Why does PLC need to be used with an isolation barrier/isolator?

01Related concepts

PLC: Programmable logic controller

The full name is programmable controller, which is an application system composed of a main control chip and peripheral circuits. It can pre-program instruction flows through programs to achieve specific functions. It is usually used in automation, industrial control and other fields. Its characteristics are high stability and easy programming and use.

Q: Why does PLC need to be used with an isolation barrier/isolator?

A: Because the working environment of the field equipment is relatively bad, if the signal sensor line to the PLC is broken and connected to other lines with different voltages or shorted to the equipment at the same time, strong electricity will enter the PLC input port, causing the entire module to burn and causing great losses. If this happens with the signal isolator, only the isolator will be damaged without damaging the PLC. In addition, it can prevent interference and stabilize the signal.

Analog signal:

The basic process signal (voltage, current, pressure, temperature, flow, etc.) input form in automated process control systems.

The process signals in the system are converted into unified voltage and current signals through the isolator, and these signals are transmitted to the controller (PLC) in real time.

DC 0~20mA, 4~20mA,

DC 0~5V, 1~5V, 0~10V

Pressure, temperature, flow, speed, valve opening, etc.

02. BM200 series isolated safety barrier

BM200 series isolated safety barrier:

Connected between intrinsically safe circuits and non-intrinsically safe circuits. A device that limits the voltage or current supplied to an intrinsically safe circuit to a certain safe range. Safety barriers are a general term, divided into Zener safety barriers and isolation safety barriers. Isolation safety barriers are referred to as isolation barriers.

Detection end isolation safety barrier:

Equipment that receives signals from hazardous areas, isolates them, and then transmits them to safe areas can effectively limit the energy in hazardous areas from being transmitted to intrinsically safe circuits.

Operation end isolation safety barrier:

The control signal received in the safe area is isolated and then transmitted to the control equipment in the dangerous area for reception. It can be used to limit the amount of electrical energy supplied from the control room in the safe area to the field instrument, that is, to prevent the generation of sparks that can detonate dangerous gases, thereby eliminating the detonation source.

Selection related parameters

03. BM100 series signal isolator

BM100 series signal isolator:

Increase the safety and reliability of the system, and improve measurement accuracy and anti-interference capabilities.

It is essential to isolate the analog signal measurement circuit to ensure the safety and reliability of the output.

Three types of current input

Two-wire system: mainly for the front-end output power supply products (pressure sensor)

Three-wire system: one power line, one signal line, and one reference ground (reference ground for power and signal)

Current source: two signal lines, two independent power lines (four-wire system)

Output power supply type

There is no separate power terminal, power and output share the same terminal.

Passive type

No power terminal required

Input: DC 4~20mA

Output: DC 4~20mA

04. Functions of Isolation Barrier and Isolator

In practical applications, the signal interference problem of control systems is sometimes very serious.

Wiring method using signal isolation barrier/isolator

• Cut off the propagation path of conducted interference,

• Reduce signal distortion caused by ground loops directly connected to the device,

•Improved the anti-interference ability of analog signals

Once an accident occurs and the device is damaged, replacing the isolator is more time-saving and cost-effective than replacing the PLC/DCS module. Therefore, it is recommended to determine the installation based on the actual situation.

05. Application scenarios

06. The difference between isolated safety barriers and signal isolators

Isolation fence application scenarios:

Petroleum, chemical industry, thermal power, gas, metallurgy

Isolator application scenarios:

Wind power, new energy, water treatment, robots, automobiles

The isolated safety barrier can replace the signal isolator, but the signal isolator cannot replace the isolated safety barrier. However, the costs are different, so it is recommended to choose according to actual usage needs.