Regarding the classification of wiring signals in junction boxes
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Dear seniors: In terms of instrumentation, the digital and analog signals from the instruments should be connected separately, in order to prevent the digital signals from interfering with the analog signals. But after connecting to the junction box, the buses coming out of that junction box still converge together at the cable tray; so won’t there be any interference at that point? I would appreciate some guidance from a senior, thank you.
It won’t cause interference; the signal cable is shielded!
If you really insist on being precise, keep them separate; generally, strong electricity and weak electricity should be kept apart
Different signals are introduced into separate junction boxes as well, for safety and management purposes, to prevent them from mixing together and causing failures.
No, otherwise there’s no need to proceed with the project. 1. The cable has a shielding layer. 2. Analog signals are usually intrinsically safe, while digital signals are typically flameproof; these two types of cables are laid in different sections of the cable tray, with an additional partition in between as well
I know about shielding. Then why do the wiring boxes need to be divided into analog and digital types? Don’t they also have a shielding layer?
That’s because the junction box is designed for centralized management of wiring on-site; it makes sense for intrinsically safe and flameproof designs to be separated. Moreover, in typical junction boxes, especially those intended as spares, only one multi-core cable is installed, so there can only be one type of signal within such a box – usually analog signals that are intrinsically safe, while digital signals are flameproof. That’s indeed the logical approach. You can think about why analog signals are often intrinsically safe, while digital signals are usually flameproof
I don’t know why either, but at first I thought it was because, from the perspective of intrinsically safe energy limits, it’s easier to apply energy limitations to current signals and to calculate whether they meet the requirements of intrinsically safe circuits. When digital switches experience contact sticking, it is easy for the signal to exceed the threshold. However, it seems that there are also non-NAMUR-type intrinsically safe digital switches; I think this is a legacy issue. . . It has been designed this way throughout the whole process. Additionally, a few more remarks on the original poster’s question: some regulations that should be taken into account in engineering work are to be cautious about the mixing of intrinsically safe circuits and non-intrinsically safe circuits. The intrinsically safe cables and non-intrinsically safe cables running from the control room to the field are laid in separate trunking systems, separated from each other by partitions; the trunking systems are covered to prevent damage from external mechanical actions. Intrinsic safety cables and non-intrinsic safety cables shall not share the same metal conduit or the same field terminal box. Multiple intrinsically safe circuits or related circuits should not share the same cable (except when the cable conductors are individually shielded) nor be placed within the same steel tube (except when shielded wires are used)