Fault Finding Electrical Circuits

Presented by an Electrician for Electricians


Please note that this presentation is aimed at Electricians that are suitably qualified to carry out repairs to electrical installations and/or equipment. Please ensure that you always follow the correct procedures and observe all legal and safety requirements. Always ensure you are aware of the safety and other regulations on every site you intend to work at

In saying that, we all know how much pressure is put on you when a piece of equipment fails during production to get it back into a running state again. This does not mean we are allowed to overlook safety because of time constraints. Safety must always be first and production second!

Understandably the author does not accept any responsibility for any damage and/or injury by a person using any information published on this site and/or any training materials

About the Author

I am a qualified Electrician and served my Electrical Apprenticeship on a Platinum Mine. I passed my Trade Test in 1980 and have been involved in the Electrical Trade since then with a large number of years as an Electrical Contractor catering for both Domestic and Industrial sectors. My company became known as the specialists in fault finding on machinery. We were called in when others failed in getting a machine running.


National Technical Certificate V

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Key Elements to Fault Finding

In order for you to be able to walk up to a machine and locate the fault, you must have a good understanding of Electrical Circuitry. You may at times be fortunate and have a schematic electrical drawing and at times there will be nothing. If you do not understand an electrical circuit, you will be at a major disadvantage.

So let’s consider the electrical circuit first. Apologies to the more experienced readers but we need to cater for our entire audience so please bear with me. I will just touch on the basics to give you a brief idea of what we will be talking about. If you want for information/training materials on general circuit design, please post your request and I will set up a package for you to help.

It is important to note the difference between the control and power circuits. When we fault find a machine, our main focus will be on the control circuit. This is the circuit that has all the stop and start buttons, limit switches, pressure switches, flow switches and the like connected to it. This will originate from a power supply and follows the control path through all the various switches up to either a relay or a contactor. Once the contactor receives power to the coil, it will energise and provide power to the motor or other device connected to it. If you follow this you can already see what our pattern will be to find the fault on the circuit right? Now let’s break it down into a series of steps I would follow when arriving at a breakdown.

Step 1

Familiarise yourself with the operational surroundings

Always inform the manager in that are that you are there to work on the equipment

Ensure the equipment is isolated, locked and tagged out to prevent someone turning the power on while you are working on it

Talk to the operator! The operator can tell you exactly what happened when the machine stopped, explain to you how the machine works if you are not familiar with it and maybe also tell you what normally goes wrong on the machine. These guys are your best friends at a breakdown! They tend to watch the sparky working on the machine and even though they may not fully understand what he/she is doing, they can point you to the spot that was worked on the last time. This is one of my GT RulesJ Failing that, I will proceed to my more structured approach to troubleshoot the circuit.

A very important point to remember here is that you may have multiple supplies in the cabinet. One for the power circuit, and another for the control circuit. Please make sure you follow the correct testing procedure before touching anything. Your voltage tester is the most important item in your toolkit, USE IT!

Step 2

Check for any visible damage to the machine. This sort of damage may have caused a limit to shift etc. Ensure all the guarding on the machine is in place as these are generally connected to safety switches that will shut the machine down when not safe to operate

This is also where I would have a quick look at the safety relay which has indication lights on it to indicate ready state or fault state. If there is no fault, proceed to the next step. However, if you have a fault indication you are almost there. Check each safety switch for operation and repair the faulty unit. Please NEVER bypass a safety switch!

If you have found the problem and repaired it, move to step 5. If not, continue on to step 3 of fault finding the machine

Step 3

If our fault finding did not highlight the fault in step 2, we will continue on here. This is where you need to understand how the machine works and what it is supposed to do. Is there sequence starting of equipment i.e. is there an oil pump that needs to be started before the machine can be started. Are there any other interlocks on the machine etc. Once you have this information, proceed to isolate all power from both the control and power (motor) circuits. PLEASE, Never Work On A Live Board!

Connect one lead from you continuity tester on the load side of the control circuit breaker and make sure you are set to the Ohm Scale. What we will do now is take the other lead and follow each wire in the circuit. Have a look at the following rough sketch

Notice the logic? You simply start at the beginning of the circuit and work your way through. With the lead not connected to the circuit, go to position one shown on the sketch. If there is no break in the wire, you will obtain a reading of virtually 0 ohm. Now we will move to the other side of the thermal overload and look for the same reading, then on to the stop button. Again, we test both sides of it. Don’t be tricked into thinking you see the wire from one connection to the other and as such there simply can’t be a fault. Test it and make sure. Trust your instrument!

Where the picture changes a little is when you get to position 6. When you test here, you must press the start button to obtain a reading. If the circuit has tested fine to this point, I do make one change and that is to move the “fixed” lead from my tester to position 6 and continue on from there using the same principal. I do this simply so that I don’t have to keep on pushing the start button every time

Keep on working your way through the circuit until you find the fault. Even when I found it, I tend to continue through to the end of the circuit as there may be more than one fault. Trust me, it does happen

This method of fault finding works well even if you do not have a circuit diagram. What you need to do is physically trace the wire from one connection to the next. This is where I will make a drawing of the circuit as I trace it along for future reference and to make it easier for the next Sparky that has to work on the machineJ

Seeing that you have now found the fault, let’s proceed to the next step

Step 4

  • Repair the Fault
  • Test continuity after repair to make sure
  • Close/replace all covers and/or guarding you may have removed
  • Inform the relevant manager you are going to restore power to test the machine
  • Ensure it is safe to do so and restore the power to all circuits
  • Get the operator to start the machine and check it is operating correctly

Step 5

Once everyone is happy that the machine is working as it should, stop it and clean the area in which you have worked. Never leave bits of wire, insulation tape and the like behind on site

Step 6

Inform the relevant manager that you have completed the work and the machine is ready to be put back into production

This last section is almost as important to me as finding the fault in the first place.

I like to analyse the cause of the breakdown to see if there is any way that it can be prevented in future. It is not enough to merely find the fault, fix it and then walk away from the machine only to have the same fault again

I can almost hear someone saying the limit switch failed because of the conditions it has to operate in? This might be the case. What we need to look at now is whether this is the best type of switch to use in this application or is there a better alternative? Maybe not but I still feel the need to analyse the fault and look for ways to improve machine functionality and reliability


Well, that is it for this post. I hope you can find some benefit from it and please note, this is how I approach fault finding and it is not a standard written by someone. It has served me well in my career as a Sparky and I hope it will do the same for you.

Till next time, and as always, please work safe!

8 thoughts on “Fault Finding Electrical Circuits”

  1. I found your site on technorati and read a few of your other posts. Keep up the good work. I just added your RSS feed to my Google News Reader. Looking forward to reading more from you down the road!

  2. Pingback: Tips on Fault Finding « sparkyhelp

  3. Fault finding seems to be synonymous with the word logic.

    In IT we have frameworks for understanding how technology is layered on top of each other, which incidentally is also the best approach for finding faults (working from a bottom up approach). Most of the frameworks are built on top of the OSI Model… with the very bottom layer being physical – what can you see? is it plugged in? is it actually receiving power? If the answer is yes, you move up the stack.

    Great tips!

  4. nice tips. surely they are good.
    hope to read more tips from you. i have a problem when the circuit gets complicated

    1. Francois Korf

      Hi Fidelis
      No problem at all. When the circuit gets complicated, work with a notepad and break the circuit into different sections. We will spend a bit more time on this method soon

  5. Do individuals still use these? Personally I adore gadgets but I do prefer something a bit much more up to date. Still, nicely written piece thanks. 493291

  6. I used this method today before reading this….. On a Lenze canvasing network coupled with HMI, Servo drives and asynchronous servo motors overlooked by a programmable pilz safety relay. Its hard to establish the fault without the software or required cables required for the diagnostic approach. Turns out it was bad sparky who put a wrong wire in the wrong place and topped it off by crossing over two plug in light guard omron units.
    2 Lessons
    If you sell Hardware… provide the software for free and USB compatible out of the box please! Gone are the days of rs232 / 485, expensive cables, dedicated/compatible laptops, expensive or subscribed software.
    Be careful of your greed or you will be replaced by a less glutinous manufacturer.
    Sparkys. dont put wires where they dont belong and never bypass safety circuits!!!!!!!!!!!

    1. Well said Stuart. You make a very important point about the safety circuit as well – never ever bypass a safety circuit! In my opinion, a machine without a safety device ends up being safer than one with a safety device that has been bypasses for the following reason:- The operator of the machine may not know the device has been bypassed and relies on it to react when supposed to. This means he/she may move into an unsafe position thinking it is still safe and that is when you have an accident on the machine.

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