Video: Understanding the Wires of a Plug

Which of the following statements correctly explains the purpose of the earth wire in a standard UK plug? [A] The earth wire is for safety; it prevents the casing of the device from becoming live. It only carries a current when there is a fault. [B] The earth wire completes the circuit of the device. [C] The earth wire provides the alternating potential difference that powers the device.

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Video Transcript

Which of the following statements correctly explains the purpose of the earth wire in a standard UK plug? A) The earth wire is for safety; it prevents the casing of the device from becoming live. It only carries a current when there is a fault. B) The earth wire completes the circuit of the device. C) The earth wire provides the alternating potential difference that powers the device.

Okay, so let’s start by imagining that we’re looking side on at an electrical socket. So here’s the wall. Here’s the socket. And here is the switch on that socket in the on position. So let’s say that we now plug a toaster into the socket. Now, that toaster’s plug is going to contain three wires, firstly, the neutral wire, secondly, the live wire, and, thirdly, the earth wire. Now, let’s say that this toaster has a metallic casing. And let’s also say that something goes wrong with the wires in this toaster, such that the live wire happens to be touching the metal casing of the toaster. And of course, these three wires are actually inside the toaster. So the live wire is touching the metal casing from the inside.

Now, let’s pretend for a second that the earth wire didn’t exist. Well in this situation, if someone was to come along and touch the toaster, then because the live wire is touching the casing of the toaster from the inside that would mean that a current would be able to travel through this person’s fingers through their body and down to the earth. This is a very dangerous situation. The person could be killed because of this. And so that is actually why we have the earth wire in this toaster in the first place.

When things are working well, the earth wire doesn’t carry any current. However, in a situation such as this, the earth wire provides a better route for current to flow and because the earth wire is earthed or grounded, the current flows along the earth wire and down to ground this way, rather than through the person’s fingers. This is because the earth wire is a better conductor than a human being say. And so it’s highly likely that any current will flow through the earth wire.

So based on this information let’s have a look at options A to C and see which one sounds correct. Option A says that the earth wire is for safety; it prevents the casing of the device from becoming live. It only carries a current when there is a fault. Now, this description actually does match what we’ve been saying about the earth wire until now. So let’s quickly go through options B and C to make sure that they’re incorrect.

Option B says that the earth wire completes the circuit of the device. Well actually, that sounds more like the neutral wire because let’s recall that the live wire provides an alternating potential difference that powers a device. And even though that potential difference is dropped across the device, there needs to be another wire coming out of that device to complete the circuit. And that’s the neutral wire. And option B sounds more like a description of the neutral wire.

So let’s look at option C. This one says that the earth wire provides the alternating potential difference that powers the device. Well as we’ve just seen, this sounds like a description of the live wire. So that’s not the answer to our question. Therefore, the statement that correctly explains the purpose of the earth wire in a standard UK plug is option A. The earth wire is for safety; it prevents the casing of the device from becoming live. It only carries a current when there is a fault.

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