How can birds land on a live wire and not get shocked? The answer is complicated, yet simple to most experienced home inspectors. It’s a matter of understanding how electricity operates.
There are few tools in a home inspector’s toolbox that don’t require interpretation or have more than one explanation. But when it comes to electricity, we have solid, black-and-white guidelines to help us figure out what’s what. Using these “laws” can help us understand, figure out or diagnose why electricity acts the way it does or, you might say, how it “conducts” itself.
Keep in mind that there is no actual movement in a conductor when electrons are flowing, at least not in the way we might think. No atoms within the copper are carrying electricity and moving forward. Electricity is an electromagnetic force that runs along the circumference of a conductor, transferring the electrical energy from one atom to another.
Voltage and Amperage
What are the two most dangerous forces a home inspector will come across when inspecting electrical systems? Voltage and amperage.
We can thank Georg Ohm, a German physicist (1787-1854), for developing the laws of voltage and amperage. I’m sure he got zapped many times before he figured them all out. Ohm’s Law, described below, is one law, but for the sake of clarity, I’ve
separated it into two parts.
Ohm’s Law, Part 1, states that current (amperage) flowing across two points in a conductor is directly proportional to the potential difference across those two points. The key phrase in this part of the law is “potential difference across two points.” In other words, electricity flows from high to low potential, just like water that flows downhill. And the greater the potential difference (or the steeper the hill), the more the electricity wants to flow from high to low.
Ohm’s Law, Part 2, states that the current in a conductor that flows between two points is inversely proportional to the total resistance in that circuit. The key phrase in this part of the law is “inversely proportional to the total resistance.” In other words, when current goes up, there’s less resistance. Less resistance in a conductor means there is less heat and better electrical conduction.
It might sound complicated, but it really isn’t. By the time you finish reading this article, it will all become translucent. (Yes, that’s right, translucent, not transparent. Still, it’s better than having it be opaque.) Keep in mind that learning and understanding electricity is a process that takes time and experience.
Using this thought process might help:
Think of voltage (V) as being equivalent to pounds per square inch of force and think of amperage (A) as being equivalent to gallons per minute. They have to work together to have electrical flow and its accompanying danger. Without flow (amperage), there is little danger—electricity just sparks and we are shocked. It also might help to think of the electrons that flow through a conductor like bowling balls. You could manually roll bowling balls uphill, but there would be a lot of resistance (which would produce heat). But once your bowling balls are up the hill, you can release the balls from there and they will roll downhill freely (with less resistance and less heat).
So, with voltage and amperage in perspective, what about grounding? Grounding is the path electricity wants to take so that it can get back to its source. What is its source? Is it the transformer? Or, more likely, is it the main service panel (MSP)? For all practical purposes, home inspectors should only be concerned with the electrical mast, the MSP and the branch circuits, which together create a controlled path for electricity’s entry into the building and distribution throughout the house via various circuits.
However, electricity does not have to follow the prescribed paths that are created via the MSP and the branch circuits. We want it to, and we “encourage” it to follow those paths by having the appropriate wire size and the ability to create a direct path back the MSP.
As long as all circuits and breakers (or fuses) are as they should be, then everything is safe, right? Yes and no. Things can get out of hand sometimes, so the folks who design electrical schematics put in backup systems. What’s an example of a backup system? A ground wire? In fact, all modern 120-volt branch circuits have a grounded wire called the white or neutral wire. That’s the return path for electrical flow.
The backup path I will explain now is the green or bare wire—a system that is there in case something goes wrong with the neutral. The green (or bare) wire is dormant until a short occurs. Then, it’s there to offer an alternative path back to the MSP. What happens then? A breaker trips and shuts off the electrical flow. It’s all about safety–by keeping electricity contained within each individual circuit, a breaker will sense an imbalance and shut itself off. In a properly wired home, the imbalance is in the white or neutral wire or, if that fails, the backup green wire kicks in (Figure 1).
Electricity will take whatever conductive path that is available to it. And because electricity is invisible, we don’t always know or sense what path it might take. That’s what makes it dangerous. But understanding its components and how they function will help us minimize our exposure.
Take birds on a wire, for example. Birds can sit on a live wire because the difference potential between their legs or other body parts is zero. The electrical flow prefers staying in the wire instead of going through the bird (Figure 2). Birds on a wire are not grounded; that is, no part of them touches the ground, so they don’t act as conductors.
This concept also applies to people. It’s the nature of electricity that it is always trying to reach its starting point or to ground. This brings up another point that directly affects home inspectors: step potential. Step potential is the difference between two points of an energized source.
Take, for example, a barn owl landing on a live wire (Figure 3). When the owl landed, it made contact with one wire while its feet were in contact with a second wire. The difference potential between the two wires caused part of the current to flow through the bird, killing it. If the barn owl had landed on only one of the wires, making no contact with the other wire, it would have lived. This explains one of the reasons why high-voltage wires on poles are spaced apart as far as they are—so that any large bird, when spreading its wings, will not make contact with more wires than the one on which it lands or is perched (Figure 4).
As home inspectors, we have to be aware of the difference potential as well as the grounding rules. Electricity wants to go back to its source in any way that it can or by discharging into the ground (that is, from a place of high voltage to a place of low or no voltage, such as the ground). The function of an electrical system is to make sure it goes through controlled pathways so that a breaker is tripped. The electricity is then shut off and no one is injured or killed. Figure 5 shows an example of a person making contact with wire, and the ground and the factors that can affect the voltage.
What’s the best way to avoid a difference potential between your legs in an energized situation?
- Be aware of your surroundings at all times when inspecting, especially in abandoned or foreclosed homes.
- Never turn on anything electrically if you don’t know why it’s off.
- Hop away (“bunny hop,” keeping both feet together) from the danger for at least 30 feet. Keeping your feet together prevents creating any difference potential between your legs.
Let’s not forget touch potential! Touch potential is voltage that occurs between an energized object and the feet of a person in contact with that object. Why? Because we are usually standing in contact with ground.
Figure 6 shows three ways that a home inspector can create a conductive path through the body. One more thing…electricity can jump or arc. The amount of jump is directly proportional to the amount of voltage, as illustrated in Figures 7 and 8. I imagine the voltage jump that can occur when dealing with very high voltage, such as in an electrical room or when standing next to a high-voltage transformer.
Finally, using common sense and understanding is how we protect ourselves during an inspection. Carry a voltage sniffer with you and use it instead of your fingers (or tongue) to “sniff” out voltage where you suspect it. Use caution and think before you act. Electricity can do substantial damage and it can be a cause of death when it is not respected. Respecting electricity might save your life. Remember, as little as 10amps (and in some cases even less than that) can kill you with continued exposure. What’s the minimum amperage found in an average residential home? Fifteen (15) amps. Stay safe out there.
Mike Conley has been home inspecting since the mid-1980s. He has served on the ASHI Board of Directors several times and as Secretary in 2018. He also has served on various committees and task forces, chairing some of them. Mike writes many informative articles on various subjects to advance the inspection profession and help other home inspectors. He is one of the founders of his local ASHI Chapter and is active in education at InspectionWorld® and the ASHI School, as well as at conferences and seminars around the country.