Fault Finding Techniques

Hi All. It is time for another post and it may be a good time to link the PLC into the circuit to see how it will affect your fault finding skills.

As the PLC becomes more popular on an installation, it is necessary for us to have a better understanding of what to do when something goes wrong. What I would recommend is that you set up a list showing you exactly what is connected to the different input and output terminals of the PLC. Let’s create an example:

Digital Input Device Description
00 Limit Switch Hydraulic Clamp   Open
01 Limit Switch Hydraulic Clamp   Closed
02 Pushbutton Start
03 Pushbutton Stop
04 Overload Thermal Overload   Normally Closed Contact
05 Overload Thermal Overload   Normally Open Contact


I have just set random inputs up here with no thought of a program just to give you an idea of what I mean. By having these set up in a list, do the same with all of the outputs. Create a functional description of exactly how the machine needs to operate. Make sure you look at sequences and interlocks. In other words, the hydraulic clamp must be open before the conveyor can run. By simply having that information and the input number of the limit switch that is going to provide the signal, you can trace the circuit.

If the clamp is in the open position, the limit must be activated and as such you will see the input 00 light on the PLC turn on (should the PLC not have an indication, you can simply test for voltage on input 00 to determine if the limit switch is providing the signal)

We are only just scratching the surface here. What you need is a full functional description that will include every input and output connected to the PLC. Armed with this, work your way through the sequence; one input/output at a time. What is also very handy to have here is to make a drawing of the installation. In other words, show every single device as it is installed. This together with the description will make it easier to understand how it is meant to operate. This is where logic plays a big part. You will need to understand the process when you look at it. The position of limit switches, sensors and any other field device will aid you to understand it.

Looking at the installation, you see the limit and ask yourself why it is there. Use the example above and ask yourself what would happen if the conveyor started with the clamp in the closed position. The consequence of this will tell you that it must be open otherwise the load will not be released and will cause a “jam” on the conveyor. This is how you get to understand the machine. These devices almost need to “talk” to you when working through them if you understand what I mean. I know I am trying to give you a lot of information in a very short post, but hopefully this will help you to work past the fear of seeing a PLC in the cabinet because you don’t have the software for it.

Till next time, you guessed it by now but still, work safe and always consider others around the installation you work on. Have a great week!