 Understanding the relationship between how a voltmeter works and a schematic circuit works is so incredibly important when it comes to troubleshooting. In this video I'm going to walk through with you exactly what to expect in a simple three-wire circuit and show you exactly how at different points and in different scenarios the voltmeters have different readings. So let's get going. So what we have here is your basic start-stop circuit. I've got a stop button which has got normally closed contacts, start button, normally open contacts. I've got a set of holding contacts. I've got my coil here so it's my M coil which will close the power contacts as well as my holding contacts. I have a green run light so that when the M runs or when the M coil pulls in this is in parallel so it will turn on and turn green. I have a set of normally closed overload relay contacts and down below I have another M contact and a red light. So when the motor is not running the light will be red. So what we'll do here is let's talk about these different voltmeters. I have voltmeter A, voltmeter B, voltmeter C, voltmeter D, voltmeter E and voltmeter F. We're going to assume that I've got a line here and a neutral here. Let's assume that our control power is on. So let's just give an assumption that it's going to be 120 volts. Now let's look at voltmeter A. This voltmeter A is reading from this point to this point in reference to each other. Now with this normally closed contact this might as well be just a wire going through. So from this point to this point I have no potential difference because there's no resistor in the way, there's no load, it's just one straight wire. So it would be the same thing as taking this lead and putting it over here. So if I have no potential I have zero volts. So in this situation voltmeter A is going to read zero. Moving over to voltmeter B. I've got potential right up to this line here and then we have this open here and this is where I see many people make a mistake because they would assume that on this side here I've got some sort of references there as well. But if you look here this goes to the load and this goes to this light and these are not engaged at this time. So what's going to happen is that I'm going to read zero volts because I have zero reference point on this side. So whilst I have a voltage reference there this will have no reference to that point there. Then I look on this side. Voltmeter C. If I look to voltmeter C I go to here I have no reference because there's an open here and an open here. I do have a reference on this side however because I have my normally closed overload relay contacts but again to neutral but I have no reference point here. When we're using our voltmeters we need to have both probes referencing something. So just like this one had a reference point to here but not on this side this one has no reference point on this side and a reference point here so zero volts. And if we look across voltmeter D these are all normally closed contacts so this point here and this point here are basically the same electrically so we're going to have a zero volt drop. Down here same idea normally closed contact so on this side in this side are the same points electrically basically zero volts on voltmeter E. However if we look at voltmeter F I have got a load here it goes all the way through so I've got a reference point to that side and I've got a neutral reference point to this side. I should read 120 volts at voltmeter F in this situation. So let's see what happens when the circuit's running. We'll take a look and see what's what's going on at that point. Okay here we have your basic motor control circuit we've got a stop button which is normally closed. We've got a start button which is normally open. I've got ourselves a why is it on? Fart. So here what we've done is we've pressed down in the start we've engaged M coil M is fired up it is closed this M here it is also green the green light here is in parallel so it's running we again none of the overloads are open so we've got a straight line there so that's engaged. Now when this is energized the M coils energized this closed but then this opened so now I have an open here and I have no reference point sorry we have no red light running at that point. So let's go through and again see what's going on with our voltmeters. So voltmeter A again same point electrically so I'm going to have zero volts at that point. Voltmeter B this is normally closed so it's the same thing as having a normally closed contact here I'm going to have zero volts there electrically. Voltmeter C however if you look here I've got a reference point going through going through to this side and engaging so I've got a reference point at that point and then on this side over to the neutral I have a reference point there so therefore voltmeter C at this situation will read 120 volts. Voltmeter D at this point here will read zero volts because again these are all normally closed contacts so this point and this point are the same electrically. Voltmeter E will read zero volts because I have a reference point to this point here but then on this side I don't because this load is not engaged so I have no reference point at that point so basically E is going to be a useless voltmeter at any given time so I've got zero volts here and again because this is open I have a zero reference point here and I have a reference point on this side so voltmeter here voltmeter F is reading zero volts as well so let's take a peak C around the circuit one last time here here the situation that we're running into now is our overloads have tripped so that is opened up these contacts here and that's going to change a lot so again voltmeter A basically it's a useless reading because we're reading across the same point electrically as long as that stop button is closed it's always going to be zero volts so you don't really need to put anything in here for that's just a useless gesture. Voltmeter B when we look at voltmeter B here again it's got a reference point to this side but because this is open on this side it's going to be a zero volts reference point as well. Voltmeter C I could what I could do here is and this is what I might do on the field is I would automatically I'm reading zero you can see why because I've got the open overloads what I might try to do is press down on the start button and that way I would know that I should have power coming to this point here but if I read zero volts there then I know that there's something wrong on the overload side same thing with the green light doesn't turn on well that's because the overloads are tripped and opened on that side. Voltmeter E again that's one of those useless positions because no matter what we're going to read zero volts because again this is closed that point in that point are the same electrically. Voltmeter F we see that we've got this side is running fine this side we've got power all the way through and we're back to what we were in our original situation reading 120 volts so that's how you kind of reading through you notice that as we went through each scenario you're going to find that some volt meters readings are not unnecessary this one here it's always going to be zero volts basically this one here you'll always have some sort of zero volt situation because even when it's engaged it's gonna be closed it will be the same when it's disengaged it means that I have nothing happening on this side so I have no reference this one does tell me a little bit of something but for the most part actually I'm going to read zero volts what I would generally do at this point if I was reading across here is I would not be reading voltage I would be reading continuity to see if I had continuity from this point to this point if I don't these contacts are open if I do have continuity between that point in that point these ones are closed this will tell me if this is running or if it's engaging now if I press down on this button the start button and I have voltage at volt meter C that would tell me that there's in this doesn't gauge that tells me that the coil might be burnt out and that I would need to replace the coil and I should see the green light turn on anyway so if I press down on that I see 120 volts there and the green light doesn't turn on that tells me that the green lights probably burnt out and then again down here this one's going to read zero no matter what and this one will read some sort of voltage so again if I press this stop button which will stop the circuit this red light should be on as long as I have control power if it's not then that tells me that that possibly the red light is burned out now why this is important for troubleshooting is while it's really easy to see these circuits the control wires when they're actually out in the field themselves can be all over the place so what you would do is knowing that we want to find physically where this point to this point is so where these power wires are being wired to to see this so what I like to do with my classes is make them go through an exercise such as this so they understand what they should be reading at certain given moments and so we go through the exercise of trying to figure that out with a schematic and then we go out to the lab itself and try it out and so it is a helpful exercise in understanding where we should be reading voltages and where we definitely don't think we'll be reading voltages at all if you found value in this video make sure you hit subscribe if you want to check out last week's video hit the previous video and if not you can go ahead and hit the recommended video YouTube weeks of choice for you see in the next video