Difference Between Isolated and Non-Isolated PLC Modules

11 Oct 2020.

In this post, we will clarify the difference between isolated and non-isolated PLC analog modules; and understand why isolation plays an important role in 4-20 mA current loops. (Click on the images for a more zoomed view)

Hello programmers. You all must be familiar with the two widely used types of analog signals – 0-10 V and 4-20 mA. Of these, the most popular one that is used is the 4-20 mA type one. The wiring is simple; you have transmitters where only two wires are needed (loop-powered). It can be run over long distances with minimum drop; voltage drops much across long distance wires as compared to current drop. A varying current loop load impedance will not affect the signal if it does not exceed required component limits. Electromagnetic interference affects current loop at a much lower rate as compared to voltage circuit. 

All the above advantages can be explained by simple Ohm’s law – V=I*R. Let’s see the below image. The circuit consists of a power supply and three loads. The power supply will give the voltage required to drive the current loop. Through current passing across each load, voltage drop occurs each of them and will vary according to the resistance available. However, current remains the same in the whole circuit. That is why; current is the most reliable source of information available for data. 

                     

Now consider the below image for the understanding of current loop. The sensor provides the physical signal and the transmitter acts as a transducer which converts this physical signal into electrical current signal. For this signal to be produced first, a power source is required. The loop is nothing but the wiring which carries the signal from transmitter to receiver and then back to it. The value of the cable resistance is directly proportional to the distance of transmission. If 0-10 V is transmitted over long distance, there will be a drop in voltage due to resistance of the cable; and the signal received will not be accurate. So, 4-20 mA is the main reason for it’s wide scale use.

But every technology has it’s pros and cons. The current type has high power consumption compared to other analog signal types. Increasing circuit load resistance will reduce the supply voltage available to power the transmitter that is generating the current signal. One of the major concerns of 4-20 mA is non-isolation. This single reason affects the use of this signal. We will investigate this theory and understand how an isolated PLC module improves the overall efficiency of the signal transmission as compared to a non-isolated PLC module.

Theory of analog isolation:

What is isolation in PLC?

First, what is isolation? Isolation is used to protect circuitry and human beings, break ground loops, and improve common-mode voltage and noise rejection performance. See the below image. The left-hand side image shows that a sensor is directly connected to analog input channel; whereas the right-hand side image shows that the sensor is connected to the channel via an isolator. The isolator protects the channel from any power/noise fluctuations on the sensor side. It only filters the proper analog signal to the channel and removes the spikes and fluctuations in it’s input side.

Ground loop leakage in 4-20 mA:

It’s other important use is to avoid ground loop leakage. A ground loop current is nothing but the current flowing from one signal ground in a circuit to another signal ground at a different voltage potential. Most analog (4 to 20) mA loops are grounded at a single point. Problems occur when there is more than one grounding point because earth potentials will not be the same. This means current will flow between earth points causing errors or noisy signals. This causes unpredictable signal fluctuations and will often average the process signals, which causes reduction in actual input/output with respect to channel. It can be like; you are providing 16 mA from PLC analog output, but the field device is functioning at 12 mA. This problem is mostly faced in loads with high impedance. So, installing an isolator in between will eliminate ground loop leakage, as the isolator isolates channel and ground. 

What are the different types of isolation?

PLC channels can be isolated either by connecting an isolator in the current loop circuit; or using an isolated PLC module. Isolation is mostly achieved by galvanic isolation. Galvanic isolation is the principle of physically and electrically separating two circuits, so that there is no direct conduction path, but data and power can still be exchanged. This is typically achieved by using transformers, optocouplers, or capacitors. Refer the below image for understanding. As we know an individual isolator, let’s investigate an isolated PLC module. There are two types of isolation generally used in PLC modules – group isolation and channel-to-channel isolation. In group isolation, all the channels are grouped together to share a single isolation barrier, including power isolation and signal isolation. But here, common-mode voltage difference arises between channels in the group; meaning they should be placed in same zone. To eliminate this problem, channel-to-channel isolation is used. In this, every channel has it’s individual power isolation and signal isolation. This means that channels are isolated from each other and any damage to a particular channel won’t affect the other ones.

            

Isolated vs non-isolated analog input:

A common question most programmers ask is how to choose between an isolated and non-isolated analog input. Mostly, in all the PLC’s, when you choose a module between isolated and non-isolated; remember that non-isolated means there is no isolation between channels and isolated means there is isolation between individual channels. Remember that channel to ground isolation is provided by default.

I would not go deep into ground loop theory, isolation methods and need for isolation. It is very vast and would take a lot of time for you to study. But it’s important to first understand the concept behind isolation in analog circuitry. It basically protects the channel from external damage. Also, this does not mean that you don’t have to use a non-isolated module. It is cheaper than an isolated module and will work properly too; provided the loop design has been thoroughly analyzed and made accordingly, to avoid problems that are only realized when trying to get the system up and running. There are numerous electrical methods in grounding the system properly and securing it with safety too, which will properly utilize the non-isolated module. The topic that I have written covers both analog inputs and outputs.

I have covered the general theory related to isolation and the difference between isolated and non-isolated modules. I have also not attempted to cover every type of features deeply. I have just given you an insight of this type of study. Learn the basics and explore a new type of theory in this type of automation.

Thank you, guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.