Classification and working principle of safety barriers

http://bbs.hcbbs.com/viewthread.php?tid=57576 Comrade XXGF has been registered for quite some time now; he should learn how to use search functions. In the future, before posting such threads, he should first check what’s already available.

A Zener safety barrier operates based on the reverse breakdown behavior of Zener diodes. A Zener safety barrier consists of a voltage limiting circuit, a current limiting circuit, and fuses, etc. The function of the voltage limiting circuit is to keep the voltage on the safety circuit side at the rated operating voltage. When the supply voltage V1 equals the rated operating voltage of the Zener safety barrier, neither Zener diode DW1 nor DW2 conducts; at this point, the voltage V2 on the safe spark circuit side equals the voltage required to power the field instruments. When the supply voltage V1 exceeds the rated operating voltage, the Zener diode DW1 breaks down and conducts first, so the voltage V2 on the safety spark circuit side remains unchanged. When V1 rises to equal the maximum safe holding rating voltage, the current flowing through DW1 and DW2 causes the Zener diodes to heat up and get damaged; at this point, the fuse RD blows, cutting off the power supply and thereby providing protection for the safe spark circuit side. The fuse blow time Tf and the Zener diode blow time TZ must satisfy the requirement of 10Tf < TZ. In the Zener safety barrier, the use of thick-film fast-blow fuses ensures that the maximum safe operating voltage for the voltage V1 on the non-safe spark circuit side is 340V DC. The function of the current limiting circuit is to limit the current on the safe spark circuit side to below 35mA DC. For the Zener safety barrier used in conjunction with the transmitter, as long as the transmitter’s output current is within the range of 4–20 mA DC, the safety barrier has no impact on the operation of the transmitter. However, in the event of an accident with the transmitter, such as a short circuit, the current-limiting circuit limits the current on the safe spark circuit side to below 35 mA DC. Overview of Isolated Safety Gates: In industrial environments, two-wire transmission systems are generally used for power distribution units. These units are required to supply a 24V power supply to primary instruments such as pressure transmitters, while also collecting, amplifying, processing the incoming current signals, and filtering out interference before outputting isolated current and voltage signals for use by subsequent secondary instruments or other devices. However, some special industrial sites require not only two-wire transmission that provides both power supply and signal isolation, but also explosion-proof properties of the safety spark type to reliably prevent contact between high voltage from the power supply and the signals. These sites need special-purpose power distribution devices – safety barriers – that utilize dual control of current and voltage to limit the energy entering hazardous areas to levels below safe thresholds. Isolated safety barriers basically come in two types: detection-side safety barriers and operation-side safety barriers. The detection terminal safety barrier is used in conjunction with two-wire transmitters ； The operational safety barrier is used in conjunction with electrical converters or electrical valves. There are also isolated safety barriers of types such as signal input. Thanks to measures such as voltage and current limiting as well as isolation employed in isolated safety barriers, not only is it possible to prevent dangerous energy from entering the hazardous area through intrinsically safe terminals, thereby enhancing the system’s intrinsically safe explosion-proof properties, but the system’s resistance to interference is also increased, **which improves the reliability of its operation. Working principle: The 24VDC power supply is converted through DC-AC-DC conversion to generate various voltages required by the module circuit. The principle of the isolated safety barrier at the detection end is as follows: The modular circuit converts the current or voltage signals supplied through the intrinsically safe energy limiting circuit to 0.2–1 VDC, then sends them into the module where they are collected, amplified, processed to eliminate interference, and subsequently modulated by a transformer into isolated current and voltage signals for use by subsequent secondary instruments or other devices. The module must also output a isolated 18.5∽28.5VDC voltage, which is used via an intrinsically safe energy limiting circuit as the operating voltage for two-wire transmitters. The intrinsically safe energy limiting circuit can prevent dangerous signals with high current or high voltage from entering hazardous areas. The principle of the isolated safety barrier at the control side is to isolate the 4-20mA DC signal generated by the regulator or operator, and then output another 4-20mA DC signal, which is supplied through an intrinsically safe energy limitation circuit to the electrical converter or the on-site electrical valve positioner for use. The intrinsically safe energy limiting circuit can prevent dangerous signals with high current or high voltage from entering hazardous areas.

The answer is truly classic; it provides a great learning platform for us beginners. Thank you