Issues with three-wire combustible gas detectors and safety barriers
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For combustible or toxic gas detectors that are connected to the DCS or PLC via safety barriers, how is wiring done in general? It is powered independently by 24VDC; the positive terminal of 24VDC should definitely be connected to the combustible gas detector. Where should the negative terminal of 24VDC be connected? If the negative terminal of 24VDC is connected to the negative terminal of the safety gate signal on the field side, it will be isolated by the safety gate, preventing a circuit from being formed. In such a case, is it necessary to connect a ground wire? The negative terminal of 24vdc is connected to the negative side of the safety gate signal terminal block – has anyone taken care of this?
I encountered this problem last time, and later we switched to a two-wire system; the three-wire system requires connecting to their control unit.
Three-wire combustible gas detectors are connected to the system via safety barriers: they require a separate 24V power supply, with the 24V positive terminal connected to the positive terminal of the gas detector; the signal output from the gas detector is connected to the positive terminal of the safety barrier. The negative terminals of both the gas detector and the safety barrier are connected to the negative terminal of the 24V power supply system. I hope this can help you.
For three-wire combustible gas detectors, there are hardly any intrinsically safe products available on the market; most of them are explosion-proof types, and safety barriers cannot be used when connecting them to systems or instruments. Since its power consumption exceeds the limits of intrinsically safe products, the safety barrier cannot supply it with power. Wiring must be carried out in accordance with the regulations for flameproof products.
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Three-wire safety barriers did not exist in the past. There are some now. It is said that it existed a few years ago, and that a safety certificate was obtained for it. The three wires of the instrument are connected to the non-intrinsic side of the safety barrier; the intrinsic side of the safety barrier features 2-wire power supply and 2-wire signaling, resulting in a total of 4 wires. Is the combustible gas alarm of the third type intrinsically safe? What signal is the output? Instruments generally adopt a three-wire or four-wire configuration only when it is not possible to achieve 2-wire intrinsically safe operation.
A two-wire safety barrier is a device that operates on the principle of constant power output. I have never used this three-wire safety barrier. It hasn’t been taken apart either. I guess
A two-wire safety barrier is a device that operates on the principle of constant power output. I’ve never used this so-called three-wire safety barrier. It hasn’t been taken apart either. I guess it’s not a device that delivers a constant power output. I guess it’s something like a (stable voltage regulator + linear optocoupler + operational amplifier) combined to create that device. It
The three-wire system consists of 2 power wires and 1 signal wire, with the negative power terminal also serving as the negative signal terminal. When the positive terminal of the safety barrier input is connected to the signal positive, and its negative terminal is connected to the power supply negative, a circuit is formed. Some of the gas detection signals from our company are still sent to the data acquisition device for transmission to the environmental protection agency. For this situation, our company uses a one-in-two-out safety barrier.
Three-wire probes are basically explosion-proof. If it is intrinsically safe, the manufacturer will provide a dedicated safety barrier.
Split the negative wire in two, changing the instrument from a three-wire system to a four-wire system; the 4-20mA signal is sent to the safety barrier