Selection of safety barriers and engineering implementation issues: The Zener safety barrier section

Thread Content

www.273cn.com – signal isolators; www.chinafb123.com – safety barriers. 1. Zener-type safety barriers should be used. The safety of intrinsically safe explosion-proof systems depends primarily on the associated devices, that is, the safety barriers. The selection principles for safety barriers are as follows: 1.1. Determine the explosion-proof rating of the safety barrier required based on the on-site explosion-proof requirements; the explosion-proof rating of the safety barrier must be no lower than the requirements specified for that specific on-site environment ； 1.2. Check the highest voltage that may exist or be generated in the control system, and determine the maximum allowable voltage for the safety barrier; for some devices that operate at high voltages, isolation from intrinsically safe systems is required ； 1.3. The maximum open-circuit voltage, maximum short-circuit current, and maximum power under fault conditions shall not exceed the corresponding values of the intrinsically safe field device under fault conditions ； 1.4. Determine the polarity of the safety barrier based on the signals from the field devices and the polarity of the power supply with respect to ground ； 1.5. Select a suitable safety barrier based on the power supply and signal transmission method of the intrinsically safe field instruments ； 1.6. Consider the effect of the voltage drop across the safety barrier to determine whether the system can operate properly; the terminal resistance and loop resistance of the safety barrier should be sufficient to meet the minimum operating voltage requirements of the intrinsically safe field devices ； 1.7. Influence of common-mode voltage and leakage current on the accuracy of signal response ； 1.8. Are the distributed parameters permitted by the safety barrier satisfactory? ； 1.9. When selecting the associated equipment, namely the safety barrier, for an explosion-proof system composed of transmitters, consideration should be given to the circuit’s operational performance, particularly in terms of 20 milliampere-hours, as well as safety performance ； 1.10. Before selecting a Zener-type safety barrier, it is necessary to know the minimum operating voltage of the transmitter as well as the supply voltage for the secondary instruments, which is usually 24V. Then, a safety barrier with an appropriate internal resistance should be chosen to ensure that the transmitter can function properly ； 1.11. Attention must be paid to the selection of the extreme resistance values for the internal resistors of the safety barrier (it is assumed here that the resistance value of the DCS cannot be reduced). If the terminal resistance is too high, excessive voltage drop may occur at signal levels of around 20 milliamperes, preventing the system from functioning properly. On the other hand, if the terminal resistance is too low, the maximum short-circuit current of the safety barrier will be high, potentially exceeding the maximum current allowed by the field-mounted intrinsically safe instruments to meet their intrinsically safe requirements, thus compromising the safety performance. Therefore, selecting an appropriate terminal resistance requires calculating the allowable voltage drop ； www.jjksmz.com – Health pillows; www.gouwenr.net – Solutions for combating canine distemper; www.ggjjzx.net – Canine distemper; China Massage/ www.chinadfree.com. 1.12. From the perspective of the safety performance of intrinsically safe systems, it is advisable to choose safety barriers with low maximum open-circuit voltages and high maximum short-circuit currents. However, in terms of the practical applicability of Zener safety barriers, it is better to opt for those with low internal resistance. Therefore, when the application requirements correspond to IIB standards, IIB products should be used whenever possible, as they have much lower internal resistance than IIC products. This results in a smaller voltage drop across them, as well as a higher minimum short-circuit current, thereby enhancing their ability to drive loads ； 1.13. Minimize the capacitance and inductance within the intrinsically safe instruments; use connection cables with low distributed capacitance and inductance. In the event of a fault, the maximum external capacitance and inductance of the safety barrier must be at least equal to the maximum equivalent capacitance and inductance values within the intrinsically safe field devices and in their signal circuits. A summary of the information, shared with everyone.