I have a question regarding the instrument wiring box

It is best to set them separately. If it cannot be installed separately, it can also be placed in a junction box, with metal partitions used to separate sections within the junction box ； The 24V digital and analog signals are set separately, as the digital signals can cause electromagnetic interference to the analog signals.

It’s generally better not to use a box, as it causes interference and there isn’t enough space in the wiring box

Well, if there are very few points – only 4 or 5 in total – there’s no need to use two boxes. The intrinsically safe and flameproof devices can be placed in the same box; just arrange the intrinsically safe devices together and the enhanced safety devices together, with them separated from each other or not on the same terminal block, just like in the diagram on the first floor

Intrinsic safety and explosion protection need to be separated; an intrinsic safety instrument, together with an intrinsic safety cable and a safety barrier, ensures that the circuit is completely safe. The advantage of intrinsic safety is that there is no current in this circuit that could generate sparks. However, if explosion protection is used as well, then intrinsic safety cannot be guaranteed. Digital and analog signals can actually be used together, as both operate at 24V.

Let’s see if it’s useful HG-T 20512-2014 Code for Design of Instrument Piping and Wiring

1. Can Intrinsic Safety and flameproof signal types be placed in the same junction box? 1) Please refer to the two specifications, HGT20512 and SH3019. 2) In principle, it is possible, but as mentioned in the information posted above (HGT20512 9.0.4 and SH3019 9.4). However, it is generally difficult to achieve a spacing of more than 50 mm in junction boxes, or to have sufficient isolation measures; and even if this is possible, the issue of cable routing still needs to be taken into consideration ； 3) HG20512 8.1.8 Intrinsic safety and non-intrinsic safety signals should not share the same multi-core cable or the same junction box (note that it says \"and\", but I think it should be \"or\"); as per 9.0.4, conduit systems are separate trunking systems (this clause does not use \"SH\" to describe them, as they are all the same). 4) With current technology, there is little difference between intrinsic safety cables and flameproof cables (low inductance, low resistance, low capacitance, and the purchase prices are also the same). 5) But rules are rules; to avoid trouble and inspections, in new projects, intrinsically safe and non-intrinsically safe wiring boxes should not (in my opinion, they shouldn’t be used together). 2. Can 24V digital and analog signals be placed in the same junction box? 1) It is also the same two specifications mentioned above. 2) Taking HG20512 as an example, per 8.1.7, signals of different levels should not share the same multi-core cable or the same junction box; that is, if they are all 24V, they can be placed in the same cable. 3) In this case, the shielding of the main cable should consist of a separate shielding for each circuit plus a general shielding, as specified in 7.2.2. 4) From a management perspective, if the project is large enough, it is better to distinguish between DO/DI and AI/AO signals when connecting the terminal cabinets and system cabinets. 5) In terms of numbering the junction boxes, separating EJB and SJB makes things clearer as well. 6) Therefore, theoretically it is possible to put them together, but from a management standpoint it is recommended to keep them separate. 7) In the absence of any mandatory requirements, the best approach is the one that suits one’s own project – considering factors such as the number of points required for the project, the aspects involved in new construction or technical upgrades, and the use of existing junction boxes to determine an appropriate solution

It’s better to not rely solely on what is written in books. When studying standards, one should not merely consider their literal meaning, but also strive to understand why those rules exist. Above all, it is essential to understand the standards for execution and the order in which tasks should be carried out (which are mandatory and which are recommended). By understanding the reasons behind things and knowing their consequences, one can adapt to any situation. Only by having a broad perspective can one remain invincible.

The guy upstairs explained everything very comprehensively and in detail; it’s indeed the case.

This post was last edited by Hariky on 2021-11-26 21:39. Question 1: Can intrinsically safe and flameproof signal types be placed in the same junction box? Reply 1: Intrinsic safety and flameproof signals must be connected to separate junction boxes. Intrinsic safety explosion-proof measures involve limiting energy. That is, the sparks generated when electronic equipment fails will not ignite the flammable gases in the surrounding environment. Therefore, the junction box for intrinsically safe signals can have its cover opened while it is still under voltage for maintenance. The explosion-proof measure for flame isolation is to limit the scope of the explosion. That is, in the event of a failure in the electronic device, it will ignite the combustible gases present in its surroundings (those that enter the enclosure during maintenance when the cover is opened), but these gases are contained within the explosion-proof enclosure and do not spread outward. Therefore, in most cases, it is not allowed to open the cover while it is powered on; power must be disconnected during maintenance. Therefore, the signals of these two explosion-proof types must be installed separately. ---------------------------------------------------------------------------------- Question 2: Can 24V digital and analog signals be placed in the same junction box? Answer 2: It can be connected to the same junction box, but it is better to use different terminal blocks. Because these two signals are at the same voltage level, provided that the intrinsically safe/intrinsically safe configuration is the same. Considering that the wiring layout on the control system side is often arranged according to signal types, it is advisable to use multi-core cables to connect to the control system separately. However, it is still best to set up junction boxes separately according to the signal type, as this facilitates project implementation and maintenance management; this is a principle widely adopted in current projects. ----------------------------------------------------------------------------------