[Music]
Narrator: We all know that electrical energy travels best when it has two or more conductors
or wires called a power transmission line to guide it.
But this energy exists as electric and magnetic fields that fill the space near the power
line conductors.
Each of these fields can interact with objects located near the power line.
Today we will focus on objects that interact with the electric field.
This process is often called electrostatic induction.
[What is electrostatic induction?]
Electrostatic induction is defined as a redistribution of an electrical charge in an object, caused
by the influence of an electric field.
When applied to the power system, this means that a conductive object in the vicinity of
a high-voltage power line may have energy coupled to it even if it is not connected
to that power line.
Take this fluorescent light tube for example.
There are no hidden wires here.
The electric field associated with the energized power line above us causes a voltage across
the tube that causes the gas inside it to light.
This is an example of electrostatic induction.
[Is it dangerous?]
These same electric fields interact with people who stand beneath a power line.
In fact, they can cause large voltages between a person’s head and feet, as we’ll show
later.
But under properly designed power transmission lines, this has no effect on the person.
This is because it is electric current that causes perceptible effects such as shocks,
not voltage.
The presence of the insulating air between power line and the person limits the current
to a very small level and hence the effect of this electric field.
Dr. Robert Olsen, Associate Dean for Undergraduate Programs, College of Engineering and Architecture,
Washington State University: Even though we know that electric fields generally are harmless
to the health of people underneath the line, there are situations where it can be hazardous.
Consider, for example, the situation behind us.
Here we have a sport-utility vehicle that’s parked in an electric vehicle that’s comparable
to what you would find under a 500,000 volt power line.
In this case, we know the electric field can affect the charges on the vehicle and actually
cause it to rise to a voltage above the ground which can be as high as several thousand volts.
Narrator: Let’s do an experiment where we can see how much of a hazard this really is.
Remember that voltage alone cannot hurt you.
The shock you get when dragging your feet on carpet is a good example.
Even though the voltage is very high, we all know such a shock is not harmful.
What really matters is how much current there is.
Let’s setup this meter to measure the current between the vehicle and ground.
A milli-amp is one thousandth of an amp.
We would have to have at least 100 milli-amps before this could be a life-threatening shock.
Dr. Robert Olsen: We just found that this vehicle, in the electric field that we have,
can actually be at a voltage of 2,800 volts with respect to the ground.
That sounds quite high.
But it’s not voltage that’s hazardous; it’s current that’s hazardous, as we learned
before.
In this particular case, we found that the current was about 1.6 milliamps.
1.6 milliamps is high enough to feel but it’s not high enough to be hazardous.
Narrator: We can use tools like this electric field meter to determine how much coupling
there might be in a given area.
This meter measures the intensity of the electric field and displays it in kV per meter.
Dr. Robert Olsen: I have to be quite a ways away from the meter because my body will actually
affect the electric field.
That’s why I have the meter on the end of an insulated rod here.
Narrator: Engineers can use such a meter to be sure that fields are within acceptable
parameters.
As you can see, utilities work hard to ensure that power lines are operated safely for power
system workers and for the general public.