Issues with selecting safety barriers?
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There are many types of safety barriers. What are the main factors to consider when choosing one? What are the differences between different safety barriers when in use?
Select different safety barriers based on the signal type indicated on the field panel.
Selection principles for safety barriers: a. Determine the explosion-proof rating of the required safety barrier based on the type of hazardous area and the site’s explosion-proof requirements； b. Equip corresponding types of safety barriers based on the type of equipment on site ； c. 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 ； d. 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 ； e. 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 ； f. Whether the distributed parameters allowed by the safety barrier meet the requirements.
Safety barrier; safety holder; safety barrier. Safety barrier – thermocouple input/single-channel/circuit-powered (NPEXA-C11L) safety barrier. Transmission accuracy of the safety barrier: ±0.2% F.S. (excluding cold-junction compensation accuracy). The standards for safety barriers were drafted under the leadership of Nanjing Youbei Electric, under the title “Safety Barriers for Industrial Process Measurement and Control Systems”. A safety barrier is also known as a safety holder. A safe interface for intrinsically safe circuits, which enables two-way transmission of electrical signals between the safe area and the hazardous area, and can prevent the transfer of energy from the safe area to the hazardous area in the event of a fault. Common safety barriers include Zener-type and isolation-type ones. Intrinsic safety barriers are used in the design of intrinsic safety explosion-proof systems; they are devices installed in safe areas and contain both intrinsic safety circuits and non-intrinsic safety circuits. Current-limiting and voltage-limiting circuits are used in these circuits to restrict the amount of energy sent to the intrinsic safety circuits on site, thereby preventing dangerous energy from the non-intrinsic safety circuits from entering those circuits. In intrinsic safety explosion-proof systems, they are referred to as associated equipment and constitute an important part of such systems. Equipment associated with intrinsically safe explosion-proof systems refers to electrical devices that are installed in safe areas and serve as connectors between intrinsically safe electrical devices and non-intrinsically safe electrical devices. Since the safety barrier is designed as an energy-limiting interface between field devices and control room equipment, it ensures that the energy transmitted to the field devices through it is intrinsically safe, regardless of whether the control room equipment is in normal operation or in a fault state. The China **Instrumentation Explosion Protection Safety Supervision Station is the authoritative body in the People’s Republic of China responsible for overseeing the production of explosion-proof products. It establishes strict, scientific, and detailed regulations regarding intrinsically safe barrier products. Only those enterprises that have passed the certification issued by this supervision station, as well as the products they develop and manufacture, possess safety characteristics that meet the standards; otherwise, it could cause immeasurable damage to the equipment, personnel, and production processes of those who use such products. Installation location: The safety barrier is installed in a safe area; it receives signals from the hazardous area and outputs safe signals to either the safe area or the hazardous area.
Structural types of safety barriers: The common structural types of safety barriers are Zener-type and isolation-type.
Zener-type safety barrier: Fast fuses, current-limiting resistors, or voltage-limiting diodes are used in the circuit to limit the electrical energy entering the system, thereby ensuring that the appropriate amount of energy is delivered to the hazardous area. Its principle is simple, its circuit implementation is easy, and it is inexpensive. However, due to flaws in its underlying principle, its reliability in practical applications is greatly affected, which limits its scope of use. The reasons are as follows: 1. The installation location must have a very reliable grounding system, and the grounding resistance of this Zener-type safety barrier must be less than 1Ω; otherwise, it loses its explosion-proof safety functions. Clearly, such requirements are quite stringent and difficult to meet in actual engineering applications. 2. The field instruments located in hazardous areas must be isolated-type; otherwise, when the grounding terminals of the Zener safety barrier are connected to the ground, the signals cannot be transmitted properly. Moreover, signal grounding reduces the signal’s resistance to interference, thereby affecting the stability of the system. 3. Zener safety barriers have a significant impact on the power supply, and they are also prone to damage due to fluctuations in the power supply. 4. Since the circuit principle of the Zener-type safety barrier requires it to absorb energy from the input circuit, it is prone to cause instability in the output. Isolated safety barrier: It adopts a circuit structure that provides electrical isolation between the input, output, and power supply, while also meeting the energy limitation requirements of intrinsically safe systems. Compared to Zener-based safety systems, although it is more expensive, its superior performance advantages bring greater benefits to users: 1. Thanks to the three-party isolation mechanism, no system grounding wire is required, which greatly simplifies design and on-site installation. 2. The requirements for instruments in hazardous areas are significantly reduced, and isolated instruments are not necessary on-site. 3. Since the signal lines do not require a common ground, the stability and interference resistance of the signals in the detection and control circuits are **enhanced**, thereby improving the reliability of the entire system. 4. Isolated safety barriers possess stronger input signal processing capabilities; they can accept and process signals from thermocouples, thermal resistors, frequencies, etc., something that zener safety barriers cannot do. 5. An isolated safety barrier can output two mutually isolated signals, which are provided to two devices that use the same signal source, ensuring that the signals from these two devices do not interfere with each other. It also enhances the electrical safety isolation between the connected devices. Therefore, by comparing the characteristics and performance of Zener-type and isolated safety barriers, it can be seen that the isolated safety barrier has significant advantages and a wider range of applications. Although its price is slightly higher than that of the Zener-type safety barrier, its overall cost may actually be lower when taking into account design, installation, commissioning, and maintenance costs. In engineering sites with high requirements, isolated safety barriers are almost universally used as the primary intrinsically safe explosion-proof instruments. These isolated safety barriers have gradually replaced Zener-type safety barriers and are being increasingly employed in the field of explosion protection.
Definition of intrinsically safe device markings
Where:
EX – Explosion-proof marking that complies with certain standards
(iiA) – Explosion protection rating
IIC – Gas group suitable for ZONE 0 hazard levels
The explosion protection rating of our company’s products is: EX(iiA)IIC
Explosion protection rating
iiA: Under normal operating conditions, the device will not ignite hazardous gases even in the presence of 1 or 2 counting faults; the circuit must maintain its safety properties even when both counting faults occur simultaneously. “Electrical equipment of the “ia” category must adopt a “triplication” design for components that are prone to interference. “Electrical equipment of the “ib” category can only ensure that no hazardous gases will be ignited under 1 counting fault condition. Gas group: Group I electrical equipment: Used in coal mining environments susceptible to methane. Group II electrical equipment: Can be used in explosive hazardous environments other than coal mines. Group II electrical equipment is further subdivided according to the ignition energy of different flammable substances. The various subgroups are distinguished by uppercase English letters. As can be seen from the table below, the IIC group requires the least ignition energy according to the standards of China, IEC, and Europe; in other words, among electrical devices in this group, those belonging to the IIC group have compatibility with devices from groups IIA and IIB. Classification of common flammable substances Typical gases, classification criteria, ignition characteristics China, IEC, Europe, North America Methane: Class I Acetylene: Class IIC, Group A Hydrogen: Class IIC, Group B Ethylene: Class IIB, Group C Propane: Class IIA, Group D Zoning of explosive hazard areas In the design of safety and explosion-proof systems as well as in the selection of explosion-proof products, it is necessary not only to classify and group the gases present in explosive environments but also to zone explosive hazard areas based on the frequency and duration of the presence of such gases: Zone 0 Areas where explosive gas mixtures are present continuously or for an extended period of time. Zone 1: Areas where explosive gas mixtures are likely to occur. Zone 2: Areas where explosive gas mixtures cannot occur, or if they do appear, it is only for a short period of time. Our products are suitable for Zone 0, as well as Zones 1 and 2. Installation of isolated safety barriers Color codes: The yellow terminal (the non-intrinsic side) is connected to the safe area. The blue terminal (intrinsic safety side) wiring leads to the hazardous area. Precautions during installation: 1. The safety barrier should be installed in a safe location, and the environmental conditions must meet the requirements specified under the \"Operating Conditions\" in the \"Safety Barrier Selection Guide\". 2. The connection wires for the intrinsically safe side (blue side) and the non-intrinsically safe side circuits of the isolated safety barrier should be laid separately in the cable tray, each using its own protective sleeve. Other power cables are not allowed in the wiring ducts on the intrinsically safe side, including those used for the intrinsically safe circuit. 3. Wires leading to hazardous areas should be intrinsically safe wires marked in blue; the cross-sectional area of the soft copper in such wires must be greater than 0.5 mm2, and their insulation strength must be above 500 V. 4. Before powering on the isolated safety barrier for testing, it is essential to ensure that its model, wiring method, and circuit polarity meet the specifications outlined in the design and product requirements; otherwise, it may cause harm to people and equipment. 5. It is strictly prohibited to use a megohmmeter to test the insulation strength between the terminals of isolated safety barriers. To check the insulation strength of the system, all connections of the isolation safety barriers must be disconnected first; otherwise, it may cause damage to the circuits inside the safety barriers. 6. Before programming the safety barrier on-site, it is necessary to disconnect all connections first before connecting it to the programmer, and then power it on to carry out programming; otherwise, it may lead to adverse consequences. 7. Field instruments connected to isolated safety barriers shall be those that have undergone explosion-proof testing by **approved relevant explosion-proof testing institutions and obtained explosion-proof certification. 8. When designing, installing, using, and maintaining isolated safety barriers, it is necessary to comply simultaneously with the instructions in this product’s user manual, as well as with GB3836.15-2000 Electrical equipment for explosive gas environments – Part 15: Electrical installation in hazardous locations (except coal mines), and GB50058-1992 Code for design of electrical installations in explosive and fire-hazardous environments.
Structural types of safety barriers: The common structural types of safety barriers are Zener-type and isolation-type.
Zener-type safety barrier: Fast fuses, current-limiting resistors, or voltage-limiting diodes are used in the circuit to limit the electrical energy entering the system, thereby ensuring that the appropriate amount of energy is delivered to the hazardous area. Its principle is simple, its circuit implementation is easy, and it is inexpensive. However, due to flaws in its underlying principle, its reliability in practical applications is greatly affected, which limits its scope of use. The reasons are as follows: 1. The installation location must have a very reliable grounding system, and the grounding resistance of this Zener-type safety barrier must be less than 1Ω; otherwise, it loses its explosion-proof safety functions. Clearly, such requirements are quite stringent and difficult to meet in actual engineering applications. 2. The field instruments located in hazardous areas must be isolated-type; otherwise, when the grounding terminals of the Zener safety barrier are connected to the ground, the signals cannot be transmitted properly. Moreover, signal grounding reduces the signal’s resistance to interference, thereby affecting the stability of the system. 3. Zener safety barriers have a significant impact on the power supply, and they are also prone to damage due to fluctuations in the power supply. 4. Since the circuit principle of the Zener-type safety barrier requires it to absorb energy from the input circuit, it is prone to cause instability in the output. Isolated safety barrier: It adopts a circuit structure that provides electrical isolation between the input, output, and power supply, while also meeting the energy limitation requirements of intrinsically safe systems. Compared to Zener-based safety systems, although it is more expensive, its superior performance advantages bring greater benefits to users: 1. Thanks to the three-party isolation mechanism, no system grounding wire is required, which greatly simplifies design and on-site installation. 2. The requirements for instruments in hazardous areas are significantly reduced, and isolated instruments are not necessary on-site. 3. Since the signal lines do not require a common ground, the stability and interference resistance of the signals in the detection and control circuits are **enhanced**, thereby improving the reliability of the entire system. 4. Isolated safety barriers possess stronger input signal processing capabilities; they can accept and process signals from thermocouples, thermal resistors, frequencies, etc., something that zener safety barriers cannot do. 5. An isolated safety barrier can output two mutually isolated signals, which are provided to two devices that use the same signal source, ensuring that the signals from these two devices do not interfere with each other. It also enhances the electrical safety isolation between the connected devices. Therefore, by comparing the characteristics and performance of Zener-type and isolated safety barriers, it can be seen that the isolated safety barrier has significant advantages and a wider range of applications. Although its price is slightly higher than that of the Zener-type safety barrier, its overall cost may actually be lower when taking into account design, installation, commissioning, and maintenance costs. In engineering sites with high requirements, isolated safety barriers are almost universally used as the primary intrinsically safe explosion-proof instruments. These isolated safety barriers have gradually replaced Zener-type safety barriers and are being increasingly employed in the field of explosion protection.
Definition of intrinsically safe device markings
Where:
EX – Explosion-proof marking that complies with certain standards
(iiA) – Explosion protection rating
IIC – Gas group suitable for ZONE 0 hazard levels
The explosion protection rating of our company’s products is: EX(iiA)IIC
Explosion protection rating
iiA: Under normal operating conditions, the device will not ignite hazardous gases even in the presence of 1 or 2 counting faults; the circuit must maintain its safety properties even when both counting faults occur simultaneously. “Electrical equipment of the “ia” category must adopt a “triplication” design for components that are prone to interference. “Electrical equipment of the “ib” category can only ensure that no hazardous gases will be ignited under 1 counting fault condition. Gas group: Group I electrical equipment: Used in coal mining environments susceptible to methane. Group II electrical equipment: Can be used in explosive hazardous environments other than coal mines. Group II electrical equipment is further subdivided according to the ignition energy of different flammable substances. The various subgroups are distinguished by uppercase English letters. As can be seen from the table below, the IIC group requires the least ignition energy according to the standards of China, IEC, and Europe; in other words, among electrical devices in this group, those belonging to the IIC group have compatibility with devices from groups IIA and IIB. Classification of common flammable substances Typical gases, classification criteria, ignition characteristics China, IEC, Europe, North America Methane: Class I Acetylene: Class IIC, Group A Hydrogen: Class IIC, Group B Ethylene: Class IIB, Group C Propane: Class IIA, Group D Zoning of explosive hazard areas In the design of safety and explosion-proof systems as well as in the selection of explosion-proof products, it is necessary not only to classify and group the gases present in explosive environments but also to zone explosive hazard areas based on the frequency and duration of the presence of such gases: Zone 0 Areas where explosive gas mixtures are present continuously or for an extended period of time. Zone 1: Areas where explosive gas mixtures are likely to occur. Zone 2: Areas where explosive gas mixtures cannot occur, or if they do appear, it is only for a short period of time. Our products are suitable for Zone 0, as well as Zones 1 and 2. Installation of isolated safety barriers Color codes: The yellow terminal (the non-intrinsic side) is connected to the safe area. The blue terminal (intrinsic safety side) wiring leads to the hazardous area. Precautions during installation: 1. The safety barrier should be installed in a safe location, and the environmental conditions must meet the requirements specified under the \"Operating Conditions\" in the \"Safety Barrier Selection Guide\". 2. The connection wires for the intrinsically safe side (blue side) and the non-intrinsically safe side circuits of the isolated safety barrier should be laid separately in the cable tray, each using its own protective sleeve. Other power cables are not allowed in the wiring ducts on the intrinsically safe side, including those used for the intrinsically safe circuit. 3. Wires leading to hazardous areas should be intrinsically safe wires marked in blue; the cross-sectional area of the soft copper in such wires must be greater than 0.5 mm2, and their insulation strength must be above 500 V. 4. Before powering on the isolated safety barrier for testing, it is essential to ensure that its model, wiring method, and circuit polarity meet the specifications outlined in the design and product requirements; otherwise, it may cause harm to people and equipment. 5. It is strictly prohibited to use a megohmmeter to test the insulation strength between the terminals of isolated safety barriers. To check the insulation strength of the system, all connections of the isolation safety barriers must be disconnected first; otherwise, it may cause damage to the circuits inside the safety barriers. 6. Before programming the safety barrier on-site, it is necessary to disconnect all connections first before connecting it to the programmer, and then power it on to carry out programming; otherwise, it may lead to adverse consequences. 7. Field instruments connected to isolated safety barriers shall be those that have undergone explosion-proof testing by **approved relevant explosion-proof testing institutions and obtained explosion-proof certification. 8. When designing, installing, using, and maintaining isolated safety barriers, it is necessary to comply simultaneously with the instructions in this product’s user manual, as well as with GB3836.15-2000 Electrical equipment for explosive gas environments – Part 15: Electrical installation in hazardous locations (except coal mines), and GB50058-1992 Code for design of electrical installations in explosive and fire-hazardous environments.