Maintenance Contracts

How To Tell If AC Fuse Is Blown

AC fuse is blown

You love it when your house is nice and cool even when the sun is on its full glory outside, increasing the temperature ominously. Needless to say, this is precisely the time when your AC decides to act up and shuts now. Now your peaceful world comes to an end. Before you panic, allow us to soothe your fraying nerves because most often than not, it is a case of the blown fuse. Now that you know it is nothing big, the question that comes to mind is - “How to tell if AC fuse is blown?”

Before we answer this question, let us take a few minutes to understand the fuse. This knowledge is critical to comprehend the reason behind a blown fuse. Once the reason is known, you can contain the problem.

The AC Fuse

During the hot summer months, when the sun is at its peak, your AC works hard to maintain the pleasantly cool atmosphere inside the home. Sometimes the continuous hours of operation with no respite in-between heats the unit beyond an acceptable level. When this happens, the circuit breaker trips or the fuse blows to prevent any lasting damage to your AC. In short, a blown fuse is just a precautionary measure to protect the machinery.

A fuse is sensitive to heat. Whenever there is a power surge or the unit heats excessively the filament in the fuse melts. This occurrence is the blown fuse. The fuse is designed in such a manner that it can bear the load of fixed amperes only. If the voltage increases beyond its amperage capacity, the fuse blows. Essentially, when the fuse blows, it cuts the power supply to the unit and thereby saving the unit from any further damage.

How To Tell If AC Fuse Is Blown

If you suspect that your AC may have blown a fuse, the first thing you shall notice is that the outside unit of your AC is not doing anything. You may hear a minor humming sound, but that is the extent of it. The humming sound is produced because, despite no power, the furnace keeps asking the AC to run.

Can I Check For A Blown Fuse?

If you understand the basic electrical system of the AC, you can indeed check for a blown fuse yourself. However, not all of us have that knowledge, and yet we try to decipher the problem on our own. This can be quite dangerous. Until and unless you are sure about what you are doing and you have taken the necessary precautions to ensure your safety, you should not try and take remedial actions to correct the problem. Most often, the current is still passing through the wires, which can give you a strong jolt or shock. So refrain from being too adventurous. Contact your local HVAC company to come and inspect your air conditioner.

Check Blown Fuse With Voltmeter

The best way to test for a blown fuse is by using a voltmeter.

1. To verify that you have a blown fuse on hand, the first step is to locate the disconnect box. It is usually grey in color and mounted near the outdoor AC unit.

2. Lift or swing open the outer and inner cover of the disconnect box to expose the cables. Locate both the incoming (line) and outgoing (load) wires. Remember both of them carry 110 volts, so be cautious.

3. Check the voltmeter to make sure that it is functioning properly. It should display a reading of zero volts or infinity depending on the model you are using.

4. Attach the voltmeters leads to the lugs joining the two line wires. The voltmeter should read somewhere between 220-240 volts. This means power is coming as far as the fuses. If the voltmeter does not read anything on this side, the problem isn’t the fuses.

5. After testing the incoming wires, it’s time to test the outgoing wires. Join the voltmeter’s leads to the lugs of the load wire. If you get a zero-volt reading, you have a blown fuse. Else, the problem is in another place.

What If You Don’t Have A Voltmeter?

Not everyone has a voltmeter at home. If you don't have one, the world does not come to an end. You can still check whether your AC has blown a fuse or not. Check the volt capacity of the fuse in the disconnect box. Buy new fuse of the same capacity. Replace the old fuse with the new one and check whether the AC works. If yes, you had indeed blown a fuse. If no, you now have an extra set of fuses on hand. The good thing is that they can be used later when your AC indeed does blow a fuse. Baad thing is that you still have a non-functioning AC on hand. 

A Few Tips

1. If you want to check for a blown fuse yourself, make sure that your hands and feet are dry. Also, see to it that so is the floor.

2. If you are not an electrician, take a pic of the disconnect box when you open it so that you bring it back to the status quo while closing it. Pictures are especially useful when you are replacing the fuse. You can verify the exact manner in which they should be placed.

3. If your AC keeps on blowing a fuse, it means there is another problem that needs to be addressed first.

4. Your local HVAC technician is the best person to inspect your AC for any troubles.

Final Thoughts

By now, the above information has given you a fair idea of how to tell if AC fuse is blown. Even if you cannot do the necessary job yourself, you at least have a good idea of what the AC technician will do. Armed with this knowledge, nobody can try and fool you. It is always better to have some knowledge rather than being completely ignorant.  

AC Fuse Frequently Asked Questions - FAQs

If you suspect your AC may have blown a fuse, the first sign that you will notice is that the compressor unit or the outdoor unit is no longer functioning. You may, in fact, hear a humming sound coming from the unit, which is its way of asking for the current to operate. But since the fuse is blown, there’s no current to give, so the compressor cannot start.

The fuse in the AC serves an essential purpose. It feeds current to the compressor so that it can work. The blowing of the fuse is a safety control that prevents serious problems like electrical fires. Once the fuse is blown, the unit does not get the current to operate. As a result, the AC stops working. Therefore, yes, a blown fuse can cause the air conditioner to stop working.

There are 3 ways in which you can test the AC fuse:

  1. Sometimes a visual inspection of the fuse is enough to judge if it is blown. There will be a black flash mark inside the blown fuse, or its window will appear obscured.
  2. The other way to come to the same conclusion is to take the old fuse to the hardware store and buy its exact replica. Insert the new fuse and check whether the AC is working. If it is, the old fuse had indeed blown.
  3. Finally, if you are comfortable using a voltmeter, you can check whether power is coming to the fuse. If yes, test the outgoing wires from the fuse. In case no current registers on the voltmeter at this end, it means you are dealing with a blown fuse.

There are a couple of reasons why the outside AC unit won’t come on, namely:

  1. The thermostat is not set to cool.
  2. You have a tripped circuit breaker.
  3. The AC has blown a fuse.
  4. The condensate line is full, which has caused the safety switch to trip.

To know the precise reason, it is best to call a professional HVAC technician.

If your air conditioner fails to start when switched on, one probable reason could be that it has blown a fuse. To confirm, go outside and look at the compressor. When the fuse is blown, the outdoor unit starts making a humming noise but does not kick in. If you find the same thing happening with your AC, its fuse is blown.