Four-wire instrument cable selection

This question was asked in detail, and it can be seen that actual contradictions were encountered during the implementation of the project. I try to talk about my understanding. For four-wire instruments, if there are no special anti-interference requirements, it is completely feasible to use a four-core cable, with two cores each for positive and negative power and positive and negative signals. It is certainly safer to use two two-core cables separately, but it mainly increases the construction volume and cost. Unless the strong current interference on site is particularly serious, it is not necessary. However, it should be noted that if the signal is 4-20mA analog, the grounding of the shielding layer is more critical than separate wiring. Combustible gas detectors are based on a split-wire system, and the most common ones are three cores (positive power, signal, and common negative). Fire codes require that power and signals be separated. This "separation" mainly refers to physical isolation or flame retardant levels, and does not necessarily require splitting into two cables. If the local fire inspection is strict and combustible gas is classified into the fire detection system pipe, it is recommended to consult the local fire acceptance department or plan review center. Some provinces recognize that "the same cable but with core wires grouped through metal tubes or flame-retardant armored" can be considered to meet the separation requirements, which is much more economical than using two two-core cables. Personal advice: If the project scale is small, for the sake of safety, the power supply and signal of the combustible gas detector should be separated by two two-core armored cables to avoid wrangling during acceptance. However, if it is a large-scale project, it is best to ask the design institute to issue a clear explanation and attach the opinions of the local fire department or drawing review to avoid taking responsibility yourself. In general, it’s not that technology can’t do it, but that implementation standards vary from place to place. The safest way is to directly ask the local fire acceptance department or let the design institute confirm. After all, the design and construction are responsible for any problems. Hope it helps.