What Does a Blown 40 Amp Fuse Look Like?


Written by

Thomas Carter



Marcos E. Williamson

what does a blown 40 amp fuse look like

Fuses are vital safety features in automotive circuits, older houses, and RVs. 40-amp fuses, for instance, are used in circuits with large current draws. Fuses blow to protect the circuit and need to be replaced to get the appliances running again, unlike in circuit breaker panels.

But, what does a blown 40 amp fuse look like? It looks like a portion of the wire inside is cut with evidence of burns. A test light or a multimeter will also indicate no current flow.

Scroll down to learn more.

What is a 40 Amp Fuse?


Fuses, in general, are overcurrent protection devices designed to cut off the circuit when the current is greater than the fuses’ rating. They are important devices that protect the electrical components of the car when a surge of current flows through the circuit.

Fuses work by taking advantage of the heat caused by current flowing through a wire. More heat is released when more current flows. If the current is too much, the conductor or solder in the fuse will melt, cutting off the circuit.

In this case, the “40 amp” designation refers to the rated current carrying capacity of the fuse. It means that the fuse is designed to handle a maximum of 40 amperes (amps) without blowing or tripping.

What is the Use of 40 Amp Fuse?


40 amp fuses are used in circuits with large current draws like ignition relays and starter motor relays. They can also be found in the circuits of the rear defroster and the power window of some cars.

40 amp fuses may also be used in RVs to protect the converter. The fuse blows in case the polarity of the battery was installed in reverse.

Why Do Circuits Need Fuses?


Here are some reasons why fuses are so important in any electric circuit:

  • Overcurrent Protection: Fuses protect the circuits against excessive current flow. Currents that exceed the rated capacity of the equipment will trip the fuse, preventing further electrical flow. This helps prevent overheating, damage to equipment, and potential safety hazards.
  • Fire Prevention: Short circuits can generate excessive heat, which can lead to electrical fires. By preventing excessive current flow, fuses help reduce the risk of fire and enhance overall electrical safety.
  • Equipment Protection: Electrical devices and equipment can only handle specific current ratings. Without fuses, excessive energy can damage or destroy the equipment.
  • Safety Precaution: Fuses prevent electrical accidents and reduce the risk of electric shocks. By promptly interrupting the circuit during overcurrent events, fuses help minimize the potential for electrical hazards, protecting individuals from harm.

Looks and Symptoms of a Blown 40 Amp Fuse


In most cases, a blown 40 amp fuse is identical to other blown fuses with different current ratings.

A blown fuse will have a disconnected conducting strip inside. A gap in the conducting strip can be seen if you look at a blown blade fuse or a JCASE cartridge fuse because they often have transparent casings.

Other symptoms of a burnt blown car fuse include a burnt color on the gap and some discoloration on the casing. Though, in some cases, the conducting strip may not be visible enough, so testing the fuse with a test light or multitester may be needed.

Identifying a Blown Fuse


Fuses can blow due to overloads, short circuits, ground faults, or arc issues stemming from faulty electrical wiring and screws. You may also have a fuse that’s insufficiently rated for your appliances, in which case, a replacement is necessary.

The fuse box contains the fuses arranged in a neat way. For cars, the fuse box under the hood usually contains the 40 amp fuses while for RVs, the box may be close to the circuit breaker panel. Simply find the fuse that belongs to the circuit supplying power to your equipment with an issue.

For instance, if your car’s AC no longer works, you can check the fuse box labels and find the spot for air conditioner fuse. Test the fuse using the steps in the next section.

Check a Bad Fuse vs Good Fuse

You can check car fuses using a test light or a multimeter. These methods keep the fuses in place for a convenient inspection. You can also remove the fuses if you want to examine them independently.

1. Check a fuse with a test light


A test light is a qualitative way to check fuses. It has a bulb that turns on when current flows to it and the fuse is working.

Otherwise, the fuse is blown.

To use the test light, connect the test light to the negative terminal of the battery. Then stick the probe to the terminals of the fuse and see if the bulb lights up.

2. Check a fuse with a multimeter


The great thing about a multimeter is that you can use it to spot a good or a bad 40 amp fuse, even if the fuses are not in the fuse box. You can use the ohmmeter or the voltmeter setting of the multimeter for this.

To test a fuse not in the fuse box, simply set the multimeter to Ohms and place the connector in each terminal of the fuse. The multimeter will read minimal (even zero) resistance on a good working fuse. Otherwise, it will read “OL” or open loop.

To test a fuse installed in a fuse box, set the multimeter to Volts-DC. Place the negative probe on the negative terminal of the battery, then use the positive probe to measure each terminal of the fuse. A good fuse will have a 12V reading on both terminals, but a blown fuse will have a reading on only one terminal.

How Do You Replace a Blown 40 Amp Fuse?

When you’re ready to replace the fuse, you can follow these steps:

1. Check the fuse you need to replace.


Identify and check the bad fuse using our previous tips. You can remove it to look for signs of a bad fuse.

If you see the fuse burnt but not blown, its holder may be loose or damaged. Test the fuse using a test light or a multimeter to be sure.

2. Purchase the fuse that matches the blown one


For cars, you can buy fuses in your local automotive accessories shop or order online from websites like AutoZone.

Make sure you check what packaging and what color the fuse is (it’s usually orange, by the way), since fuses may come in a variety of forms like a cartridge fuse or a triple 40 amp fuse. If a square fuse is blown, buy a square fuse with the same rating.

For houses, make sure you check the specifications like voltage and size. Some fuses may be rated at 125 volts while others are rated for 250 volts, and their sizes might be different.

3. Install the new fuse


Remove the old fuse with pliers (if you haven’t removed it) and plug in the new fuse in the same location. Close the fuse box after.

What Are the Other Solutions?


Before you change the fuse, have a professional check the electrical system. A blown fuse may be a sign of an underlying problem like faulty equipment or faulty wiring that may have caused a short circuit. This problem might remain even if you change the fuse.

You can also replace your blown fuse with a resettable one. A resettable fuse cuts off the circuit during abnormal current flows by rapidly increasing the resistance of the fuse. Just make sure you get a resettable fuse that matches your old one.

How to Prevent Fuse-related Issues?


  • Avoid Overloading Circuits

For homes using fuses, blown fuses can be prevented by avoiding plugging in appliances that will exceed the current rating of the circuit. Simply unplug some of the electronics from the outlet.

  • Check for Faulty Wiring or Connections and Address Short Circuits and Ground Faults

A short circuit or a ground fault might be a reason for a 40 amp fuse to blow frequently. Short circuits and ground faults happen when faulty wiring allows electricity to flow in a different path and can cause a surge in current. An electrician (for a house) or a mechanic (for a car) should check for problems to solve them quickly.

  • Use the Correct Fuse Ratings

A fuse with a lower rating may still blow. This is true for both cars and houses. The right fuse can handle the current in that circuit.

  • Regular Maintenance and Inspections

Regular maintenance can prevent faulty connections, wirings and blown fuses altogether. Your trusted mechanic can tighten any loose connections and clean your electrical components.

Frequently Asked Questions


Can I replace a blown 40 amp fuse with a higher or lower amp fuse?

No. If the fuse rating is too high, it will allow too much current during a surge or short circuit to flow to the equipment and damage them. If the fuse rating is too low, it might blow too frequently if the equipment draws a lot of power.

As long as 40 amps remain the proper fuse size for your circuit, do not deviate from it.

How can I determine if a fuse is blown without removing it from the fuse box?

You can follow our steps if you are testing fuses in cars. You can use a test light or a multimeter to check for blown fuses.

Is it safe to replace a blown fuse on my own, or should I consult a professional?

You can easily replace a blown fuse. However, a blown fuse, especially for a car, may indicate a bigger problem, so you can consult a professional.

Are there any warning signs before a fuse blows?

Sometimes, you may smell something burning or hear buzzing sounds. Your lights may also flash and dim from time to time. One or a combination of these symptoms may apply to your situation.

How often should I check my car’s fuses for potential issues?

You don’t need to regularly check and maintain your car’s fuses. They are single-use equipment and only need replacement when they’re blown.


What does a blown 40 amp fuse look like? A blown fuse looks like the conducting strip has a gap. If you cannot see the fuse well, you can test it with a light or a multimeter.

Replacing a fuse can be easy, especially if you know how to tell if a car fuse is bad. But you can talk to your friendly professional to help diagnose and repair any underlying issue in your electrical system, and prevent any more 40 amp fuse blown incidents.

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