Grounding issue with three-wire alarms? _?
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Flammable and toxic gas alarm, powered by 24V DC three-wire system. One for the signal +, one for the 24V power supply+, and one for the common terminal (signal – & 24V –). The 24V power supply is provided by the DCS cabinet, and signals are also transmitted to the DCS. The current problem is that 24V- is not connected to the ground copper bar of the DCS cabinet, whereas 24V+ has a 24V voltage relative to the ground copper bar. There is also a 24V voltage between the 24V+ terminal at the field instrument and the instrument enclosure. Is this situation normal? Why is there a voltage between 24V+ and ground? If it is put into use in this way, is it dangerous?
Based on the poster’s description, my understanding is that the gas alarm likely outputs a 4–20mA DC signal, with the common terminal serving as the negative side of this signal; therefore it is not connected to the ground copper bar. The ground copper bar functions as the negative terminal of the power supply and is connected to ground for protection. Additionally, 24V DC is a safe voltage level, which is also connected to ground for protection. I don’t think there is any risk involved.
For three-wire combustible and toxic gas detectors, the 24- terminal belongs to the working ground and must be connected to it; this is not for protective purposes, and it must under no circumstances be connected to the protective ground. If 24- is not connected to the work location, then it’s definitely wrong. This issue indicates that the negative terminal of the 24V power supply is not connected to the ground. Once the negative terminal of the 24V power supply is connected to the ground, the 24- terminal of the three-wire combustible and toxic gas detectors does not need to be grounded, as it is already connected to the ground through the negative terminal of the 24V power supply. However, this approach is not very rigorous; although it satisfies the electrical principles, it does not meet the requirements regarding grounding standards. Therefore, the 24- terminal still needs to be connected to the ground.