Safety barrier power distribution issues

A safety barrier is used to reduce voltage and prevent short circuits that could damage controllers (such as PLC cards). The wires coming out of the safety barrier are then energized, allowing instruments to transmit signals as well. Personally, I feel that in this case the instruments do not require safety barriers (signal safety barriers), and using intrinsically safe signal cables and other such circuits means that the system is no longer intrinsically safe. If that’s the case, there’s no need to use intrinsically safe instruments; ordinary ones will suffice. Intrinsic safety instruments all require a safety barrier and intrinsic safety cables to form an intrinsic safety system; this seems to have nothing to do with two-wire or four-wire systems. Generally, in four-wire instruments, the current in the power supply voltage lines (+-24V) is quite high, exceeding 20mA. In this case, security deletion cannot be added. If the safety barrier limits the current, the meter can’t function. It’s fine to use a signal line with a safety barrier. It would naturally be best if the current in the power cable were less than 20. Intrinsic safety refers not to a single instrument, but to the entire circuit system. To achieve the intrinsically safe explosion-proof level, it is required that the instruments in that circuit be intrinsically safe, that the cables used meet intrinsically safe requirements, and that a safety barrier be installed in the circuit. All three of the above are essential. I really haven’t seen any intrinsically safe meters with a 4-wire system. I have two questions: First, is it necessary to use a safety barrier for the power cable? This makes sense for 24V systems, but what about those powered by 220V? Second, whether a safety barrier (signal safety barrier) is needed for the signal line: in a 4-wire system, the signal energy is generated within the meter, so shouldn’t the safety barrier be located inside the meter?

By using an isolated safety barrier, it is only necessary to isolate the instrument signals coming from the field; no 24VDC power supply is required. The power for the field instruments is provided directly by the air switch, and the instrument measurement signals enter the DCS after passing through the isolated safety barrier.

　　The reason why instruments are designed as 4-wire systems is, in the vast majority of cases, that the power consumption exceeds the intrinsically safe standards, forcing such a design. 　　Therefore, 4-wire instruments are either intrinsically safe types or those with an intrinsically safe signal circuit while the power supply part is of an intrinsically safe type; in either case, a 4-wire safety barrier is not required. 　　For the rarely used 4-wire intrinsically safe meters, a power isolation barrier and an input safety barrier can be used respectively to isolate the power supply and the signal circuit. 　　Therefore, while it can’t be said that 4-wire safety barriers don’t exist, I really have never heard of them. 　

Indeed, four-wire intrinsically safe instruments are rare, but they do exist. For four-wire intrinsically safe instruments, attention should be paid to the certification parameters of their power supply terminals; generally, safety barriers provide only a very low power supply capacity (around 0.5W). Therefore, for such intrinsically safe instruments with a power consumption of over 30 mA, it is generally necessary to consider providing separate power supply; an intrinsically safe power supply can be used, or a Zener diode can be added to a regular power supply for protection. The selection of the Zener gate must match the certification parameters of the power port for intrinsically safe instruments. The view expressed on the 2nd floor is correct! Intrinsic safety instruments must be equipped with a safety barrier and intrinsic safety cables to form an intrinsic safety system, regardless of the number of wires used with the field devices. Furthermore, devices powered by 220V cannot be made intrinsically safe.

This won’t do! It is not sufficient to merely provide protective isolation for the signals; since the power supply comes directly from a circuit breaker or a regular power source, the entire system cannot constitute a intrinsically safe system. The power supply side also requires an intrinsically safe power supply or a safety barrier for protection.