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Have you ever turned on the toaster when the microwave is already running, or had several things running and suddenly, everything shuts off? I’ve had it happen to me several times over the years. It’s so frustrating, but I try to remind myself that it’s just the circuit breaker doing its job. Circuit breakers are designed to protect our homes by prevent electrical wiring from overheating and possibly causing a fire.

If you have a breaker that trips regularly, then you may have a wiring problem that you need to have an electrician look at and repair. On the other hand, if you have a breaker that only trips when you have several things operating simultaneously, then you likely just have breaker that is doing its job. To prevent future breaker trips, you can plug some items into receptacles that are on a different circuit in order to better distribute the load.

What is the Purpose of a Circuit Breaker?

circuit breakerMost house wiring is designed to operate up to a temperature of about 194˚ F. While copper does not melt until it reaches almost 2000˚ F, the plastic insulation on most residential wiring can only withstand temperatures up to about 194˚ F. There are two things that heat up the wires in our homes: the ambient temperature of the air surrounding the wire, and the amount of electric current flowing through the wire. Since larger wires offer less resistance to the flow of electricity, larger wires can handle more current flow than smaller wires. As a result, they will heat up less than smaller wires will for a given amount of current flow.

Breakers are designed to prevent the overheating of wires due to excessive current flow. Testing and experience have shown how much current different sizes of wire can handle without getting too hot, and the National Electric Code sets forth what size breaker is required to protect each wire size. For example, #12 wire which is one of the most commonly used wire size in many homes is not allowed to carry more than 20 amps, and must therefore be protected by nothing larger than a 20-amp breaker.

How Do Circuit Breakers Work?

Circuit breakers protect our homes by tripping and opening up the circuit to stop the flow of electricity if too much current flow is detected. Most circuit breakers have two separate mechanisms that each work in different situations to trip the breaker. These two mechanisms are a thermal trip and an electromagnetic trip.

circuit breakerThe function of the electromagnetic trip is to open the circuit in the event of a very high current which would result from a short circuit. A short circuit occurs when a hot and neutral conductor are in contact with each other, creating a circuit with essentially no resistance. Since E=IR (voltage=current x resistance), then I=E/R. If the resistance (R) is very low, then the result of the equation or current (I) is very high. When this occurs, the breaker trips almost instantaneously because of the action of the electromagnetic trip.

The function of the thermal trip is different from the electromagnetic switch. The current flows directly through a bimetallic strip of metal. Bimetallic means that two different metals have been fused together. All metals expand and contract as they get hot or cool off. Since different metals expand at different rates, the strip of two different metals that have been fused together will bend as they heat up because one metal will expand faster than the other metal. This bending will cause the unattached end of the bimetallic strip to move away from the contact.

labeled breakerOnce the strip bends enough to no longer touch the contact, the circuit will be opened. Since the bimetallic strip must heat up to some degree before it bends enough to open the circuit, the circuit does not open immediately when the current reaches a level above the current rating of the breaker. In other words, there is a time delay to the thermal tripping mechanism in a circuit breaker. If the current is just slightly above the rating of the breaker, it will take many minutes or even hours for the thermal switch to heat up enough to open the breaker. If the current is two or three times the current rating of the breaker, then the breaker can trip in just a few seconds or minutes.

In the photo below, (found at https://www.c3controls.com/blog/understanding-trip-curves/), a Time/Current curve for a circuit breaker is shown. These curves show how long it will take a breaker to open based on how much electrical current is going through the breaker. It is important to note that every make and model of circuit breaker has its own Time/Current curve, and the one below is just a random example of one such curve.

Let’s look at a couple of quick examples. Look at the number “2” along the bottom of the graph and follow the line upwards. This line shows that if the actual current is twice the rated current of the breaker that it will take anywhere from 10 – 40 seconds for the breaker to trip. For another example, follow the line up from the “1.5” at the bottom of the graph. Doing this shows us that if the current is just 1.5 times the rated current of the breaker, then it will require anywhere from 40 – 400 seconds for the breaker to trip. In other words, it can take almost 7 minutes for the breaker to open if the current is just 50% higher than the rated current of the breaker.

The point of these examples is that breakers do not always trip immediately when they detect high current, but there is a delay unless the current is very high as in a short circuit situation. This delay does not make them unsafe, and It’s not a deficiency in their design. They are made to have this delay. One reason for a delay is that motors have a high starting current. Have you ever noticed when your air conditioner comes on, or your refrigerator compressor kicks on that the lights in your house dim momentarily? This is due to the high current that occurs when a motor first comes on. You wouldn’t want a breaker to trip every time your refrigerator starts up, and the delay feature of circuit breakers prevents this.

time/current curve- circuit breaker

My purpose in showing the Time/Current curve above and of going through these two examples is to demonstrate that breakers do not trip immediately, but that there can be quite a long delay if the current is only slightly higher than the rated current of the breaker. This is why the breaker in your home does not trip immediately when you turn on that toaster or hairdryer which results in the over current situation which eventually trips the breaker in your home. You can go to https://www.c3controls.com/blog/understanding-trip-curves/ for a much more detailed explanation of these Time/Current curves.

How to Reset a Circuit Breaker that was Tripped

circuit breakerIf you have never reset a tripped circuit breaker before, and you have one to reset, it can be a frustrating experience. It is actually quite simple once you know the “secret.” Here is the secret. In order to reset a circuit breaker that has tripped, you must first move the switch all the way to the “Off” position. From the “Off” position, you simply move the switch to the “On” position.

If the breaker will still not reset after first moving the switch to the “Off” position, there are a couple of possible reasons. First, the overcurrent situation still exists. Make sure that some of the items that were running before the breaker tripped are turned off, then try again. One indication that the overcurrent condition still exists is that the breaker clicks as soon as you move it to the “On” position. Second, it is possible that the thermal trip has not cooled down enough to allow the breaker to be reset. So, allowing five minutes or so before attempting to reset the breaker may then allow it to be reset. If you still cannot reset the breaker, you may have a faulty breaker which will require an electrician to replace.

What is a Fuse?

fuses - circuit breakerLike a circuit breaker, fuses stop the flow of current, but they do so through a different mechanism – by melting the current-carrying portion of the fuse.  Fuses have a strip of metal running through them that carries the current. This metal is often an alloy of various metals (such as tin and lead) that has a low, but precise, melting temperature. The metal strip is of a precise size so that it will heat up beyond its melting point if it carries too much current. Once the fuse carries current over its rated amount, the metal strip will melt and stop the flow of the current.

What is the Difference between a Circuit Breaker and a Fuse?

So, what is the difference between a fuse and a circuit breaker? Circuit breakers and fuses both serve the same purpose: to prevent fires by stopping the flow of current if too much current is flowing for one reason or another. I have already explained that a circuit breaker works to stop the flow of current by tripping a switch which can easily be reset (similarly to a light switch) in order to restore the circuit to where it can again carry power. Once a fuse stops the flow of electricity, it must be replaced with a new fuse. This is the same whether the fuse is in your home (only found in very old homes) or in your vehicle. So, breakers are reusable while fuses are not.

Summary:

Circuit breakers have replaced fuses in our homes and are one of the most important components in a home’s electrical system. The purpose of circuit breakers is to protect our homes by preventing fires caused by overheating wires. They are designed to stop the flow of electricity if too much current is going through a particular circuit in the home. They use two mechanisms to provide this protection. The first is an electromagnetic trip to open the circuit if there is a short circuit which results in a very high current situation. The other is a thermal trip which will open the circuit if current is higher than the rating of the breaker, but not so high that it indicates a short circuit.

I hope that this information has been helpful to you. If you have any questions or comments, please feel free to comment below, and I will respond as soon as possible.

© 2020 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at mike@morganinspectionservices.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.


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