Fault Finding Electrical Circuits: Logic

Time for another post and I think we should touch on this topic again; using logic when we have to fault find a circuit.

Before we go on, just a reminder; if you have not had time to check it out yet, don’t forget to get your copy of: Manual for Electricians Volume 1. Find it at www.lulu.com/spotlight/SparkyHelp

1                    What is Logic?

The reason for this question is quite simple: In order to troubleshoot an installation/machine, you need to be able to think in a logical manner. So what exactly do I mean by that? Let’s have a look

We have a machine that is made up of a conveyor that moves a box to a specific point where it is pushed off the conveyor by a pneumatic ram. You are called to site as the machine has stopped working. When you arrive, you are shown to the machine and the operator tells you what is supposed to happen and then walks away. As usual, there are no schematics available.

Now, logic tells me that in order for the box to be pushed off the conveyor there are two essential components required:

  • Some kind of sensor to detect the box
  • The solenoid that needs to activate the ram

So without having access to a schematic diagram, we have established what needs to be there for the machine to function. That is Logic! You will also notice that we have not opened the door to the control cabinet yet. At this point I am not concerned with what is in there at all. It could be some relays or a PLC. It does not matter at this point. The only thing I am now concerned with is what makes this machine work downstream.

What we need to be able to do is to look at a machine and understand how to piece the different functions together to form a snapshot of field devices and operational sequences. Once we have this, we can move to the next step


2                    How do we Use it?

I recommend you always have a notepad and pen as part of your toolkit. Once you have had a look at the machine, draw a simple flow chart to indicate the operation. Let’s have a look at the following:

Sensor:            24V DC Proximity

Ram:                24V DC Solenoid


What I will do now is to put a box on the conveyor and run it to see what fails. Let’s assume the box moves right past the sensor without the ram pushing it off. We know the conveyor part works so we have only two things to check. The sensor and the ram. Most sensors have an indicating lamp that blinks when it detects the object, but we will assume this one does not. So how do we proceed from here? Simple, we use logic once again. Consider how a solenoid operates. It is a coil that will create a magnetic field when energised. So if the solenoid is being energised, we should be able to detect that at the solenoid as follows:

Place the tip of a non-magnetic screwdriver on the top of the coil where the locknut is. If there is power to the solenoid, the magnetism will hold down the tip of the screwdriver. You check this by lifting the screwdriver softly away from the coil. If it is energised, you will feel how the magnet holds it.

Great, we have power to the solenoid which means the sensor works and there is power to the solenoid so it is not an electrical fault. Now we can walk away from the machine and leave the client without his machine because the fault is not on our side right? Sorry, this time the answer is no!


If we have established that there is power to the solenoid, we know that the fault is on the ram. It may be as simple as an airline that has been turned off. Do a quick check to make sure you have air to the actuator. If you have air and power, then you may need to call for mechanical assistance.