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What has the Earth got to do with Electricity


ohdearme
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For safety, electric circuits are grounded, which means two of the wires on a 3 prong electric plug are attached to a 3 meter rod driven into the ground. It is sometimes called Earth ground.

PS that's a standard 120v ac circuit in the US. 240v ac may be three prong with one prong ground. I think most 240 circuits will have four prong plugs, with two connected to ground

Edited by EdEarl
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The Earth is at zero volts. The electrical system provides an earth (or ground) wire that is guaranteed to be at zero volts. This is connected to any metal parts of the system to ensure that, in the case of a fault such as a short-circuit from the "live" (120/240V) wire, the user will not be exposed to high voltages. Instead, such a fault will cause the circuit breaker to blow. 

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2 hours ago, Strange said:

The Earth is at zero volts. The electrical system provides an earth (or ground) wire that is guaranteed to be at zero volts.

Technically yes, but sometimes no. Ground is also a conductor (albeit a poor one). When a live high voltage conductor contacts the ground, there's a voltage drop radiating outward from the source, hence voltage is present. If a person was standing near the point of contact it's safe, yet walking away (in strides) can kill you.

This is known as Step Potential.

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4 minutes ago, rangerx said:

Technically yes, but sometimes no. Ground is also a conductor (albeit a poor one). When a live high voltage conductor contacts the ground, there's a voltage drop radiating outward from the source, hence voltage is present. If a person was standing near the point of contact it's safe, yet walking away (in strides) can kill you.

This is known as Step Potential.

Electricity used in homes is unlikely to create a fatal step potential. But caution is always warranted when using electricity, especially around water.

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5 minutes ago, EdEarl said:

But caution is always warranted when using electricity, especially around water.

A downed wire contacting a vehicle is safe, so long as the occupants don't step out. There's a risk for "touch potential", in some circumstances. However, exiting with one foot on the ground and one on the handle (or the other foot on the body), completes the circuit through the body and is dangerous in almost any case.

Water itself is a poor conductor. Technically, it's an insulator. Water is rarely pure though. Add a little salt or any conductive contaminate, it's dangerous.

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8 hours ago, ohdearme said:

Hello

I hope this is the right forum, apologies if not. Just a novice and reading about safety, circuits, short circuit, current etc and The Earth is often mentioned.

What has the Earth got to do with it? Hope to hear from you.

Ok so an 'earth' is not an essential part of an electrical circuit (not all circuits have one), but it is a very, very useful one.

The most important property of an earth is that is remains at constant potential and undamaged, no matter how much electricity (current) you draw from it or pour into it.

You should note that this is unlike the property of the logitech power supply you asked about in an earlier thread. This had a nominal 16 volts value, but actually varied with the current drawn from it.

For the purposes here I will not go into the distinction between potential and voltage and use the term voltage, so  Strange is correct in observing that this constant is normally taken as zero.

This property of remaining at constant voltage has three principal uses.

1) Since an Earth can accept huge quantities of electricity, devices such as fuses or circuit breakers can be arranged to divert this current to earth to disconnect the circuit in the event of a fault.

2) Electrical noise and interference is basically unwanted variations in electrical potential (voltage). So the constant voltage property can be used to shield or screen a sensitive circuit from the source of this interference. Large currents are not normally ionvolved in this.

3) Again the property of constancy of the voltage can be used as a reference or benchmark for the proper operation of the circuit itself.

 

A final note is that an earth is characterised by a single connectior or one terminal.

 

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One practical point about the Earthing on electrical gadgets, is that it can be a killer, or a life saver, depending on whether it's properly fitted.

I once touched the handle of a freezer with the tip of my little finger and got a huge shock of electricity, I was in bare feet on a stone floor. My friend had wired the plug carelessly, without securely pinching the cable with the gripping mechanism that's at the entrance to all plugs. Someone else had pulled on the plug to make it reach the power point. The earth wire had pulled out from it's terminal, and was just resting against the live wire. So, the whole chassis of the freezer was live, just waiting for someone to touch it.

It was lucky my finger only just brushed the handle. If I had got a proper grip, I would have definitely been killed. That gripping mechanism in a plug is vital, but a lot of people don't give it much thought.

On the subject of step potential, did you know that the safest thing to do, if lightning is about to strike, is to stand on one leg? People might laugh, but it's true. If your feet are wide apart, the charge can go up one leg and down the other. The further apart your feet are, the greater the voltage drop, and so the stronger the current. That's why cattle are more likely to die in a lightning strike than a human. The distance from the front feet to the back ones is so much more. A flamingo on one leg should be ok.

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10 minutes ago, mistermack said:

My friend had wired the plug carelessly, without securely pinching the cable with the gripping mechanism that's at the entrance to all plugs. Someone else had pulled on the plug to make it reach the power point. The earth wire had pulled out from it's terminal, and was just resting against the live wire.

I was trained to always make the earth wire a little longer to reduce the risk of things like that (as well as paying attention to cable grips, etc.) My first boss insisted on using a particular make of plug that had a pillar that the wire wrapped round instead of this tiny holes that the wire gets put in. He would then make us create a little loop, like a tiny lasso, so the wire could pull loose. And the cable grips were two bits of angled plastic that couldn't be avoided.

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Once, when I was young and foolish (I'm not young any more, just foolish) I decided to wire a light fixture myself.  Of course, I turned off the electricity to the fixture- at the wall switch.  I was very surprised, when I got a shock, to discover that wasn't enough.  Now I throw the circuit breaker- or call an electrician.

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2 minutes ago, Strange said:

I was trained to always make the earth wire a little longer to reduce the risk of things like that (as well as paying attention to cable grips, etc.) My first boss insisted on using a particular make of plug that had a pillar that the wire wrapped round instead of this tiny holes that the wire gets put in. He would then make us create a little loop, like a tiny lasso, so the wire could pull loose. And the cable grips were two bits of angled plastic that couldn't be avoided.

Very wise. It's hard to overstate just how lethal it can be if you get it wrong. When I was three, ( sixty odd years ago ) I was nearly killed by a basic single bar electric fire. I was holding the bar when my elder brother plugged it in. You simply can't let go. All the gripping muscles in your hand contract, so you can't release it. I remember it vividly to this day. I was jumping around like a fish, and every time I hit the floor I got a tremendous shock. Maybe a three year old heart is hard to stop, but I was badly burned.

If I had got a full grip on the handle of the freezer that time, I would definitely have died.

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2 hours ago, mistermack said:

Very wise. It's hard to overstate just how lethal it can be if you get it wrong.

 

 

That's why you should approach both with the back of the hand; one for smelling and the other for contraction in case you get bit ;).  

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