The issue of grounding the enclosure of intrinsically safe instruments
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This post was last edited by zyz8438 on 2010-11-2 at 16:47. According to GB3836 requirements, the enclosure of intrinsically safe instruments must be grounded. The intrinsically safe instruments (pressure transmitters) supplied by a certain transmitter manufacturer now come with explosion-proof certificates, but they lack bolts for grounding the enclosure as well as related accessories. Only internal grounding. This pressure transmitter is the reference model. May I drill holes on the casing by myself to install the grounding accessory? Is there any other better way to handle this?
Won’t threading the casing destroy the explosion-proof function?
I think it’s definitely not possible to tap a ground point on the enclosure, as that would violate the requirements of its intrinsically safe system. Internal grounding and enclosure grounding are different concepts. Instrument circuits are divided into working circuits and protective circuits. The working circuits are grounded for the signal loops and for shielding, with the purpose of providing a unified reference point, eliminating interference, and preventing overcurrent. Enclosure grounding belongs to protective grounding, and its purpose is to protect human safety in the event of a leakage current. Can it be grounded at the accessories of the transmitter, such as the transmitter’s mounting plate or similar locations?
Reply to 4# Qin Zhan Han Wa: In intrinsically safe systems, it is required that the enclosure be grounded. What is the purpose of grounding this enclosure? Is it to prevent energy from accumulating in the housing and thereby affecting its explosion-proof performance?
It’s sufficient to ground the cable at one end of the control room; in reality, there isn’t much difference between the working ground and the protective ground in actual use – abroad, it’s simply called grounding.
Reply to 5# yangyang012: Grounding the enclosure is primarily aimed at protecting people’s safety and preventing electric shock accidents.
I would like to ask the experts: in principle, the working ground and the protective ground of instruments should be separated. However, in practice, when some instruments are powered, their 220V grounding terminal (working ground) is connected to the enclosure, making it impossible to separate them. How should grounding be considered in this situation?
In my opinion, the cable with calcium shielding ensures electrical continuity, as the enclosure, gland, and calcium shielding connections all contribute to this; furthermore, the calcium shielding has been grounded in the control room. The table enclosure is also equivalent to being grounded. The non-calcium cable is connected in the same way to the protective tube, which is in turn connected to the steel structure; this is equivalent to a connection between the enclosure of the meter and ground.
The last edit to this post was made by Past glory on 2011-8-25 at 19:15. It isn’t specified that the field instrument enclosure of the Zina-type safety barrier must be grounded, right? It only emphasizes that equal potential connection is required in the following areas: the grounding busbar or grounding rail of the Zener safety barrier is in direct electrical connection with the negative pole of the DC power supply, and ultimately is connected at equal potential to the neutral point of the AC three-phase power supply.
The grounding of the AI/AO instrument signal wires generally does not need to be done on-site; it is sufficient to ensure proper grounding in the DCS. The DCS grounding must be a separate instrument grounding system and cannot be combined with electrical-related grounding systems. In the case of four-wire instruments with a power supply voltage of AC220V, there are generally grounding screws marked for grounding at the internal power connection points of the instruments on site.