The main function of a safety barrier is to limit the entry of hazardous energy into a hazardous area, that is, to restrict the voltage and current sent to the hazardous area. Zener transistor Z is used to limit voltage. When the circuit voltage approaches the safe voltage limit, the Zener transistor conducts, keeping the voltage at both ends of the Zener transistor below the safe voltage limit.

Basic knowledge

It adopts a circuit structure that electrically isolates the input, output, and power supply from each other, while complying with the requirements of intrinsic safety energy limitation. Compared with Zener safety, although the price is slightly higher, its outstanding advantages in other aspects have brought greater benefits to user applications:

Due to the use of a three party isolation method, there is no need for a system grounding line, which greatly facilitates design and on-site construction. 2. The requirements for instruments in hazardous areas have been significantly reduced, and there is no need to use isolated instruments on site.

2. As the signal lines do not require a common ground, the stability and anti-interference ability of the detection and control circuit signals are greatly enhanced, thereby improving the reliability of the entire system.

3. Isolated safety barriers have stronger input signal processing capabilities and can accept and process signals such as thermocouples, thermistors, frequencies, etc., which cannot be achieved by Zener safety barriers.

4. The isolated safety barrier [2] can output two mutually isolated signals to provide to two devices using the same signal source, ensuring that the signals of the two devices do not interfere with each other and improving the electrical safety insulation performance between the connected devices.

Therefore, after comparing the characteristics and performance of Zener and isolated safety barriers, it can be seen that isolated safety barriers have outstanding advantages and a wider range of uses. Although their price is slightly higher than Zener safety barriers, their overall cost may be lower than Zener safety barriers in terms of design, construction, installation, commissioning, and maintenance costs. Isolation safety barriers are almost universally used as the main intrinsic safety explosion-proof instruments in engineering sites with high requirements. Isolation safety barriers have gradually replaced Zener safety barriers and have been increasingly widely used in the field of safety and explosion-proof

Definition of intrinsic safety equipment labeling

Among them: Ex - Explosion proof mark

(ia) - Explosion proof rating

IIC - Gas Group

Product explosion-proof level: Ex (ia) Ⅱ C

Explosion proof grade

Ia: Under normal working conditions, hazardous gases will not ignite in one or two fault states, and the circuit must ensure safety characteristics even when two faults occur simultaneously. Class "ia" electrical equipment must adopt a "triple" design for components that are susceptible to interference. Class IB electrical equipment can only guarantee that it will not ignite hazardous gases in one fault state.

Gas group

Group I electrical equipment: used in coal mine environments that are 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 based on the ignition energy of flammable substances.

Each subgroup is distinguished by uppercase English letters. From the table below, it can be seen that subgroup C requires less ignition energy, indicating that among the electrical equipment in this group, subgroup C has universality.

Grouping of common flammable substances

Zoning of explosive hazardous areas

In the design of safety and explosion-proof systems and the selection of explosion-proof products, in addition to classifying and grouping the gases present in explosive environments, the hazardous areas of explosive gases should also be classified according to the frequency and duration of their occurrence: our company's products are suitable for Zone 0, Zone 1, and Zone 2.

color code

The yellow terminal (non intrinsic safety side) is wired to the safe area

Blue end (intrinsic safety side) wiring leads to hazardous area

Working principle

The resistor R is used to limit the current. When the voltage is limited, selecting the appropriate resistance value can limit the circuit current below the safe current limit.

Compared with Zener safety barriers, isolated safety barriers not only have the functions of voltage and current limiting, but also have the function of current isolation. The isolation barrier is usually composed of three parts: loop energy limiting unit, current isolation unit, and signal processing unit. The basic functional circuit is shown in Figure 2. The circuit energy limiting unit is the core part of the safety barrier. In addition, there are auxiliary power supply circuits for driving field instruments and detection circuits for instrument signal acquisition. The signal processing unit performs signal processing according to the functional requirements of the safety barrier.

Industrial sites generally require the use of two-wire transmission power distribution devices, which not only provide 24V distribution power for primary instruments such as pressure transmitters, but also collect, amplify, calculate, and perform anti-interference processing on the input current signals before outputting isolated current and voltage signals for use by subsequent secondary instruments or other instruments. But some special industrial sites not only require two-wire transmission, providing both distribution power and signal isolation functions, but also need to have safety spark type explosion-proof performance to reliably prevent mixed contact between high voltage and signals of the power supply. By using current and voltage dual limiting circuits, the energy entering hazardous areas is limited to below the safety quota with special functions of distribution devices - safety barriers.

There are two types of isolated safety barriers: detection end safety barriers and operation end safety barriers. The detection end safety barrier is used in conjunction with a two-wire transmitter; The safety barrier at the operating end is used in conjunction with electrical converters or valves. There are also signal input and other types of isolated safety barriers.

Due to the use of measures such as voltage limiting, current limiting, and isolation, the isolated safety barrier not only prevents hazardous energy from entering the hazardous site through the intrinsic safety terminal, improving the intrinsic safety explosion-proof performance of the system, but also increases the anti-interference ability of the system, greatly improving the reliability of system operation. After DC-AC-DC conversion, the 24VDC power supply outputs various voltages required by the module circuit.

The principle of the isolated safety barrier at the detection end is that the module circuit converts the current or voltage signal input through the intrinsic safety energy limiting circuit into 0.2-1VDC, which is then sent to the module for collection, amplification, calculation, and anti-interference processing. After that, it is modulated by a transformer to output isolated current and voltage signals for use by subsequent secondary instruments or other instruments. The module also needs to output an isolated 18.5-28.5VDC voltage, which is used as the working voltage for the two-wire transmitter through an intrinsic safety energy limiting circuit. Intrinsic safety energy limiting circuits can limit the entry of dangerous signals with high current or high voltage into hazardous sites.

The principle of the isolated safety barrier at the operating end is to isolate the 4-20mA DC signal output by the regulator or operator, and then output the 4-20mA DC signal, which is supplied to the electrical converter or on-site electrical valve positioner through an intrinsic safety energy limiting circuit.

Usage

1. Isolation safety barriers should be installed in non hazardous areas.

2. The cross-sectional area of the soft copper wire leading from the isolated safety barrier to the site (hazardous area) must be greater than 0.5mm2.

3. The insulation strength of the connecting wire is greater than 500V.

4. The wiring of the intrinsic safety end (marked in blue) and non intrinsic safety end circuits of the isolated safety barrier must not be connected incorrectly or confused. It is recommended to use blue as the intrinsic safety mark for intrinsically safe wires. Intrinsic safety wires and non intrinsic safety wires should be laid separately in the cable tray, using their own protective sleeves. The intrinsic safety side of the isolated safety barrier shall not be confused with other power sources, including power sources of other intrinsic safety circuits.

When the isolated safety barrier and primary instrument form an intrinsic safety explosion-proof system, it must be inspected and approved by a nationally designated explosion-proof inspection agency. The WP8000-EX series isolated safety barrier is provided by the National Explosion proof Electrical Supervision and Inspection Center, and the Co, Lo distribution parameters are the maximum allowable values relative to Class IIC (hydrogen level). For Class IIB environments, this parameter can be multiplied by 3, and for Class IIA environments, it can be multiplied by 8. When selecting cables of different specifications for transmission lines, the cable parameters themselves should be highly valued and should not exceed the specified values.

When conducting separate power on debugging of isolated safety barriers, attention must be paid to the model, power polarity, voltage level, and labeling on the wiring terminals of the isolated safety barrier shell.

7. It is strictly prohibited to use a megohmmeter to test the insulation between isolated safety barrier terminals. When checking the insulation of the system circuit, all isolated safety barrier connections should be disconnected first, otherwise it may cause damage to internal components.

8. All on-site instruments connected to the isolation safety barrier shall be instruments that have undergone explosion-proof testing by relevant explosion-proof departments and obtained explosion-proof certificates.

9. If the internal module of the isolated safety barrier is damaged and needs to be repaired or replaced, it should generally be borne by the manufacturer. When users repair themselves, they should follow the relevant precautions, and the specific methods should be carried out according to the maintenance section (the maintenance of intrinsic safety instruments is limited to the scope described, and external maintenance should be discussed with the manufacturer). Only after maintenance can it be put back into operation.

The installation, use, and maintenance of isolated safety barriers should be strictly followed The relevant provisions of GB 3836.15-2000 "Electrical Equipment for Explosive Gas Atmosphere Part 15: Electrical Installation in Hazardous Areas (excluding Coal Mines)" shall be followed.

11. Products that have obtained explosion-proof certificates are not allowed to replace components or structures that affect explosion-proof performance at will