 The crisscross method of testing fuses is utilized when the circuit is generally live. However, if at all possible, you should always kill the circuit and have it locked out for safety's sake. So in this case here, we're going to be using a 208 volt system. We have three fuses, fuse A, B, C. I've set up some testing points. So we have testing point one, two, three, four, five, and six. Right now with all fuses good, we're going to analyze and see exactly how the circuit works when we put the leads on. So the first thing I want to do when I use the crisscross method is I want to make sure that voltage is all right coming in up from the top so that my voltage incoming voltage is not my problem. So we're going to take our red lead and go to point one and we'll take our black lead and go to point two. And as long as our power coming in is all right, we should read 208. Then what I would do is take my black lead, keeping my red lead on one, take my black lead over to three. And if everything's good, I should read 208 volts. And then lastly, what I'm going to do is take my red lead and move it over to two and read from two to three. And if everything is as it should be coming in, we have 208 from two to three. So that verifies that all my lines coming in are good. So I have no problem upstream. So what we're going to do now is we're going to look at what happens to our voltage drops as we go across the fuses themselves. So here I've set up from point one to point four and I'm going to be reading the volt drop across these two points here. Now here's the thing. If this fuse is good, there's means there's continuity here, which means that this point here and this point here will be the same electrically. If that is true, there's no potential difference across this fuse, which would mean that I would have a zero volt potential. So using the volt meter in this way, one to four would read zero volts. And that would stay true for two to five as well as three to six. So you can't really go ahead and read across a fuse when you're using a live circuit. Because this point and this point, as long as they're good, are going to be the same points and you're going to have zero volt potential. Now if I look here, I've taken from point one and I've gone across to point five. Now again, if B is good, that would mean that two and five are the same point potential because it'd be like having just a wire going through there. So if that's true, what we looked at before was from one to two was 208. If this fuse is good from one to five should be 208 volts as well because points two and five are the same points electrically. So you might as well, if this fuse is good, this lead might as well be up on two. It'd be the same thing. So we have 208 volts coming in. We're going to read 208 volts like that. And the same would hold true if I went from one to six or any variation thereof. If you're reading the cross and all three fuses are good, you should read 208. So if we're crossing over here, and this is why we call it the crisscross method, if the fuse is good, it will read 208 volts. Or if we went over to six, it'd be 208 volts would tell me that C and B are both good as far as the fuses are concerned. Now let's take a look at what happens when we actually have a blown fuse. So here I've physically drawn it so that you can see that point A or sorry, fuse A is broken. You wouldn't physically see this out in the field. That's why we're using this method, but this is just to prove the point. So again, if I would read it coming across one to two is 208, one to three is 208, and two to three is 208. Then what we did was we measured one to four. And when I measured one to four, I read zero volts when the fuse was good. However, when the fuse is blown here, you notice I have a reference point here. I've got this attached to 208 volts here, and I have this attached down here. Yes, this is blown, but you notice that this is open and these contacts are open. That means that four has zero reference point. So it's not in reference to anything. So it'd be like taking this lead and sticking it up in the air, which would mean that from this point to the air, I've got a difference potential of nothing because it's not reading anything. So if I try to read my voltage across this with it blown, I'm going to read zero volts as well. When the fuse was good, I read zero volts. When the fuse is bad, I read zero volts. So we can't measure the voltage across a fuse to determine whether or not the fuse is good. We need to measure it across. And so what we're going to do now is we're going to move this black lead over to two. So I've kept my red lead on four, and I'm going from two. So we know that we tested these voltages coming in, so two has a reference voltage to it. So reading a reference voltage from that point to this point here. So when we do that, we know that there's voltage here. We know that there should be voltage there, but with this fuse blown, it has no reference as well. So we confirm that A is blown because we're trying to read from two to four. Now, if we read one to two, we should read 208. But if we read four to two, we're reading zero volts because of this blown fuse here, which in turn tells me that fuse A is blown. You could do the same thing with two to four, or sorry, three to four. You're going to get the same thing. Three to four would read zero volts as well. Let's take a look at what happens when we have another fuse blown. So let's blow fuse B. So we'll start out, we'll read across two to five. But again, that's a moot point because two and five would be either the same point electrically if this fuse was good, or because this fuse is blown, two has a reference, five has no reference, so it's going to read zero volts. So again, we have to measure across. We have to do a crisscross, so we could actually take the leads and measure from five to one, or five to three. We want to keep one lead downstream of the fuse because we want to determine if this fuse is blown. So let's move this over to three just to prove a point. So we've kept five there, and we've got our other lead going to three. So if we read two to three, we should read 208 volts, which we did earlier, because we have good power coming in. But if we see here, if this fuse was good, points two and five would be the same, so we should read 208. But with this fuse blown, there again, there's no reference at this point. So three to no reference point is going to give us zero volts, which would tell us that B is blown. We would expect if this fuse was good to read 208, but now because this fuse is blown, we're reading zero volts. And so that fuse C doesn't feel left out. Let's take a look at what happens when we blow fuse C. Again, if we go ahead and we go across here, whether this fuse is good or fuse is blown, we're going to read zero volts. So what we need to do is use the crisscross method. We're going to keep this lead on, and we're going to measure voltage across to these two points here. Before, when this voltage is good, we checked two and three. We had 208 volts coming through. If we have this point here as having a reference point, we know that there's power coming to this point, but we have a blown fuse here. Before, when it was 203 up here, it was 208. If this fuse is good, we should read 208, because this point and this point should be the same electrically, but with a blown fuse, this is open. This now has no reference point. So three and six are no longer the same, and I'm going to end up reading zero volts across there. So we've used a crisscross to get across the voltage. So that's how you use the crisscross method. Again, you'd never want to read the voltage across the fuse itself because whether the fuse is good is zero volts or if it's blown is zero volts. So that could be dangerous. So we want to make sure that you're checking all points. You want to measure voltages across the different points.