How To Check For Blown Fuses Using A Digital Multimeter.

Published: 29th March 2007
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There are several ways to check for blown fuses in a circuit. It is always best to check fuses with the power off when possible but as a maintenance mechanic in high speed production facilities for the past twenty years, I know that this is not always possible. If you consider yourself a novice when it comes to electricity, I suggest that you never check fuses with the power on until you have received the proper training and feel comfortable doing so.

I know that the safety agencies such as OSHA and others have really tightened the guidleines on how to troubleshoot circuitry with power applied. My company provides us with flame retardent uniforms, face shields, high voltage gloves, and other protective gear. Always work under the guidelines that your company requires. If you are a home tinkerer and don't know anything about electricity, I suggest finding someone who does. You should never take electricity for granted.

Let's use my air conditioning unit in my own home for an example. It was installed many years ago. I think right after Ben Franklin discovered electricity. The power was run to a fused disconnect box then to the main power panel. I have the ability turn the power off at my main power panel or the disconnect box. If my air conditioning unit has stopped working., the first thing that I want to do is make sure that I have power going to the unit. I will start the troubleshooting process by checking the fuses in the disconnect box to make sure that they are not blown.

I like to think of a fuse as abridge that the power has to cross to get where it wants to go. If a section of the bridge is missing, the power cannot get across the bridge and get to where it needs to go. The first thing that I will do is turn the disconnect off before I open it up. Most disconnects today will not allow you to open them until you shut them off. This is a safety feature. Remember, even though you turned the disconnect off, the feed side of the disconnect will still have power to it. I have the ability to turn the power to the air conditioning unit off at the main panel so I usually do that just to be safe.

Now that we have the disconnect open, you will see the fuses. i want to turn my digital multimeter on and turn the dial to the ohms function. Some DMM's (digital multimeters) have different ranges within the ohms function. I usually set it to the highest range at first. Once I have done, I will take the leads of my multimeter and place them on each side of the fuse. If the fuse is good, the reading on my DMM should be zero. Don't panic if the meter reads 1 or 2 ohms.

The fuse is still good. The meter probably needs some adjusting or calibration. Remember the example of the bridge, a zero ohms reading means that their is no resistance in the path across the bridge for the power to cross it. In technical terms I would be reading a short so the fuse is good. If the fuse is bad, the meter will read infinity ohms, mega ohms, or like my Fluke meter will say OL. OL means the resistance is too high to read and the fuse is open. If I use our example of the bridge again, a max resistance reading means the power cannot get across the bridge to where it needs to go so the fuse is blown or open.

Next I will show you one way to check a fuse with the power on. Once again, if you are not experienced with electricity get the proper training before performing this procedure. Lets assume that the power is on to my my air conditioning unit and I have opened the disconnect box. I will turn my DMM to the voltage function. My fluke meter only has one voltage function and determines wheher the voltage is AC or DC voltage. If your meter has both an AC voltage function and DC voltage function, you need to be knowledgeable enough to determine whether the circuit is powered by AC or DC power.

I know that my air condition runs on 220 Ac so thati s what I am looking for. The 220 AC is split into two 110 AC legs. Each leg is running through a separate fuse. I need to check both fuses to make sure that both legs of 110 are present because the air conditioning unit needs 220 AC to run. Before I check the fuse, I need to explain something about electricity.

Measuring electricity is actually the measurement of a difference in potential between two points. I will use the example of two points on a straight line. You can measure the distance between the points but you cannot measure the difference of potential between the two points because they are connected on the same straight line and are just an extension of the same point. If Iput a break in the straight line between the two points, they are no longer an extension of the same point and we can measure for a diffrence in potential. A good fuse is just like the points on a straight line.

Each side of the fuse is connected together by a wire or other conductor. This make the fuse the same point on each side. If the fuse is good and I put the leads of my DMM on each side of the fuse with the power on, I will read zero volts because I am reading the same point electrically. If the fuse is bad, it is like reading the points on a straight line that has a break in the line between the two points. They are no longer an extension of the same point. this gives us the opportunity to read a difference of potential across the two points. It is the same when you have a blown fuse you are checking with the power on.

The side of the fuse that is connected to the main power panel will have power on it (in our ewample, it will be 110 volts) The other side of the fuse will have zero volts because the fuse is blown and tha path for the power is destroyed. This leaves us with a difference of potential of 110 volts because one side of the fuse has 110 and the other side has 0. When you put the leads of your multimeter on each side of the fuse, you are reading that difference in potential. It is probably opposite of what you would think if you are not skilled in electricity. I will have more articles on electricity basics in the near future. until then be safe when dealing with electricity.

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Sarah on September 13, 2011 said:
I hate it when fuses blow, great advice and it seems so simple even i might be able to do it.

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