instruments; galvanized flat steel is not placed inside these trough-type cable trays； However, grounding jumpers are installed at the connections of the cable trays; grounding is carried out every 30–50 meters, and equipotential bonding is done at the inlet and outlet points of the installation. Electrical cable trays usually use ladder-type trays, in which flat steel bars are placed.
If instruments (such as thermometers, pressure gauges, etc.) are grounded and then cross-connected to the conduit carrying the explosion-proof instrument wires, which in turn are connected to the cable tray, is it necessary to use a flat bar on the cable tray with both ends grounded?
If instruments (such as thermometers, pressure gauges, etc.) are grounded and then cross-connected to the conduit carrying the explosion-proof instrument wires, which in turn are connected to the cable tray, is it necessary to use a flat bar on the cable tray with both ends grounded?
Grounding is divided into working grounding, lightning protection grounding, and protective grounding. Analyzing this, the concept of working grounding does not apply to cable trays. Lightning protection grounding is used in situations where there is a risk of lightning strikes, while protective grounding is used for metal enclosures that under normal conditions do not carry dangerous voltages; its purpose is to prevent harm to people in case those enclosures become electrified. Therefore, whether a cable tray needs to be grounded depends on its material; metal trays definitely need to be grounded, as clearly stated in the relevant standards, while non-metallic trays have no need to be grounded under any circumstances.
Instrument grounding refers to the grounding of instruments; however, it is generally not allowed to perform on-site grounding for instruments. Instead, shielding grounding is carried out on the cabinets in the control room. You need to specifically consider the material of your cable tray and the relevant drawings; metals, fiberglass-reinforced plastics, as well as trough-type and ladder-type trays, along with electrical and instrumentation types, are all different
I also want to ask how the cable tray grounding was done in the end. At our construction site, the contractor carried out static bonding of the cable trays using 6-square-millimeter wires; it was said that flat steel bars should also be placed inside the cable trays. As per the client’s requirements, the cable trays are made of stainless steel, so the flat steel bars must also be stainless steel as well. There are many such components, which results in high costs. In the drawings, there are three methods for cable tray grounding: using flat steel bars running the entire length, using cables running the entire length, or using bonding connections. However, for bonding connections, 16-square-millimeter wires are required. I’m unsure about how to arrange for proper grounding at the site
SH/T3569-2020 Standard for Construction and Acceptance of Petrochemical Cable Trays provides very clear answers to what you asked