Fuses and how to tell when they are blown

April 21, 2008 by Larry Cad · 3 Comments  
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A couple of weeks ago, before my computer crashed, we discussed special cases in electrical systems and defined these as “short” and “open” circuits. I posed a question about which of these special cases would most likely result in a blown fuse. The correct answer is that a short circuit will most likely blow a fuse because the current flow in a short circuit tries to go to infinity. The purpose of the fuse is to limit the current flow by “blowing”, thus preventing excess current from flowing in the circuit.

Fuses come in many styles, types and ratings.

Most of us are aware of fuse amp ratings such as 1 amp, 10 amp, 20 amp, etc. Although the amp rating of a fuse can be complicated, a simple interpretation would be to say that the amp rating is the current flow above which the fuse will blow. We will leave it at that for now and talk more about fuse amp ratings in a later article.

A second fuse rating that is important is the voltage rating of the fuse. Very simply, this is the maximum voltage that can appear in the circuit where the fuse is used.

There are reasons why a circuit designer chooses a particular fuse to use in a circuit. Because of this you should always replace a blown fuse with another fuse with identical or similar ratings.

Since a blown fuse can occur due to electrical faults in a system, it is important to be able to tell when a fuse is blown. There are at least three ways to tell when a fuse has blown. One is to look at the fuse to see if it appears to be blown. Another way is to check the resistance of the fuse with a multimeter. A good fuse should read 0 ohms on the lowest resistance range. A blown fuse will read infinite. A third method to check for a blown fuse is to use a voltmeter. This method gets a bit trickier and we will discuss that, and maybe get into meter usage next time.

Until then, happy camping!!

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3 Responses to “Fuses and how to tell when they are blown”

  1. Gordon thompson on May 9th, 2008 8:25 pm


    Thanks a bunch for this series of posts. I’ve just come out of a two-day struggle with an electrical problem in my fifth wheel. After getting two techs ($150.00) to look at the situation, the problem was finally traced to a bad fuse. I’m sure most would say that should have been easy but it wasn’t.

    I’m a very logical person but generally brain dead when it comes to electrical gear and issues. I did narrow down this particular problem but needed that help and two days without heat and cooling to arrive at the solution. I just read your post regarding the use of a multi meter and will buy one tomorrow. I know it won’t turn me into an electrician but with help from people like you I won’t be quite as lost as I am now.

    I’m relatively new full timer and one of the things that makes me feel good about being in the lifestyle is the brotherhood that runs common where ever RVers congregate. It’s good to know that whether in a park or on line, help is never far away.


  2. Larry Cad on May 10th, 2008 8:19 am

    Gordon, thanks for the kind words. I to am disappointed with the lack of talent in the electrical troubleshooting world. Electricity is complicated, yet simple. Hopefully we will all learn together and be more independent with our RVs.


  3. Dan Shockey on May 11th, 2008 7:42 am

    When I unplug the landline my interior lights don’t work but the battery does start the generator, and when plugged into the gen set the interior lights work. I have checked all the fuses that I can find. Any suggestions before I take it to the dealer?