【2025 Mini Knowledge Point】Differences and connections among intrinsically safe devices such as Zener gates, isolation gates, and intrinsically safe cards

Principles for selecting safety barriers: 1. Determine the explosion-proof rating of the required safety barrier based on the type of hazardous area and the on-site explosion-proof requirements. The explosion protection rating of the safety barrier must be no lower than the explosion protection rating indicated on the intrinsically safe field equipment ； 2. Equip corresponding types of safety barriers based on the type of equipment present on site ； 3. Determine the maximum voltage of the safety barrier based on the highest voltage that may exist or be generated by the instruments in the control room ； 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 ； The safety barrier must match the safe polarity and signal transmission method of the intrinsically safe field instruments. 5. Consider the effect of voltage drop across the safety gate terminals to determine the maximum short-circuit current of the safety gate and whether the system can operate properly ； 6. Are the distributed parameters permitted by the safety barrier satisfactory?

Key points to note regarding the use of safety barriers in petrochemical production technologies: 1. Regarding explosion protection, safety barriers must meet the required explosion-proof rating for the specific installation site; this is necessary to ensure the proper capacity of cables and on-site equipment, so as to maintain their capacitive and inductive properties. 2. Regarding grounding: When installing a Zener safety barrier, a very reliable grounding system is required, and its grounding resistance must be less than 1 ohm. 3. Regarding instruments, select a suitable safety barrier based on the type of instrument signals present on site. 4. Regarding cables: Intrinsic safety system equipment requires the use of intrinsic safety cables, and such cables should be laid separately from non-intrinsic safety cables during installation. And signs need to be made.

When using a Zener safety barrier, the following points should be noted: (1) The installation location must have a very reliable grounding system, with a grounding resistance of less than 1Ω; otherwise, the explosion-proof protection function will be lost. (2) It is advisable that the field instruments from the hazardous area have isolation functionality; otherwise, once the grounding terminals of the Zener safety barrier are connected to the ground, the signals cannot be transmitted properly, and signal grounding reduces the signal’s resistance to interference, affecting the stability of the system. (3) The Zener safety barrier has a significant impact on the power supply, and power supply fluctuations can easily cause damage to the barrier. (4) Due to the circuit principle of the Zener safety barrier, which requires it to absorb energy from the input circuit, this can lead to unstable output signals.

Methods for dealing with common faults of safety barriers: 1. The power indicator light does not glow or the LCD display shows nothing. (1) Check whether the power supply to the instrument is connected correctly and reliably. (2) Check whether the supply voltage is normal. (3) Check whether the positive and negative terminals of the 24V power supply are reversed.

2. No signal output (1) Has the input signal been properly connected to the input terminal? (2) Is the type of input signal consistent with the instrument’s input type? (3) Are the output terminals tightened? (4) Whether the removable external wiring terminals are properly inserted. (5) The external output circuit is open.

3. Large deviation in output value: (1) Are the positive and negative leads connected reversely when the thermocouple is connected? (2) Whether the three-wire connection is correct when a thermoresistor is connected. (3) Are the positive and negative polarities of the output wiring reversed? (4) When current is output, is the total value of the external load resistances greater than the specified value in ohms (for 4-20mA output)? (5) When voltage is output, is the external load resistance less than 500 kΩ? (6) The power supply voltage is too low.

4. Unstable output signal (1) Check whether the wiring is secure. (2) Check whether the input signal is stable. (3) Check whether the wiring of the output circuit is reliable. (4) Check whether there are high-power electromagnetic devices in the environment surrounding the instrument.