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How to Fault Trace on a Small Appliance using a Multimeter Published 10/03/2015

In this video Mat explains how to use a meter to trace a problem in a simple electric circuit like on a mower, strimmer, vacuum cleaner or other motorised device.

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Video Transcript

Hi I'm Mat from eSpares.

In this video I'll be showing you how to fault trace on a small appliance, every simple appliance works by having a current running through it. This current begins at the plug, the live pin on the plug which comes up through the cable, through the switch through the motor, back up through the switch through the cable and out through the neutral pin on the plug. If there's a break on any part of this circuit your small appliance will not work.

Safety first: always unplug an appliance before carrying out any work.

This technique can be used on vacuum cleaners, trimmers, lawnmowers, steam irons even something as simple as Christmas lights.

So what you need to do grab yourself a multimeter set it to the resistance setting and put the probes on the neutral and live pins on the plug. Here we can see we have a reading of 6.2 ohms. If I turn the switch off on this vacuum cleaner you can see that the reading changes to zero, no resistance. This is the same reading as if the probes were attached to nothing therefore this machine has no fault.

However if I attach the connectors to the live and neutral on this vacuum cleaner, as you can see there is no resistance reading even if I turn the machine on and off via the switch the reading does not change. Therefore there is a fault in the circuit of this machine.

So the first thing I need to test is the fuse within the plug, if I take that out I can again use the multimeter to test. One probe on one end and one probe on the other and you can see we have a low resistance reading, therefore there is no fault in the fuse.

If I returned the fuse to the plug, the next place we need to check is where the cable meets the switch. In order to do this I need to remove the top of the switch cover.

With the switch cover off, you can see that we actually have a loose connection to the switch. Now this is something that we fabricated in order to best demonstrate fault finding, in reality it may well be that you don't have a loose cable, in that case you'd have to check the switch itself again using the multimeter to the probes, if you have a lower resistance reading on the switch there is no problem with the switch and the problem may well be with the cable where it enters the machine. If that's the case you would measure from the switch to the plug to find out if the cable is faulty.

In a lot of motorized small appliances the main problem is with the motor itself. Here we have a typical vacuum cleaner motor, most motors contain carbon brushes, now I've started to take the carbon brush out so I'll just take that fully out to demonstrate. Carbon is an electrical conductor, the current runs through the carbon brush through the motor and back out through the switch.

These carbon brushes are in spring loaded holders, so what this will do is extend the carbon brush making sure that it's constantly in contact with the metal part of the motor to complete the circuit.

What happens is that these carbon brushes wear down overtime, so much so that the carbon no longer touches the metal part of the motor, therefore not completing the circuit and you'd have to replace the carbon brushes.

We hope this video has been helpful in helping you to identify a fault in a small appliance. Remember spares for most appliances can be found on the website.

Thanks for watching.

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