Explain electrical earthing to me
Discussion
On a car, the whole body (at least, on metal bodied cars) is connected to the negative side of the battery. It's typically called "earth" but it's actually negative.
In a house, as well as the negative "side" there's a true connection to ground (earth) which could be via a physical rod into the ground. Typically electrical fittings (certainly metal ones and those that aren't double insulated) have an earth connection so that in the case of a fault (eg a live wire shorting on a metal item) the current flows to earth via the wiring and not via you if you touch it.
Older house wiring would cause the fuse to blow in such a circumstance. In modern circa 80s onwards an RCD is fitted which turns off the power if *any* current is running to ground.
In a house, as well as the negative "side" there's a true connection to ground (earth) which could be via a physical rod into the ground. Typically electrical fittings (certainly metal ones and those that aren't double insulated) have an earth connection so that in the case of a fault (eg a live wire shorting on a metal item) the current flows to earth via the wiring and not via you if you touch it.
Older house wiring would cause the fuse to blow in such a circumstance. In modern circa 80s onwards an RCD is fitted which turns off the power if *any* current is running to ground.
"earthing" is broadly because the main voltages supplied to your house are, at their fundamental level AC voltages at a dangerous potential to ground, whereas your car is a completely isolated, low votlage DC system.
If we start with your car, and DC, because it's easiest.
The battery creates a positive potential difference between it's positive and negative terminals. Because it is a battery, those voltages are isolated from any voltage carried by any other part of the car, or it's environment. For current to flow, the load must be connected between those +ve and -ve points. Because car bodies are mostly steel and hence conductive, it's become normal to arrange for the negative terminal of the battery to be connected to the cars metal chassis, meaning the whole chassis is held at that potential. Because a car battery is just 12v (nominal) and a human body has too high resistance, you cannot get electrocuted whatever you touch. (ie you can touch both battery terminals, or the chassis and the positive terminal, and no (significant or harmful) current will flow. In this case, "earth" is therefore irrelevant and hence omitted.
But for your house, that is supplied with a high AC voltage, things are very different.
At the power station, large (megawatt power level) generators create an earth referenced AC voltage, and that power is sent out across the distribution grid at a range of voltages (as high as 400,000 volts on the big cross country pylons, and as low as 240v where it enters your house).
UK houses are only fed with a single phase (called "Live"). In order therefore to provide a return path for the current a conductor called "Neutral" is introduced. This conductor is rated to carry the full return current from the loads in your house, but nominally is fixed to EARTH where the power enters your house (next to your electricity meter). A long metal pole is driven into the ground to provide a local earth, and neutral is electrically tied to that pole. Because of this, theoretically Neutral and Earth are at the same voltage, but because Neutral is carrying current, and all wires have a voltage drop, there can be a small voltage difference between those conductors (under non-fault conditions)
Back in the day, when most consumer goods had a metal cover (cookers, toasters, water heaters) there was a significant risk that should a fault cause the live wire to touch the metal cover (lose wire, chaffed insulation etc), if you were to touch that cover, standing on earth (same potential as neutral remember) then you would receive an electric shock that could kill you. To avoid this safety "Earth" is also distributed around your home, and despite being nominally the same (zero) voltage as the neutral conductor, is never set up to carry any load current under non fault conditions (that's what the neutral wire does). Now, if the live wire touches the metal case, there is a direct short circuit to earth, and therefore a fuse will blow, hopefully before you get electrocuted. Additionally, modern earth leakage circuit breakers monitor the current flowing out the earth wire at all times (instead of the neutral wire where it should be flowing) and if that exceeds a dangerous level (typically 20mA) that circuit breaker trips to interrupt the live supply. That system should help avoid most serious electrocutions by turning off the power before too much current can flow and electrocute you.
So in summary we have three conductors:
1) Live, carries the current to the loads
2) Neutral - returns the current from each load to the supply
3) Earth - carries no current during normal operation, but acts as a safety net to prevent un-intended voltages being created where they shouldn't.
Any metal object in your house should be earthed to prevent that object sitting at dangerous voltages due to a misfire, or a fault or in fact for any reason. For example, if you look at your central heating, you will find all the copper pipes are earthed to prevent them becoming a hazard!
If we start with your car, and DC, because it's easiest.
The battery creates a positive potential difference between it's positive and negative terminals. Because it is a battery, those voltages are isolated from any voltage carried by any other part of the car, or it's environment. For current to flow, the load must be connected between those +ve and -ve points. Because car bodies are mostly steel and hence conductive, it's become normal to arrange for the negative terminal of the battery to be connected to the cars metal chassis, meaning the whole chassis is held at that potential. Because a car battery is just 12v (nominal) and a human body has too high resistance, you cannot get electrocuted whatever you touch. (ie you can touch both battery terminals, or the chassis and the positive terminal, and no (significant or harmful) current will flow. In this case, "earth" is therefore irrelevant and hence omitted.
But for your house, that is supplied with a high AC voltage, things are very different.
At the power station, large (megawatt power level) generators create an earth referenced AC voltage, and that power is sent out across the distribution grid at a range of voltages (as high as 400,000 volts on the big cross country pylons, and as low as 240v where it enters your house).
UK houses are only fed with a single phase (called "Live"). In order therefore to provide a return path for the current a conductor called "Neutral" is introduced. This conductor is rated to carry the full return current from the loads in your house, but nominally is fixed to EARTH where the power enters your house (next to your electricity meter). A long metal pole is driven into the ground to provide a local earth, and neutral is electrically tied to that pole. Because of this, theoretically Neutral and Earth are at the same voltage, but because Neutral is carrying current, and all wires have a voltage drop, there can be a small voltage difference between those conductors (under non-fault conditions)
Back in the day, when most consumer goods had a metal cover (cookers, toasters, water heaters) there was a significant risk that should a fault cause the live wire to touch the metal cover (lose wire, chaffed insulation etc), if you were to touch that cover, standing on earth (same potential as neutral remember) then you would receive an electric shock that could kill you. To avoid this safety "Earth" is also distributed around your home, and despite being nominally the same (zero) voltage as the neutral conductor, is never set up to carry any load current under non fault conditions (that's what the neutral wire does). Now, if the live wire touches the metal case, there is a direct short circuit to earth, and therefore a fuse will blow, hopefully before you get electrocuted. Additionally, modern earth leakage circuit breakers monitor the current flowing out the earth wire at all times (instead of the neutral wire where it should be flowing) and if that exceeds a dangerous level (typically 20mA) that circuit breaker trips to interrupt the live supply. That system should help avoid most serious electrocutions by turning off the power before too much current can flow and electrocute you.
So in summary we have three conductors:
1) Live, carries the current to the loads
2) Neutral - returns the current from each load to the supply
3) Earth - carries no current during normal operation, but acts as a safety net to prevent un-intended voltages being created where they shouldn't.
Any metal object in your house should be earthed to prevent that object sitting at dangerous voltages due to a misfire, or a fault or in fact for any reason. For example, if you look at your central heating, you will find all the copper pipes are earthed to prevent them becoming a hazard!
Miniman and Max_Torque said:
Lots of good stuff
Thanks for that.I'd guessed that was probably how vehicles work, because I was sorting out a cooling fan problem on my Honda Deauville recently. One wire from the fan goes through the fuse and back to the battery, the other goes to a switch which connects directly to the radiator once it gets hot.
The domestic point was something I'd never understood, nor what the difference was between live and neutral on an AC system.
Edited by Dr Jekyll on Sunday 6th January 20:19
There are several different earth types in houses in the UK. This guide helps to explain them:
http://wiki.diyfaq.org.uk/index.php/Earthing_Types
http://wiki.diyfaq.org.uk/index.php/Earthing_Types
The gas company are replacing the old metal supply pipes in my street, replacing them with modern, presumably plastic, ones. At the moment, there is an electrical earth connection on the metal gas pipe between the meter and the house.
I presume this grounds the system through the gas pipe. If the gas supply becomes a plastic pipe, will the affect the earthing?
I presume this grounds the system through the gas pipe. If the gas supply becomes a plastic pipe, will the affect the earthing?
In old installations the gas and/or water pipe was often used to earth the supply and swapping for plastic could cause problems but in more modern wiring the earth connection is a separate thing and the reason the gas and water pipes are earthed is because they are exposed metal that could become live in the event of a fault and are connected to earth for safety reasons. Otherwise you could potentially end up with live radiators or live water coming out of the taps in the event of an electrical fault in your boiler.
, in a three phase supply from a delta transformer, each house is supplied from a different phase and if all were perfectly balanced, you actually get no neutral current 
One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
trick3000tt said:
In old installations the gas and/or water pipe was often used to earth the supply and swapping for plastic could cause problems but in more modern wiring the earth connection is a separate thing and the reason the gas and water pipes are earthed is because they are exposed metal that could become live in the event of a fault and are connected to earth for safety reasons. Otherwise you could potentially end up with live radiators or live water coming out of the taps in the event of an electrical fault in your boiler.
I think I had better get an electrician to check this out for me.
The house was built in 1840, so I think I can assume it is an 'old installation'!
Roofless Toothless said:
trick3000tt said:
In old installations the gas and/or water pipe was often used to earth the supply and swapping for plastic could cause problems but in more modern wiring the earth connection is a separate thing and the reason the gas and water pipes are earthed is because they are exposed metal that could become live in the event of a fault and are connected to earth for safety reasons. Otherwise you could potentially end up with live radiators or live water coming out of the taps in the event of an electrical fault in your boiler.
I think I had better get an electrician to check this out for me.
The house was built in 1840, so I think I can assume it is an 'old installation'!
Gary C said:
, in a three phase supply from a delta transformer, each house is supplied from a different phase and if all were perfectly balanced, you actually get no neutral current
One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
On a delta system there is no neutral - each house would have to have two phases as there would be no neutral present. delta is generally used for three phase motors or other balanced loads. Or you could supply a transformer that would give you a star connected supply.One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
trick3000tt said:
In old installations the gas and/or water pipe was often used to earth the supply and swapping for plastic could cause problems but in more modern wiring the earth connection is a separate thing and the reason the gas and water pipes are earthed is because they are exposed metal that could become live in the event of a fault and are connected to earth for safety reasons. Otherwise you could potentially end up with live radiators or live water coming out of the taps in the event of an electrical fault in your boiler.
One occasion when it worked to opposite way - I once got a mild shock from a radiator when lightning hit a pylon 50m away in the field behind my house. I guess the strike briefly raised the local earth voltage.ruggedscotty said:
Gary C said:
, in a three phase supply from a delta transformer, each house is supplied from a different phase and if all were perfectly balanced, you actually get no neutral current
One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
On a delta system there is no neutral - each house would have to have two phases as there would be no neutral present. delta is generally used for three phase motors or other balanced loads. Or you could supply a transformer that would give you a star connected supply.One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
Gary C said:
ruggedscotty said:
Gary C said:
, in a three phase supply from a delta transformer, each house is supplied from a different phase and if all were perfectly balanced, you actually get no neutral current
One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
On a delta system there is no neutral - each house would have to have two phases as there would be no neutral present. delta is generally used for three phase motors or other balanced loads. Or you could supply a transformer that would give you a star connected supply.One interesting thing about this, is if you measure the voltage between your live and your neighbours live, you are likely to get 415V
I had a delta transformer in Canada, one winding was centre tapped. this was grounded. I had 110-0-110v 220v 3ph and 208v all from one transformer.... had me as a british sparkie flummoxed for a bit lol
ruggedscotty said:
your a star !
I had a delta transformer in Canada, one winding was centre tapped. this was grounded. I had 110-0-110v 220v 3ph and 208v all from one transformer.... had me as a british sparkie flummoxed for a bit lol
I bet. I had a delta transformer in Canada, one winding was centre tapped. this was grounded. I had 110-0-110v 220v 3ph and 208v all from one transformer.... had me as a british sparkie flummoxed for a bit lol
Auto transformers always got me head scratching at college.
At work we have two 3300v to 121v transformers, now they can't half deliver current.
We also have our 415v supplies as three phase, neutral and Earth (TPNE). This is because the star/delta transformers straight off the generator have their neutral to earth through a resistor and transformer protection set to operate off the voltage the produces. This means their maybe a potential between actual earth and neutral, so our equipment is directly earthed and neutral is not connected to earth, but used to derive 240V supplies off TPNE boards.
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