A Brief Overview of Safety Barriers and Isolators
1. Definitions of Safety Barriers and Isolators
Safety Barrier
A safety barrier is a device used in hazardous areas to receive signals from dangerous zones and limit the electrical energy in the circuit, thus ensuring intrinsic safety for explosion-proof purposes. Safety barriers are categorized into two main types: Zener Barrier and Isolator Safety Barrier.
Zener Safety Barrier
A Zener safety barrier utilizes components like fast fuses, current-limiting resistors, or voltage-limiting diodes to restrict the electrical energy input, ensuring that the energy transmitted to the hazardous zone does not trigger sparks that could cause explosions. This method guarantees the safety of both personnel and the environment.
Key Installation Requirements and Features for Zener Barriers:
The installation location must have a reliable grounding system, and the barrier itself must be grounded with a resistance of less than 1Ω to maintain its explosion-proof properties.
Instruments in hazardous areas must be of the isolating type, or else the signal transmission will be ineffective due to grounding.
Zener barriers are sensitive to power fluctuations and are more prone to damage if the power supply is unstable.
They may cause output instability due to the need to absorb energy from the input circuit.
While Zener barriers are cost-effective, their installation requirements are stringent, and they may not be the best solution in terms of long-term durability.
Isolator Safety Barrier
An isolator safety barrier uses a circuit design that electrically isolates the input, output, and power supply, meeting the intrinsic safety energy limitation requirements.
Advantages of Isolator Safety Barriers:
No need for system grounding, which simplifies design and installation.
It reduces the requirements for instruments in hazardous areas, allowing for more flexible system configurations.
The system’s stability and anti-interference capabilities are significantly enhanced, improving the overall reliability of the setup.
It has a stronger signal handling capacity, supporting thermocouples, switches, and other signals which are not feasible with Zener barriers.
Isolators can output two independent signals, providing electrical isolation for two devices connected to the same signal source.
Although Isolator safety barriers are more expensive than Zener barriers, they offer clear advantages in terms of performance and flexibility. It is recommended to use Isolator barriers in most applications.
Signal Isolator
A signal isolator serves to isolate, filter, amplify, convert, and distribute input signals to other instruments or systems, thereby improving electromagnetic interference (EMI) resistance.
Signal isolators are primarily used to eliminate various interferences during operations, significantly reducing electromagnetic disturbances. This is crucial for ensuring stable electronic device operation and preventing malfunctions.
Key Benefits of Signal Isolators:
Provide electrical isolation to prevent ground loops.
Reduce the risk of damage from voltage spikes or transients.
Maintain signal integrity for accurate measurements.
Eliminate electromagnetic interference, enhancing safety in industrial applications.
In comparison, Zener barriers have strict installation conditions and are more vulnerable to external interference. Signal isolators, while used for general interference elimination and signal amplification, are better suited for stable and long-term field operations. Safety barriers are primarily used in hazardous environments with explosive gases or dust, providing safety protection in chemical, gas, and mining industries.
2. Common Types of Safety Barriers and Isolators
(Here, we take the ZheJiang University Control Safety Barrier as an example.)
1. Switch Input Type
Switch input isolators are less commonly used but can be useful to understand. This is an active isolator, where pins 6 and 8 serve as the DC24V power supply, pins 5 and 7 are for output, and pins 1 and 2 are for input from field relays or proximity switches. When the relay or proximity switch closes, the isolator’s output terminals (pins 5 and 7) close simultaneously.
2. Current Input Type
This type is used for collecting current signals from transmitters. In this active isolator, pins 6 and 8 supply DC24V, pins 5 and 7 are for output, and pins 1 and 2 are for input from field transmitters. It can handle multiple current input types, such as for pressure, flow, level, temperature, and water quality transmitters.
3. Current Output Type
Used to output analog signals for DCS or PLC systems. This active isolator works with a 24V DC power supply and outputs current signals to control valves or frequency converters.
4. Thermocouple Input Type
Thermocouple input isolators, typically used less frequently, can measure temperature from -200°C to 1600°C. The isolator converts the thermocouple mV signal to a 4-20mA output.
5. RTD Input Type
This type is used to collect temperature signals from resistance temperature detectors (RTDs). RTDs have a narrower measurement range than thermocouples (e.g., -200°C to +850°C for platinum RTDs). This isolator converts RTD signals to 4-20mA outputs.
Passive Isolators
These isolators do not require separate power supply and are typically powered by the control room system, reducing energy consumption. Similar to active isolators, passive isolators are used for various signal types, such as analog inputs, thermocouples, and RTDs, depending on the manufacturer’s product offerings.
3. Selection of Intrinsic Safety Barriers
Intrinsic Safety Parameters
Maximum Voltage (Um): The highest voltage that can be applied to a non-intrinsically safe device without compromising safety performance.
Maximum Output Voltage (Uo): The highest output voltage of the intrinsic safety circuit under open-circuit conditions.
Maximum Output Current (Io): The maximum current output from the intrinsic safety circuit.
Maximum Output Power (Po): The maximum power that can be delivered from the intrinsic safety circuit.
Maximum External Capacitance (Co): The maximum capacitance that can be connected to the electrical equipment without causing failure of intrinsic safety.
Maximum External Inductance (Lo): The maximum inductance that can be connected without compromising safety.
Matching Requirements for Intrinsic Safety Devices
When purchasing intrinsic safety instruments, it is essential to compare the parameters with those of the safety barriers to ensure compliance with safety standards. Failure to match these parameters can lead to safety hazards and failure to pass inspections.
Symbols for Intrinsically Safe Equipment and Barriers
Safety barriers are marked with specific explosion-proof symbols. Intrinsically safe equipment is indicated with the “Ex” symbol, followed by the type of protection (e.g., ia for intrinsic safety), and the protection level (e.g., Ga for equipment protection level).
Conclusion
In conclusion, Zener safety barriers are cost-effective but have higher installation requirements and lower durability. For more flexible and reliable performance, isolator safety barriers are recommended in most cases. Signal isolators are also essential in enhancing system reliability and protecting devices from electromagnetic interference.