Electrical Power Transmission

Lessons

Senior 3 · Physics

Electrical Power Transmission

4 lessons

Lesson 4 of 4

Dangers of High-Voltage Power Lines

20 min

The kite in the wires

A boy's kite drifts into the wires running along the road and sticks there, its string hanging down within reach. His friends say: "Just pull it, the string is only cotton." That advice could kill him. In this unit you have seen that many of the lines along roads carry 15 000 or 30 000 volts, and the pylon lines 110 000 or 220 000 volts. At these voltages, electricity can flow through things we normally think of as insulators, and can even jump through the air.

Why high-voltage lines are so dangerous

The wires are bare. Overhead lines are not covered with insulation: they are held up on long insulators of porcelain or glass and kept safe only by their height and their distance from everything else.
You do not need to touch them. At high voltage, the air near the wire can break down and conduct: a spark (an arc, or flashover) can jump several centimetres or more to a person, a ladder, a crane or a branch that comes too close.
"Insulators" can conduct. A damp kite string, a wet branch, a wooden ladder with dirt or water on it, or a person's body all pass a deadly current at these voltages.
Current through the body kills. A current of a few tens of milliamperes through the chest can stop the heart; a high-voltage contact also causes deep burns inside the body.

The bird and the ladder

A current flows through a body only if there is a potential difference across it. A bird sitting on one wire has both feet at the same potential, so no current flows through it, however high the voltage of the wire. A person standing on the ground is at earth potential (0 V). If he holds a ladder that touches the wire, or if a spark jumps to him, there are thousands of volts between the wire and his feet, and a large current flows through his body to the earth. (A large bird that touches two wires, or a wire and the pylon, at the same time is killed for the same reason.)

Definition

Flashover (arcing): an electric discharge that jumps through the air from a high-voltage conductor to a nearby object at a lower potential, without direct contact.

Right-of-way: a strip of land along a power line, centred on it, that must be kept clear of buildings and other unsafe uses so that the line can operate and be maintained safely.

The law in Rwanda: right-of-way

RURA's guidelines on the right-of-way for power lines (adopted on 30 November 2015) set the width of the strip that must be kept clear along each line:

Line voltage	0.4 kV	15 kV to 30 kV	110 kV	220 kV
Right-of-way width	3 m	12 m	25 m	30 m

The same guidelines forbid anyone to build, farm or dig in the right-of-way without the licensee's consent; to light a fire within 60 m of a transmission line's right-of-way; to climb on, attach or hang anything on a tower, pole or line; to cause anything to touch a line; to bring a crane or similar machine near a transmission line without written consent; and to carry out blasting within 100 m of any power line.

Worked example 1: where may the house go?

A family plans a house beside a 110 kV line. Using RURA's right-of-way width, how far from the centre of the line must the house stay?

\text{distance} = \frac{\text{width}}{2} = \frac{25\ \text{m}}{2} = 12.5\ \text{m}

The line runs down the middle of its right-of-way, so the house must be at least 12.5 m from the centre line; for a 220 kV line, at least 15 m.

Worked example 2: is the bird really safe?

A bird's feet are 4.0 cm apart on a 30 kV aluminium conductor whose resistance is $0.20\ \Omega$ per kilometre, and the conductor carries 100 A. Find the potential difference between the bird's feet.

R = 0.20 \times \frac{0.040}{1000} = 8.0 \times 10^{-6}\ \Omega \qquad V = IR = 100 \times 8.0 \times 10^{-6} = 8.0 \times 10^{-4}\ \text{V}

Less than a thousandth of a volt: far too small to drive any harmful current through the bird. The danger is not the 30 kV of the wire itself, but the potential difference between the wire and something else the body touches.

Important

If you see a fallen or low-hanging line

Treat it as live, even if it is not sparking or humming. Lines can be switched back on automatically.
Stay at least 10 m away from the wire and from anything it touches, such as a fence, a vehicle or a puddle, and keep others away.
If you are already close, move away with small shuffling steps, keeping your feet together: the ground near a fallen wire can have a large potential difference between points a stride apart.
Never touch a person who is in contact with a line, and never try to move the wire with a stick.
Report it at once to REG on its toll-free number, 2727, and if anyone is hurt call 912 (ambulance) or 112 (emergency).
If a line falls on a vehicle you are in, stay inside until REG says it is safe. Only if there is a fire, jump clear with your feet together, without touching the vehicle and the ground at the same time, and shuffle away.

Warning

Everyday rules near power lines: fly kites and drones only in open spaces far from lines; never climb poles, pylons or trees that grow near lines; carry ladders, long pipes and poles horizontally and look up first; never build, dig or burn rubbish under a line; and never touch, tamper with or "borrow" electricity from a line or a pole transformer. REG warns that stealing electricity through illegal connections carries deadly risks, and only REG may work on its lines.

Do power lines harm the health of people living nearby?

The currents in power lines produce weak electric and magnetic fields around them. The World Health Organization reports that even directly under a high-voltage transmission line, the currents these fields induce in the body are very small compared with the levels that cause shocks, and that no adverse health effects of long-term, low-level exposure to power-frequency fields have been confirmed, although research continues. The real dangers of power lines are contact and flashover, which is why the right-of-way rules exist.

Tip

Current needs a difference. Electricity flows through you only if you bridge two points at different potentials: a live wire and the earth, or two wires. Keep everything, you, your ladder, your kite and your crane, far enough away that no bridge, and no spark, can form.

High-voltage safety demonstration (Lincoln Electric System). A power company's crew shows on a live demonstration line what happens when objects touch high-voltage wires; note each safety rule they give.

✦

Practice

Name four activities that are dangerous near overhead power lines.
Explain why a bird can sit on a high-voltage wire unharmed, while a person on the ground holding a ladder that touches the same wire is killed.
Explain how a person can be killed by a high-voltage line without touching it.
Using RURA's right-of-way widths, how far from the centre of (a) a 30 kV line; (b) a 220 kV line must a new building stay?
After a storm, a broken wire is lying across a path to your school. List what you should do and what you must not do.
Why is it dangerous to try to pull a kite out of power lines by its string, even if the string is made of cotton?
A pupil says: "Wooden electricity poles are safe to climb, because wood does not conduct electricity." Evaluate this statement.
A family worries that living 50 m from a 110 kV line will make them ill. Using the WHO's findings, what would you tell them, and what would you warn them about instead?

Show answersHide answers

For example: flying kites or drones near lines; climbing poles, pylons or trees near lines; carrying ladders or long metal pipes upright under lines; building, digging or burning rubbish under lines; driving cranes or tall loads near them; making illegal connections.
Both of the bird's feet are on the same wire, at the same potential, so there is no potential difference across it and no current flows. The person is at earth potential, so there are thousands of volts between the wire and the ground; current flows down the ladder, through the body and into the earth.
At high voltage the air near the wire can conduct: if the person (or something held) comes too close, a spark (flashover) jumps the gap and current flows through the body.
(a) Half of 12 m: 6 m from the centre line; (b) half of 30 m: 15 m.
Treat it as live; keep at least 10 m away and keep others away; report it to REG on 2727 (and the police if anyone is in danger); use another route. Do not touch it, poke it with a stick, step near it or touch anyone in contact with it.
At such high voltages, a cotton string, especially if it is damp or dirty, can conduct enough current to kill, and a spark can jump from the wire to the string or to the person. Let the kite go and report it to REG.
Wrong. Poles carry live wires at the top, and a climber can touch them or get close enough for a flashover. A wooden pole may also be wet, and it carries metal fittings and an earth wire. Only trained, authorised REG workers may climb poles, after the line has been switched off and earthed.
The WHO reports that no harmful health effects of long-term, low-level exposure to power-line fields have been confirmed, and the currents induced in the body are very small, so living 50 m away (outside the 25 m right-of-way) is not known to harm health. The real dangers are contact and flashover: they should never build in the right-of-way, never let children fly kites near the line, and never climb the pylons.

Key points

Overhead lines are bare; at high voltage a spark can jump to a person or object that comes too close.
Current flows through a body only when it bridges a potential difference, such as a live wire and the earth.
RURA sets clear strips along lines: 12 m wide for 15–30 kV, 25 m for 110 kV, 30 m for 220 kV.
Stay at least 10 m from fallen lines, never touch anyone in contact with a line, and report it to REG on 2727.

1The kite in the wires

2Why high-voltage lines are so dangerous

3The bird and the ladder

4The law in Rwanda: right-of-way

5Do power lines harm the health of people living nearby?

The kite in the wires

Why high-voltage lines are so dangerous

The bird and the ladder

The law in Rwanda: right-of-way

Do power lines harm the health of people living nearby?