| Ever take a peek inside the toaster while you’re waiting for the toast to pop up? If so, you would have noticed a bright orange glow. That glow is produced when the toasting wires heat up, which in turn creates a nice crusty surface on your bread or waffle. It’s the same phenomenon as when the filament inside an incandescent bulb glows. The light and heat produced in both these cases are the result of the Joule, pronounced “jewel,” effect at work.
To understand Joule heating, let’s first refresh our memories as to electrical current resistance. We learned previously that wire is not a perfect conductor, and as such resistance to flow is encountered. This resistance causes power to be lost along the length of wire, in accordance with this equation:
Power Loss = I2 × R
Where I is the electric current flowing through a wire, and R is the total electrical resistance of the wire. The power loss is measured in units of Joules per second, otherwise known as watts, “watt” denoting a metric unit of power. It is named after the famed Scottish mechanical engineer, James Watt, who is responsible for inventing the modern steam engine. A Joule is a metric unit of heat energy, named after the English scientist James Prescott Joule. He was a pioneer in the field of thermodynamics, a branch of physics concerned with the relationships between different forms of energy.
Anyway, to see how the equation works, let’s look at an example. Suppose we have 12 feet of 12 AWG copper wire. We are using it to feed power to an appliance that draws 10 amperes of electric current. Going to our handy engineering reference book, we find that the 12 AWG wire has an electrical resistance of 0.001588 ohms per foot, “ohm” being a unit of electrical resistance. Plugging in the numbers, our equation for total electrical resistance becomes:
R = (0.001588 ohms per foot) × 12 feet = 0.01905 ohms
And we can now calculate power loss as follows:
Power = I2 × R = (10 amperes)2 × (0.01905 ohms) = 1.905 watts
Instead of using a 12 AWG wire, let’s use a smaller diameter wire, say, 26 AWG. Our engineering reference book says that 26 AWG wire has an electrical resistance of 0.0418 ohms per foot. So let’s see how this changes the power loss:
R = (0.0418 ohms per foot) × 12 feet = 0.5016 ohms
Power = I2 × R = (10 amperes)2 × (0.5016 ohms) = 50.16 watts
This explains why appliances like space heaters and window unit air conditioners have short, thick power cords. They draw a lot of current when they operate, and a short power cord, precisely because it is short, poses less electrical resistance than a long cord. A thicker cord also helps reduce resistance to power flow. The result is a large amount of current flowing through a superhighway of wire, the wide berth reducing both the amount of power loss and the probability of dangerous Joule heating effect from taking place.
Our example shows that the electric current flowing through the 12 AWG wire loses 1.905 watts of power due to the inconsistencies within the wire, and this in turn causes the wire to heat up. This is Joule heating at work. Joule heating of 50.16 watts in the thinner 26 AWG wire can lead to serious trouble.
When using a power cord, heat moves from the copper wire within it, whose job it is to conduct electricity, and beyond, on to the electrical insulation that surrounds it. There the heat is not trapped, but escapes into the environment surrounding the cord. If the wire has low internal resistance and the amount of current flowing through it is within limits which are deemed to be acceptable, then Joule heating can be safely dissipated and the wire remains cool. But if the current goes beyond the safe limit, as specified in the American Wire Gauge (AWG) table for that type of wire, then overheating can be the result. The electrical insulation may start to melt and burn, and the local fire department may then become involved.
That’s it for wire sizing and electric current. Next time we’ll slip back into the mechanical world and explore a new topic: the principles of ventilation.
Posts Tagged ‘electrical fire’
| Whether or not you live or work in a city, you are probably aware of rush hour traffic and how frustrating it can be. As a matter of fact, this traffic is the number one reason many choose to live within cities providing public transportation. Instead of watching the cars pile up in front of you, you can be checking your email or reading the paper. And no matter where you live, you’ve probably encountered a narrow one-lane road at some time. If this road were to be spotted with traffic lights and double parked cars, the resulting frustration would reach a new high, one which has you craving the freedom of a crowded three-lane expressway. At least there’s the possibility of movement there.
Generally, the wider the road and the fewer the impediments, the better traffic will flow. The problems presented by vehicular traffic are analogous to those present in electrical wires. For both, obstructions are impediments to flow. You see, the thicker the metal is in a wire, the more electrical current it can carry. But before we explore why, let’s see how electric wires are classified.
If you’ve ever spent any time hanging around a hardware store looking at the goodies, you’ve probably come across wire gauge numbers, used to categorize wire diameter. American Wire Gauge (AWG) is a standardized wire gauge system, used in North American industry since the latter half of the 19th Century. Handy as it is, the AWG gauge numbering system seems to go against logic, because as a wire’s diameter increases, its gauge number decreases. For example, a wire gauge number of 8 AWG has a diameter of 0.125 inches, while a gauge number of 12 AWG has a diameter of 0.081 inches. To make things easier on those who need to know this type of information, wire diameter is tabulated for each AWG gauge number and readily available in engineering reference books.
So what does this have to do with electric current? To begin with, the larger the AWG number, the less current it can safely carry. If we turn to an engineering reference book, and look up information relating to an 8 AWG insulated copper wire, we find that it can safely carry an electrical current of 50 amperes, while a 12 AWG insulated copper wire can safely carry only 25 amperes. This information allows us to make important and relevant design decisions regarding a myriad of things, from electrical wiring in electronic devices, to appliances, automobiles, and buildings.
So, why are bigger wires able to carry more current? Well, as you’ve heard me say before, no wire is a perfect conductor of electricity, but some metals, take copper for instance, are better conductors than others, say steel. But even the best conductors are inherently full of impurities and imperfections that resist the flow of electricity. This electrical resistance acts much like traffic lights and double parked cars that impede the flow of traffic. The larger the diameter of the wire, the less electrical resistance is present. The logic here is simple. Wire that is larger allows more paths for electrical current to flow around impurities and imperfections.
The congestion present in rush hour traffic results in travel delays and hot tempers, and heat is also present in electric wires that face resistance to electricity flow. If the resistance to electric current flow is high enough, it can cause overheating. Road rage within the wires is a possibility, and if the wires get hot enough, electrical insulation can melt and burn, creating a fire. Known as the “Joule heating” effect, this phenomenon is responsible for its share of building fires.
We’ll learn more about Joule heating and how wires are sized to keep electrical current flow within safe limits next week. Until then, try to keep out of traffic.
Property damage and loss of lives, these are often the result of fires. But did you know that one of the leading causes of fire is electricity? Residential electrical fires claim the lives of nearly 500 Americans each year and injure another 2300. Annually, these fires result in over $800 million in property losses.
Approximately one third of the nearly 70,000 home electrical fires that occur each year are traceable to design and manufacturing defects in electrical products. The rest are caused by the misuse and poor maintenance of electrical products, overloaded circuits and extension cords, and incorrectly installed wiring.
The three components that must be present in order for a fire to manifest and sustain itself are well known. These components make up the “Fire Triangle,” a potentially lethal combination of heat, fuel, and oxygen. If any one of these three components is missing from the triangle, a fire can’t be started or sustained. In the case of an electrical fire, it’s electricity that creates the heat component of the Fire Triangle.
The Fire Triangle
How does electricity contribute to fires? One example would be an overloaded extension cord. Homeowners are sometimes unaware that extension cords must be sized appropriately for their ultimate usage. If not, they can overheat, particularly if they are damaged. Damage to cords can result from a myriad of factors, from factory production errors to kinking when heavy furniture is carelessly placed on top of them.
The same principle holds true for electrical products. If their internal wiring or a component is insufficiently sized or damaged, overheating can result. If things get hot enough and there is sufficient airflow (oxygen) and combustible material (fuel) in the vicinity, then the fire triangle is complete. The fire starts internally and can soon spread to other objects in the area.
Electrical arcing can occur when an energized electrical circuit is broken. For example, suppose a wire carrying current is suddenly broken in two. If the voltage is high enough, the electricity will want to continue to flow through the air across the break to form an electrical arc. If the power flowing through the arc is great enough, heat can once again complete the Fire Triangle, resulting in fire.
Forensic engineering analysis of evidence collected from a fire scene often yields telltale signs of overheating due to overloaded electrical circuits or damaged wiring in components. Under close examination by an experienced professional, even the smallest strand of wire can point to the cause of an electrical fire.
CSI skills aren’t only employed at crime scenes. Forensic engineers also use similar techniques to get to the true story of cause and effect.