Selection of 24VDC-powered instrument cables
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The last edit to this post was made by The project’s progress on 2023-10-14 at 22:35. Question: Do instruments powered by 24VDC use multi-core cables for both power and signals? Additional info: From a functional perspective, it’s better to run them separately; however, it’s also acceptable to run them together, as many signals at 24VDC are also at 24V
When selecting 24VDC-powered instrument cables, you need to consider several factors. Whether the power supply and signals share a multi-core cable depends mainly on your specific application and system design. In some cases, you may choose to use a multi-core cable to share both power and signals. This saves space and installation time, but it should be noted that the power cable may cause electromagnetic interference with the signal cable, leading to a decline in signal quality. If your system has high requirements for signal quality, or if the current supplied to the power supply is high, it may be necessary to use separate cables to supply power to both the power supply and the signals. This can reduce electromagnetic interference and improve signal quality. Therefore, the decision of whether to use a multi-core cable or not mainly depends on your specific requirements and system design. If you decide to share it, you need to ensure that appropriate shielding and grounding techniques are used to reduce electromagnetic interference. At the same time, you also need to ensure that your cables and connectors can handle your power current and signal current. .
1. According to SH/T3019-2016 \"Code for Design of Instrumentation and Pipeline Systems in Petrochemical Industries\", 7.1.7 Signals with different voltage levels and frequency characteristics shall not share the same cable or the same junction box. HG/T20512-2014 Code for Design of Instrument Piping and Wiring, 8.1.7 Signals of different levels shall not share the same multi-core cable or junction box. Both the 24VDC power supply and 4~20mA are DC ； It’s again a signal at the same level (high level). If the distance is too large, it is necessary to check the voltage drop generated by the 24VDC power supply to ensure that it meets the minimum voltage requirement for the instrument. (The power supply and signal have the same wire gauge). If this requirement is not met, another cable must be used for power supply. 2. For the definition of levels, refer to 5.1.12 of the \"DLT5182-2021 Code for Installation of Instrumentation and Control Local Equipment, Piping, and Cable Design in Thermal Power Plants\" ； 3. For instruments powered by AC supply, refer to Section 5.3 of the \"DLT5182-2021 Code for Installation, Piping, and Cable Design of Instrumentation and Control Local Equipment in Thermal Power Plants\".
Okay, I see these two specifications; I didn’t pay attention to the level definitions at that time. What I’m dealing with now is the stall switch of PNP; the signal should be a cut-off signal, with a high level corresponding to a value greater than 1V. Is it still the same voltage level?
I personally think it’s possible; for example, toxic and flammable gas detectors use a three-wire system (24V DC and 4~20mA). In newly designed projects, 24V DC is routed through separate junction boxes (PJB), while signals are generally handled using SJB (analog) and EJB (digital) junction boxes.
Requirements of relevant specifications: According to Section 7.1.7 of the SHT3019-2016 Standard for the Design of Instrumentation Pipelines in the Petrochemical Industry, signals with different voltage levels and frequency characteristics should not share the same cable or the same junction box. According to clause 8.1.7 of the HGT20512-2014 Standard for Design of Instrument Piping and Wiring, signals of different voltage levels shall not share the same multi-core cable or wiring box. According to 10.4.1.7 of GBT50892, signals of different voltage levels should not share the same cable nor the same junction box. The signals from safety instrument systems, fire and gas systems, and process instrument control systems must not be shared on the same cable. The communication bus cable should be installed separately.
The poster’s question is whether a 24V power supply and signals can be run together. The above specifications all refer to signals and signals, without explicitly mentioning power supplies and signals. But in practice, three-wire instruments don’t present any problems in use (four-wire systems can also be configured as three-wire systems, with just an additional common terminal). Therefore, it’s not possible to separate them in a project; for 24VDC and analog signals, a split-screen + main screen cable can be used, while for 24VDC and digital signals, a main screen cable can be utilized.
As for the understanding of levels and frequencies: The voltage levels of instrument signals can be divided into low-voltage levels, such as those from thermocouples TC and pulse signals PC； Medium-level voltage ranges: such as 4–20mA DC signals AI/AO, thermal resistance signals RTD, 24V DC switch signals DI/DO, etc ； High-level voltage ranges: such as DC switch signals greater than 28V, DC control signals less than 250V, AC signals less than 28A, etc. There are four types of frequency characteristics for signals: the spectrum of power signals, the spectral density of energy signals, the power spectrum of power signals, and the energy spectral density of energy signals. The signals to which these four frequency characteristics are adapted are, respectively: periodic signals with limited power, energy signals, power signals, and energy signals. The signals from the measuring instruments we commonly use are all medium-level voltage signals; therefore, signals such as AI/AO/DI/DO/RTD can all be placed on the same main cable or in the same junction box (except for 220VAC solenoids)
What I have here is a stall switch, PNP
Mine is a PNP stall switch