Shedding Light on Electricity Episode 5: Electrical Safety

The Shedding Light on Electricity series exposes the shocking truth about electricity. Yes, positively a bad pun to begin with, but we promise to conduct ourselves really well from now on. This series teaches students watts of stuff (sorry, couldn’t resist) about electricity including how it’s produced, how it’s used in our homes, how it’s controlled, and how we keep ourselves safe from nasty shocks. Ohm my goodness, this series is without parallel!

In Episode 5, Electrical Safety, we first look at the dangers of electric shocks by actually demonstrating an electric shock. We then look at the safety features built into our homes and electrical devices that keep us safe from electric shocks.

A 3-minute excerpt followed by a 1 minute trailer.

The Question Sheet for students.

Contents:
Part A: Introduction
Part B: Light Globes, Motors, and Toasters
Part C: Short Circuits
Part D: Circuit Breakers and Fuses
Part E: The Earth and the RCD.


The Transcript (more or less) (Screen shots coming soon)

Part A: Introduction

Electricity is one of the greatest inventions ever. However, with great power comes… great… responsibility to keep things safe. That’s a Spiderman reference by the way. This little light globe is drawing a current of about 0.29 Amps. If the same amount of current was going through my body right now, say from one hand to the other across my chest, it would kill me. In fact it would take only about 0.1 of an amp or 100 mA to kill me. My heart would stop beating properly and I would collapse and die.

So though 0.29 Amps is going through the light globe, much less is obviously going through me because I’m not dying. And why is that? Because a human body has a much higher resistance than a light globe so a 6 V battery can’t push all that much current through me.

However, if I connected to a 240 V supply, by say cutting this cable open and touching the bare wires, there’s an extremely high chance of dying.

To demonstrate an electric shock, safely, I can pass a small amount of current through my forearm muscles.

To make a really good connection I placed two sponges on my forearm and then covered the sponges with copper plates. The sponges had been soaked in a special chemical that was a very good conductor.

Every time I then connected to the power pack, my muscles went into a spasm, a really creepy spasm and the voltage the power pack had been set to was only about 18 V! Electricity is not something we should mess with! Some of the spasms were painful. I then connected an ammeter to the circuit and the peak current was only about 10 mA, 0.01 of an amp. So if 18 V has this effect, imagine the effect of accidentally connecting to a mains 240 V supply.

240 V from the mains supply would cause complete paralysis. If the current was running across my chest, the muscles that control my breathing wouldn’t be able to function and my heart which is a muscle would almost certainly stop beating.

In our last episode we saw how manufacturers of light globes design light globes with different resistances depending on how much current they need to operate correctly. They also have to ensure that the electrical equipment is safe of course. The electrical connections in our homes also have to be safe.

So even though electricity can be dangerous, and lethal in fact, very few people are ever harmed by it. And why is that? Well, in this episode of the Shedding Light on Electricity series, we’re going to quickly recap what we learned about voltage, current and resistance in previous episodes and then look at some of the safety features that are built into homes and electrical devices to keep us safe. Let’s begin.

Part B: Light Globes, Motors, and Toasters

These kinds of light globes are called incandescent light globes. They produce light because the electric current makes the very thin filament wire so hot that it starts glowing. I can use a powerpack that produces a variable voltage to change the current flowing in this car headlight and we can see the filament getting brighter and dimmer. The word incandescence means the production of light from a hot object. It comes from the Latin “incandescere” which means to glow. The word candle also comes from this word, inCANDescence, because candles glow thanks to the hot gases that are produced as the wax burns.

We’ve seen that the resistance of a wire is determined by four main factors: length, width, temperature, and the nature of the material. The resistance of the headlight is only 3 ohms, because it needs to draw a fairly large current to produce the desired amount of power since it’s designed for a 12 V car battery. The resistance of this household globe is about 960 ohms. It doesn’t need much current because it’s designed for a 240 V power supply. A 240 V source gives more energy to each electron than a 12 V source does, so to get a similar amount of power you don’t need as many electrons per second to flow.

The manufacturers change the resistance of incandescent light globes by changing the length and the width of the filaments. The fatter and shorter filament of the headlight has a much smaller resistance so more current flows in it.

It’s important to remember that whenever electric current flows through any wire, heat is generated, and the amount of heat generated depends on its resistance and on the current flowing in it.

Now while incandescent light globes still have their place in many applications, LED light globes have almost completely taken over in our homes because LED light globes can produce the same amount of light as incandescent light globes using only about 1/10 of the electricity. So LED light globes are designed with a much higher resistance because they don’t need that much current.

The copper wires used in power lines, extension cords, and in the cables in buildings are fairly thick so they have extremely low resistances. This allows them to carry the electric current without getting hot and therefore wasting the electrical energy.

The wires in the windings of electric motors are also made of copper. By changing the length and thickness of the wires, the motors can be made with different power outputs. A small motor has thinner wires to restrict the current flow, and larger, more powerful motors have thicker wire to allow more current to flow.

The elements in toasters are made of a metal called nichrome, which is an alloy made up of about 80% nickel and 20% chromium.

In the early 1900s, American scientist Albert Marsh discovered that this metal has an unusually high resistance for a metal so it was perfect for use in the elements that are found in toasters, and grillers, heaters, and kettles. Its relatively high electrical resistance enables the manufacturers to produce heating elements with thicker wires. Thicker wires are stronger and so are less likely to break every time you move an appliance. If it wasn’t for Albert Marsh, we wouldn’t have any of these things.

Thanks, Albert! Legend.

However, nichrome wires, and wires in any electrical device such as in the motor of a washing machine, can still break and this can occasionally lead to what are called “short circuits” which can cause fires and other issues. So what are short circuits?

Part C: Short Circuits

Let me demonstrate a short circuit. Here I’ve connected two light globes in series to a battery and the current is flowing through the circuit. Each light globe is getting only about 3 V because they have to share the voltage of the 6 Volt battery. The filaments remember are made of really really thin wires. Now what will happen if I connect this wire across the terminals of the first light globe? The wire is a fairly thick wire which means that it has a very, very low resistance.

When I connect the wire in parallel across the first light globe, we can see that the light globe turns off. This is because most of the electric current passes through the low-resistance thick wire rather than through the ultra-thin filament wire. So, the light globe on the left has been “short circuited” by the wire. A short circuit is often just called a “short” for short. There’s actually a little saying that goes “electricity follows the path of least resistance”. Some current is still flowing through the light globe, but most, 99.9% or whatever is flowing through the connecting wire.

A short circuit occurs when a big fat low-resistance wire is connected across an electrical device and the current flows through it instead of what it’s supposed to flow through.

In this case, there is no real damage, because the second light globe can handle the extra current and this is not really a practical circuit anyway.

However, in many circuits, like this one where a resistor is connected in series with an LED, then short circuiting the LED causes the LED to turn off and short circuiting the resistor causes too much current to flow into the LED which can burn it out. You can see that the current jumps up from about 14 mA to about 140 mA when the resistor is shorted. Loose wires in electronic devices often cause short circuits and the device basically stops working or stops working properly anyway.

Now short circuits can be very dangerous if the short-circuiting wire is placed directly in contact with a power source. In a typical circuit, you have a power source, a load (or loads), wires, and usually a switch. As we’ve seen, the load is designed to allow the right amount of current to flow. If, though, a low-resistance wire is placed in direct contact with a power source then you’ll get a huge amount of current flowing that can cause damage. The lower the resistance the higher the current, and the higher the current, the more heat is generated.

If I connect this low-resistance metal shaving directly to a 6V battery, a lot of current flows out of the battery and the metal starts glowing because it gets red hot. And it’s only getting 6 V.

This powerpack is delivering about 20 V so much more current flows and much more heat is generated. So faulty equipment where wires come loose and short circuits are created can get so hot that they become a fire hazard.

This thin piece of graphite which is used in these refillable pencils, is actually a pretty good conductor. When I connect it to a 6V battery it starts smoking. When I connect it to a power pack delivering 20 V, it get so hot that it gives off bright light much like an incandescent light globe. What are usually called “lead” pencils are actually graphite pencils. Graphite is a form of the element carbon that conducts electricity and it’s often used in electrical motors and generators.

Now the battery produces only a limited amount of current because it’s fairly small and the chemical reactions that produce the current can only happen so fast. Essentially, batteries have their own internal resistance which limits the amount of current that they can deliver. The powerpack is also limited because it has an automatic safety switch that switches off the power if it detects that too much current is flowing.

However, a large car battery can easily deliver 500 Amps or so which is a huge amount of current because the chemical reactions can all happen very quickly. The battery’s internal resistance is extremely low.

A short circuit across a car battery can produce a huge amount of current and a huge amount of heat. It can melt wires and cause fires very easily, even though the voltage across the terminals of a car battery is only about 12 volts.

At least one terminal of a car battery is typically well covered so that a mechanic, or someone working on their car at home, doesn’t accidently place a spanner down across uncovered terminals and cause a fire. In the old days, that occasionally happened but laws now specify that battery terminals have to be covered.

Placing a spanner or any metallic tool down across the terminals of a car battery can be very dangerous. In only a few short seconds the steel spanner got really hot and had little bits of it melted and blasted away. If fact, car batteries themselves have been known to get so hot in these situations that they burst open and the hot battery acid inside them sprays out all over the place.

Wherever possible in the filming of this series, we used either smaller batteries, or the power pack, because as I said, it has a switch that cuts off the power if it detects that too much current is flowing.

Our homes also have switches that cut the power if a fault in an electrical device causes a short circuit. Let’s take a look at some of the safety features that are built into the wiring of a typical home that save lives and property.

Part D: Circuit Breakers and Fuses

Short circuits can occur in our homes where they’re especially dangerous since the voltage supply into our homes is a relatively high 220 to 240 Volts.

When you’re using a toaster, for example, the current flows into the toaster through one of the copper wires in the cable, through the nichrome element, and then back out through the other copper wire. A toaster connected to a 240 V supply draws about 4 Amps so, using the V =IR equation, which means that R = V/I, we can calculate that the resistance of the element is 60 ohms. The resistance of the copper wires is very very low.

Sometimes though, and it’s very rare, a wire inside the toaster may break in a way that short circuits the element. Suddenly you have a 240 V supply connected to a low resistance of let’s just say (for example only) only 1 ohm. This would cause a huge current to flow through the lead and through the hidden wires inside the walls which would cause a fire. But in fact this doesn’t happen!

Obviously a fire would be a bad thing, and that’s where these things come in.

They’re called circuit breakers, and they break the circuit if they detect that too much current is flowing.

All the electricity that enters our homes or any building has to pass through these circuit breakers which you can find in the meter box, which is usually near the front of your house. There’s a little electromagnet inside them and if the current gets too big, the electromagnet triggers the switch. In this diagram, I’m showing only one circuit breaker and only one power point, but of course multiple cables fan out from all the circuit breakers to all the electrical points in the house.

Typically, lighting circuits and power circuits have their own circuit breakers and the number of circuit breakers just depends on the size of the building.

All the current that goes to the lights in the northern section of this house goes through this circuit breaker. These two service the lights in two other sections of the house. These three circuit breakers service about 10 power points each that are in different sections of the house. Electric ovens and air conditioners also typically have their own circuit breakers.

Circuit breakers used for lighting circuits are typically rated at 10 Amps, which means that they flick off if the current exceeds 10 Amps. Circuit breakers used for power circuits are designed to carry up to 20 Amps before they trip.

So, if you ever go to switch on an appliance and it doesn’t turn on then it might just be broken but if you notice that the television has also turned off, it means that there is a dangerous short in the appliance.

The circuit breaker will have tripped so you’ll have to reset it and you’ll have to replace or repair the appliance.

Damaged cords, like this one, rather than the device itself, are a major cause of short circuiting so they should be repaired.

This circuit breaker acts as a master switch. All the current going into this house goes through this circuit breaker first before going through the other circuit breakers. It’s rated at 40 A. If I flick the switch off, the whole house will be without power.

Circuit breakers don’t just protect against short circuits. They also protect against any current overload. If I connect a light globe designed for a 6 V power source to a 6 V battery, it draws 0.28 amps. If I then connect two light globes (in parallel), the total current coming out of the battery increases to 0.54 amps. When I connect three light globes in parallel, the total current coming out of the battery increases to 0.76 amps. Within limits, the more devices you connect, the more current flows and this of course applies to household circuits as well.

This electric radiant heater would draw about 10 Amps if it was on. If I then turned the kettle on, another 10 amps or so would be drawn bringing the total to about 20 amps which is about at the limit of the circuit breaker. If I then went to use the toaster as well, which draws about 4 amps, the total current would exceed 20 amps and so the circuit breaker would trip and cut the power.

Fuses, these things here, are similar to circuit breakers. Found in older buildings, the electricity flows into the building through a fuse wire that is enclosed in the ceramic shell. The wire is specifically designed to melt if too much current flows through it which then results in the power being cut off.

In all my life, I’m going to guess that I’ve only had about 5 incidents where a fault has caused a circuit breaker to trip.

When earthquakes occur, not only do buildings collapse or get damaged, quite often, wires are ripped from their connections and short circuits occur. This often causes fires. Fires can sometimes cause more deaths than the collapsing buildings.

Throughout the electricity grid, there are huge circuit breakers that cut the power to largish areas if there’s a surge of electricity due to a major short circuit in the overhead wires caused by, for example, a pole falling over and wires touching each other. Fortunately, this doesn’t happen very often.

Faults are probably most common in electric motors, which have really thin insulated wires wrapped around a steel frame. Over time, the insulation might flake off and the motor shorts out. So, though faults don’t develop very often, there are millions of buildings and electrical devices and circuit breakers have saved countless numbers of them from fires.

However, circuit breakers don’t necessarily save humans from electric shocks because humans have a very high chance of dying if only about 100 mA passes through them, much much less than the 10 A or 20 A trigger points of the circuit breakers. However, our homes have other safety features which are just as important.

Part E: The Earth and the RCD.

So we’ve seen that electric current has to flow into an appliance, through it, and then back out again.

That means that the cord you plug into the power point has to have at least two pins. However, appliances that have a metal case usually have three pins. The third pin is called the Earth or the Earth pin and it’s also called the Ground. Earth, Ground, same thing. The Earth pin plays no role at all in the operation of an electrical device. It’s there purely for safety. So what does it do?

Well, occasionally, a wire in an electrical device will come loose or break and come into contact with the metal case of the device which means that the metal case becomes electrified or “live”. In this situation, if anyone touches the metal case (which because of the fault has a direct connection to a source of electricity), the person may get an electric shock when he or she touches it.

Basically it’s like having an exposed wire that’s plugged into the socket and you can’t have that.

So if an electrical device has a metal case, an “earth” wire is included. One end of the Earth wire is connected directly to the case of the device and the other end is connected to the Earth…literally to the Earth.

Let me redraw the diagram of the set up but spread out the wires. These are the two wires that come from the street which by the way are called the active and the neutral. The wires connect to the circuit breaker and then continue inside the home. I’ve coloured the active brown and the neutral blue because that’s the colour that is typically used for the insulation of the wires. Colouring the insulation makes it easy to identify which wire is which. The current flows into the device through the power point and the cord and then back out again.

When the device is on and working normally you don’t need any other wires. So let me explain the theory behind the Earth wire.

The metal case of the device is also connected via the Earth wire (which is usually coloured green and yellow) to a copper spike that is buried into the Earth under the building. This is what it looks like but usually less of it is visible. This is the symbol for an Earth connection. The supply wires in the street are also connected (on the neutral side) at regular intervals to the Earth. Now the Earth is not a brilliant conductor, but it’s not an insulator either. It’s a medium conductor with a low(ish) resistance. So, what happens if a fault develops and the metal case of a device becomes live? FLICK, the circuit breaker trips in a fraction of a second.

Why? Let me rewind. As soon as the case becomes live, the circuit immediately short circuits through the Earth (the Earth wire) and a large current flows through the Earth (literally the Earth) back to the pole because the Earth has, as I said, a low(ish) resistance. The current spikes upwards in a fraction of a second (not slowly like I’m showing here) and the circuit breaker (or the fuse) cuts the power when the current reaches 20 A.

Now that’s the theory, and it works… most of the time.

Sometimes though, if the ground is really dry, its resistance is too high and so it might not conduct well enough to cause the circuit breakers to trip when there’s a fault. So, in the 1980s in Australia, they started connecting the Earth wire back to the neutral in the meter box and this also allows the faulty device to short circuit back to the supply wires in the street which trips the circuit breakers and cuts the power. So the Earth wire and the circuit breakers work together to keep us safe if a fault develops.

The earth wire allows the faulty device to immediately short circuit if a wire inside the device come into contact with the metal casing and the circuit breaker trips before any current can pass into you if you touch it. The Earth wire would be ineffective if you didn’t have a circuit breaker or a fuse to cut the power when a fault occurred.

Electrical devices with plastic casings don’t have an Earth connection because plastic casings don’t conduct electricity so even if there’s a fault inside, you’re not touching any conducting metal on the outside.

Now this is an Australian plug which fits into this Australian socket. Other countries may have plugs and sockets with different shapes so if you ever want to travel internationally, you need to make sure you have adapters if you need them. The adapter on the left allows an Australian appliance to plug into English sockets, and the one on the right is an adapter for European sockets. And here’s some more travel advice; take along a powerboard so that you can charge all your phones and laptops and things at the same time. New Zealand uses the same shaped plugs as Australia so no adapters are necessary.

Now, in addition to circuit breakers and the Earthing system, many homes and building also use what are called Residual Current Devices: RCDs. Residual Current Devices don’t cut the power if too much current flows, they cut the power if there’s a difference between the amount of current flowing into a building and the amount of current flowing back out of the building. These devices here that I said earlier were circuit breakers are also RCDs, in a two in one arrangement.

In Episode 3 of our series, we saw that in any electrical circuit, the current flowing out of the power supply is exactly the same size as the current flowing back into the power supply. I actually made a big deal of it in that Episode. For example, the current coming out of the car battery when the headlight is connected to it, is 3.2 amps which is equal to the current going back into the battery.

The same is true for buildings. The amount of electric current flowing into a building is the same as the amount of current flowing back out of the building.

In the case of this toaster, if it was on, about 4 amps would be going in and then 4 amps would be coming back out again.

However, if some part of an exposed wire comes into contact with a person who, for example, is using a metal knife to get some toast out of a toaster, some extra current will flow into the person and go to the Earth. It typically isn’t much, because the human body is not a very good conductor, but you have to remember it takes only about 0.1 amps to get killed, an amount that can easily flow in this situation. 0.01 amps was enough to cause spasms and paralysis in my arm. However, 0.1 amps of current is nowhere near enough to trip a circuit breaker. So, let’s just say 0.1 amps of current flows into the person, we now have 4.1 amps coming in and only 4 amps going out. The Residual Current Device senses the difference and switches off the current in a fraction of a second.

Let me demonstrate…no only joking. Residual Current Devices have saved a lot of lives since they were introduced, but please don’t go experimenting at home by sticking metal objects into electrical equipment.

In Australia, RCDs became compulsory in the 1990s in all new buildings and in any buildings being renovated where the renovations included electrical work. I’ve never seen one trip.

Of course, you should still always avoid using mains-powered appliances near water because even if the case is plastic, electricity might still be able to conduct through the water if the appliance gets wet.

Also, always make sure that the plug is pushed fully into the socket because if the plug is too loose, there’s a good chance that as the metal connections connect and disconnect, sparks might be generated in a process called arcing. Arcing is great in arc welding but it’s not so good inside a power socket.

So now that we’ve covered all the essentials of electricity, let me finish off by saying that electricity is obviously a huge part of the society that we live in, so electricity is not really just a science topic. Governments, like the Victorian government that sits here in Victoria’s Parliament building, have a responsibility

  • to keep the electricity supply going and
  • to ensure that the electrical equipment that we buy is safe.

But the people in government, the politicians, are not necessarily electricity experts so they typically appoint people who are to make many of the decisions regarding electricity matters. For example, in Victoria, where I live, we have a government agency called Energy Safe Victoria (although this name might change).

This agency is responsible for (for example), licensing Victoria’s electrical workers, ensuring electrical appliances are tested and that they are safe for use, and investigating electrical incidents such as fires and deaths caused by electricity. Other states have similar agencies, and there’s a federal agency that covers the whole of Australia as well.

They all communicate with each other and make decisions which are then passed into law. Other countries have similar agencies as well.

There’s always a lot of important stuff going on behind the scenes that most of us just don’t see, but we can be pretty confident that any electrical equipment we buy from an Australian supplier is very safe.

And that brings us to the end of the Shedding Light on Electricity series. I hope you liked it. See you next time.