Party Building Garden

12

2013-07

The function of a safety barrier

Currently, the main types of safety barriers in use are resistive, Zener, relay‑amplifier, and isolating barriers. Among these, Zener‑type and isolating‑type barriers are the most widely applied. Zener‑Type Safety Barrier A Zener‑type safety barrier operates based on the reverse breakdown (conduction) characteristics of Zener diodes. (1) When the supply voltage is normal—typically rated at 24 V with a maximum of 28 V—the Zener diodes VDW1 and VDW2 remain non‑conducting, and the loop current is determined by the transmitter within the 4–20 mA range. In the event of a short circuit on-site, the presence of resistor R limits the short‑circuit current to below the safe rated value, thereby ensuring safety. (2) When the barrier terminal voltage V1 exceeds the safe rated voltage V0 but remains below the discharge voltage Var of the spark gap—that is, V0 ≤ V1 ≤ Var—the Zener diodes break down (conduct), causing the current through fuse F1 to increase. Once this current rises above 125 mA, the fast‑acting fuse F1 blows first (within microseconds), instantly isolating the potentially hazardous high voltage from the field. Prior to F1 blowing, the voltage‑regulating action of VDW1 and VDW2 continues to maintain safety in the hazardous area. (3) If V1 reaches or exceeds Var, the spark gap discharges immediately, reducing the terminal voltage to a very low level (below 10–20 V). At this point, the current through fuse F2 rapidly increases; when it reaches 1 A, F2 blows, cutting off the dangerous high voltage and safeguarding production safety. Under normal operating conditions, thanks to the current‑limiting effect of resistor R and the voltage‑regulating function of the Zener diodes, fluctuations in current and voltage on the hazardous side are effectively contained, minimizing frequent blowouts of fuses F1 and F2. Zener‑type safety barriers are compact, lightweight, highly accurate, versatile, and more affordable than isolating‑type barriers. They also offer relatively high explosion‑proof ratings. However, their key component—the fast‑acting fuse—is extremely difficult to manufacture, requiring stringent process controls and high‑quality materials. The fuse must blow at least ten times faster than the Zener diode could burn out. Zener‑type safety barriers require a separate intrinsically safe grounding system. Isolating‑Type Safety Barrier This type employs a circuit architecture that electrically isolates the input, output, and power supply circuits from one another, while simultaneously meeting intrinsic safety requirements for energy limitation. Compared with Zener‑type barriers, although slightly more expensive, isolating barriers offer distinct advantages that deliver greater benefits to users: 1. Due to their three‑way isolation design, they eliminate the need for a common ground connection, greatly simplifying both system design and on‑site installation. 2. Instrumentation requirements in hazardous areas are significantly reduced, removing the necessity for intrinsically safe instruments on site. 3. With no shared ground required for signal lines, the stability and noise immunity of sensing and control loops are markedly improved, enhancing overall system reliability. 4. Isolating barriers provide superior input‑signal processing capabilities, capable of accepting and handling thermocouple, RTD, frequency, and other signals—functions beyond the reach of Zener‑type barriers. 5. They can output two mutually isolated signals, enabling simultaneous use by two devices sharing the same signal source while preventing cross‑interference and improving electrical insulation between connected equipment. Thus, comparing the features and performance of Zener‑type and isolating‑type safety barriers reveals that isolating barriers possess notable advantages and broader applicability. Although their price is slightly higher than Zener‑type barriers, when considering design, installation, commissioning, and maintenance costs, their total cost may actually be lower. In demanding industrial environments, isolating barriers have almost universally replaced Zener‑type barriers as the primary intrinsically safe explosion‑proof instrumentation, gradually gaining widespread adoption in the field of explosion protection.

2013-07-12

17

2013-04

Explosion-proof Knowledge for Safety Barriers

2013-04-17

22

2013-03

Installation of Surge Protective Devices

2013-03-22

08

2012-01

Field Applications of Signal Isolators

2012-01-08

20

2011-11

How to distinguish between genuine and counterfeit current and voltage transmitters

2011-11-20

27

2011-10

Key Considerations for Selecting a Signal Isolator

2011-10-27

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Liaoning Advance Foundation Semiconductor Materials Co., Ltd.

Address: No.89, Shendong 4th Road, Wanghua District, Fushun City, Liaoning Province

Telephone: +86 24 56856688

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