In this video, we're going to talk about the low voltage fuse, why it blows what that means and how to test it. First, we're going to show a fuse inside of a fan, coil or air handler. Your low voltage fuse is going to blow anytime any of your energized circuits, make contact with common or ground without first going through a load like this. We can see that is a telltale sign of a blown fuse before we go any further.

Let's show what this would give us with a volt meter with a circumstance like this always do so safely, with the power off and safety glasses in place, we're going to set our voltmeter onto the vac or volts ac scale. I also before i use a voltmeter. I like to test it against a known power supply, so it's good to start with the high voltage and just see if we have that present. So, let's check that we have 213, because this is a 208 building, meaning we're using two legs of three phase kind of throws: people off, sometimes because you'll actually notice that we have 120 from each leg to ground and together that doesn't add up to the 212.

That we're actually seeing, but that's because it's a three-phase power supply. If you want to check this out 208 versus 240. I have a video on this separately now that we know that our meter is working, we're going to take some measurements, knowing that we have a blown fuse under normal circumstances. If you see that you've got a blown fuse visually you're going to go and try to identify the cause, but let's trace this back first off.

This conductor here comes directly off of the secondary of the transformer. So this is our 24 volt hot, at least that's what we normally call it and then, on the other side of our transformer secondary, we have a 24 volt common, which is this brown wire here now this transformer has what we call a grounded secondary. So the common on the secondary is actually connected to ground via a green wire. So let's just see what we have.

I measure from brown, which is common coming from the transformer, and i go all the way back in and i measure from our hot getting good access points is the tricky part for this test. So now i'm going directly from red to common on the transformer, and i have 25 volts. If i come out of my fuse out of the other side to this red wire up here and measure that to common, i have nothing and that's because this fuse another name for it is a fusible link, has broken the circuit in order to protect it. And it's broken it because it has a 3 amp rating.

When i made a path between my common going back to my transformer and my hot coming from my transformer, we had too much current current well above 3 amps, which caused the fuse to blow when we say fuse fuse literally just means disconnect. So it caused it to automatically disconnect to protect everything else in the circuit. Now there are some fuses that it's hard to tell if they're blown or not - and this one is quite easy and quite obvious, but if you ever want to check 100, you can remove your fuse from its holder. These two spades are connection points, so in order to test it, we put our meter onto the ohm scale, specifically we're measuring for continuity.

Continuity means that there's a path, a path being a very, very low, ohm or very low resistance connection, with a fuse that does have a proper path and is not blown. We will ring out with continuity because we have a very low ohm path, but with a fuse, that's blown. If we do the same measurement, we will not get a path which is known as infinite ohms. Some meters will show ol here for open line, and in this case this meter just shows dashes when there's an open path.

When a system has a blown fuse, you want to pay attention to all the connectors and anywhere that the wires are moving through the cabinet. You can see here these thermostat wires, don't have a proper grommet protecting them to be very easy for this sharp metal to damage the jacket which could cause a short circuit. We want to inspect the places that their previous technicians strip back the outer jacket to make sure they didn't ring around the wire and create any nicks in general. We would want to clean up all the wiring and make it look neat and find anything that could have caused this problem before we go ahead and put a new fuse in check.

Your float, switch wires, make sure there's no damage to any of those. Maybe they could be chafing up against some tubing if they're routed, improperly inspect your control conductors outside ensure that there's no damage from weed eaters or other lawn equipment. Look inside of your condensing unit and ensure that none of the conductors inside are rubbing against any of the copper tubing causing chafing and damage, which can often lead to a low voltage fuse short inspect your thermostat. All the connections should be neat and tidy and connected to the proper terminal.

If you have bare wires, that could be the cause of your problem also ensure that your common is connected to the c terminal. Some people will accidentally connect it to the heat pump b terminal. The b terminal is not common, but rather it's for heat pump systems that energize the reversing valve in heating mode rather than cooling mode, and is a common mistake that people make when connecting a thermostat check, how the wires are routed if they're pinched. That can also cause a short circuit check inside the electrical panel on your condensing unit to ensure that there are no obvious signs of shorts.

Here's an example of some nicked and damaged conductors. That could potentially be a problem only once the short circuiter overload has been found and fully rectified. Do we replace the blown fuse with the new fuse and then retest the system? Now there are situations where low voltage, fuses, tripping or low voltage breakers stripping can be caused by difficult to find problems in a lot of cases, that's due to vibration or water being necessary in the equation to cause the short circuit to show up. In other cases, it can be because certain low voltage loads could actually be overloading, such as in cases where a contactor isn't allowed to pull fully in, but you want to follow best practices as far as use an ohm test check.

All of your loads check your circuits for shorted or grounded conditions and also utilize isolation, diagnosis, we've done another video on isolation, diagnosis of low voltage circuits and we have a nine panel process for that on the hvac school app for finding low voltage shorts, but it All starts with understanding fuses understanding how to use your meter, how to test them and then how to do a solid visual inspection to find the most likely cause of the low voltage short before you just start replacing fuses over and over again, if a fuse tripped, It did so for a reason, and it's there to do its job thanks for watching and we'll catch you on the next video thanks for watching our video. If you enjoyed it and got something out of it, if you wouldn't mind hitting the thumbs up button to like the video subscribe to the channel and click, the notifications bell to be notified when new videos come out, hvac school is far more than a youtube channel. You can find out more by going to hvacr, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more. You can also subscribe to the podcast on any podcast app of your choosing.

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