【2026 Wiring Systems】Differences between bus-type and branch-type gas detectors
Thread Content
The bus system is also known as RS485, while the distributed system is known as 4–20mA. These two wiring methods each have their corresponding alarm control host. Generally, the vast majority of bus systems use 4-core shielded wires, with 2 power lines and 2 signal lines, allowing for a relatively long transmission distance of about 1–2 km. The branch circuit system uses 3-core wires: 2 power wires and 1 signal wire. The negative power terminal is shared with the signal wire, and the transmission distance is relatively short, within 1 km.
Advantages of the bus system: ① Unified signals, resulting in a lower probability of failures: Data is transmitted over the data lines in a consistent format, which enhances the reliability of the data. ② Simple wiring and low workload: A clear advantage of the bus system is the reduced amount of wiring required, simpler wiring processes, and lower costs. Four-wire system: two signal lines and two power lines, offering simple and convenient wiring.
Disadvantages of the bus system: ① Signal delay: \"Data refresh is too slow, and the situation gets worse the more urgent it is,\" which is a common complaint from operators regarding bus control systems. It is not possible to transmit detector signals in a timely manner. If there are many sensors, the amount of data to be transmitted from each sensor is large, which results in longer signal transmission delays. This prevents an effective and timely response to the conditions on site. Especially for safety-related control instruments, this can lead to serious accidents. ② Concentration of hazards: With the rapid development of the petrochemical industry, plant scales are becoming increasingly larger and more continuous. Moreover, chemical processes are characterized by flammability, explosiveness, high temperatures, high pressures, toxicity, harmfulness, and corrosivity. Even a minor malfunction in the production control system can lead to catastrophic accidents, causing irreparable severe losses in terms of production, equipment, personnel, and the environment. Therefore, for the control systems of large-scale petrochemical production facilities, safety and reliability are of paramount importance and necessity; even if a failure or damage occurs in the control system (as no control system is free from such issues), it is essential to ensure that the number of affected control points is as small as possible and that the scope of production impacted is as limited as possible. In this way, after the defective areas are removed, production staff can maintain safe production by adhering to the relevant process parameters ； Maintenance personnel can handle individual defects in a more relaxed and straightforward manner, which of course also makes it safer.
How to configure fieldbus instruments? Each trunk segment (Segment) leading to the field (four buses) drives multiple bus-type instruments at the field. The four-wire bus serves both as the power supply bus for multiple sensing detectors and as the bus for transmitting signals upward and downward; undoubtedly, it is used to power multiple gas detection instruments in a production area or production unit. Any failure in this fieldbus backbone will affect all nodes on the four bus backbones. For petrochemical production plants that operate on a continuous basis, it would be a terrifying sight if multiple monitoring points and gas detectors in a single production area or unit suddenly lost control all at once in such a large-scale plant. This is a typical example of concentrated risk.
Although the user guide warns that twisted pairs must not be grounded, short-circuited, or left open – rules that users are required to follow \"absolutely\" – how can this be ensured \"absolutely\" in the highly variable production environments? Even if that is achieved, will the related cards, isolation barriers, components, terminals, etc. of the bus system never experience any problems? The aforementioned failure case is a typical example of \"concentration of hazards\".
③ Power supply issue: The power supply problem has also long been a challenge for combustible gas detectors. In environments where explosion protection is required, DC24V power supply must be used. If all instruments are powered via a centralized bus, the voltage drop on that bus may prevent the instruments at the ends from functioning properly. Therefore, bus-type products often have specific requirements regarding power supply; generally, a single DC power supply should be capable of powering no more than 30 devices, in order to reduce the power current and minimize voltage drops along the wires ； The power supply needs to be located near the site, which requires it to be installed in an explosion-proof box on-site. This is detrimental to the power supply’s heat dissipation, significantly reducing its lifespan and even posing a risk of accidents!
Advantages of the branching system: ① Good data synchronization: Unlike the bus system, in the branching system each sensor communicates separately with the control unit, allowing real-time transmission of information from the field to the control section. This enables monitoring personnel to make timely and effective decisions and take appropriate control actions to prevent dangerous incidents. ② Risk dispersion. Multiple detectors are installed in the same production area or unit; if there is a malfunction in one detector’s circuit, it does not affect the proper operation of the other detectors, thus still providing protection against leaks in that area. ③ There are no restrictions on power supply.
Disadvantages of the branch wiring system: ① Complex wiring: a large amount of wiring is required, resulting in heavy workload and intricate wiring paths. ②Installation is complex, and both installation costs and material costs are high. ③There is a lot of signal interference. The signals transmitted include 0-5V, 4-20mA, pulse signals, level or contact-type digital switch signals, and power supply resistive current signals (greater than 50mA), among others. Interference between these signals is inevitable, and as a result, issues such as false alarms and zero-point drift may occur to some extent.
Selection between bus-type and branch-type gas detectors: 1. The number of sensors to be connected. If there are more than 8 probes, it is recommended to choose a bus system. This reduces the complexity and cost of wiring, as all probes are connected to the host via a single cable ； If the number of probes is less than 8, a distributed wiring system can be chosen, with each probe connected to the host via a separate cable.
2. Distance: In actual operating conditions, since the installation positions of each probe vary and the distances between them differ, it is recommended to use a bus system when the distances are large. The typical transmission distance for a bus system is 1–2 km; only one main cable is needed, and all probes can be connected to this main cable. Additionally, the transmission distance of RS485 can be increased by adding repeaters; theoretically, there is no limit to the distance ； In short distances, a branching system can be used.