We have covered a little bit about the basics of Fault Finding and Electrical Circuit and we can now have a look at a specific circuit and the steps we would follow. The aim here is to focus on what I said about understanding the machine operation. In order to explain this better, we will work with the circuit below that is used to maintain constant pressure in a water line. Bear in mind, I have just drawn this circuit as an example and if you are planning to use this, please ensure you make the necessary changes to suit your application.
Step 1: Understanding the operation of the machine
By looking at the circuit you will notice a few things that are different. On the first section we have an E Stop that has been wired to the normally open of Relay 1 and also a reset pushbutton. The other side of the normally open and pushbutton is connected to one side of the normally closed of the overload. The other side of this goes to the coil of Relay 1. This tells us that had the E Stop been pushed, we will need to release it and then push the reset button so as to energise Relay 1.
Also look at the next section where we have the normally closed of Relay 1 connected to an indication lamp. This is great as it will tell us that there is an E Stop fault. The next part of the circuit is all wired via another normally open of Relay 1. This tells us immediately that should there be a E Stop fault, the rest of the circuit will not operate. From this normally open, we go to one side of a normally closed of a pressure switch with the other side going directly to the coil of Relay 2. That means that should the pressure in the line drop below the setpoint, the contact will be closed and Relay 2 will be energised.
At the same time we go from the same normally open Relay 1 to the normally open of the pressure switch. The other side of this normally open goes to the coil of Relay 3. This means that should the pressure reach our setpoint, the contact will close and Relay 3 will be energised. Note that Relay 2 is called the Start Relay and Relay 3, the Stop Relay. We also have a wire from the same normally open of Relay 1 to the normally open of Relay 2. The other side of this normally open Relay 2 is connected to one side of the normally closed Relay 3 and also to one side of the normally closed of the flow switch. The other side of the flow switch goes to the coil of Timer 1. (Now you will remember that in a normal circuit the start button is a normally open and the Stop button a normally closed. That is exactly what you have here, nothing out of the ordinary) The other side of the normally closed Relay 3 goes to one side of both a normally closed timer and normally open of Q 1. The other side of these contacts go to the coil of contactor Q 1. So, that tells us that if the system starts and there is no flow by the time the timer times out, it will shut down. The good thing here is that if you look at the next line of the circuit, you will note that we will have a low flow indication and the next one will give us an indication of should the overload be tripped. The next line may be something new to you and that is the reset magnet. This device mounts onto the overload and when energised, it will reset the overload for you without having to open the cabinet
Now, after looking at the drawing we know the following:
On low pressure we will energise Relay 2 which will then allow the pump to start. Once pressure has been reached, Relay 3 will be energised and the pump will stop. Great, now let’s have a look at one fault:
The pump starts but shuts down after 10 seconds. We are told that this is strange as there seems to be enough water when the pump runs yet it shuts down on low flow?
Remember the part about talking to the operator? He has just told you exactly what to look for! You know the operation based on what you saw in the circuit diagram and that tells you that should the pump start and then shut down on low flow, it means that there was not sufficient flow before the timer timed out. Yet, the operator just told you that there seems to be enough water right? So, where is your fault? My call will be to go directly to the flow switch, open it and check if it is operational. Depending on the type used, it may be a broken paddle on the switch and all done. Fix or replace the flow switch and you are done!
Hope you find this useful. Now for the usual, Please, ALWAYS TEST BEFORE YOU TOUCH! There is no substitute for Safety