Earthing

[Napa2010]

This will sound completely stupid, but i'm studying Electrical and Mechanical Engineering at college and i'm grasping everything in both the Electrical and Mechanical sides pretty quickly, apart from one thing, I understand how to wire the earth to different components, I just don't see why, it's just an extra wire to me, as I said i'm picking everything up pretty quickly but it's just this that I keep questioning, and please, tell me if i'm being stupid and missing a veryy easy point which I know is probably the case.



Thanks for all replies in advance,
Joe.

 

[flameport]

Earthing example:
Metal case of washing machine not connected to earth. Fault inside means live wire touches metal case, case becomes live, person touches it and dies from electrocution.
If the metal case is earthed as it should be, a large current (100s of amps) flows via the earth wire, the fuse/circuit breaker trips and the danger is removed. This will also alert the owner that something is wrong with the washing machine.

Protective Bonding:
Connecting extraneous conductive parts to the main earth terminal in the building. (ECPs are conductive items which enter the building from outside, and typically are connected to earth such as metal water pipes).
This ensures the pipe is at the same potential as the main electrical earth for the building, and it has a low resistance path to earth.
This means that if (example) a live wire touched a water pipe, the voltage on the exposed pipe is very low, reducing or eliminating the risk of electric shock. In most cases, it would also cause the circuit breaker to trip.
Without this, the pipe could remain live, as the current might not be enough to trip the circuit breaker. Anyone touching the pipe would get an electric shock.

 

[ricicle]

Protective Bonding:
Connecting extraneous conductive parts to the main earth terminal in the building. (ECPs are conductive items which enter the building from outside, and typically are connected to earth such as metal water pipes).
This ensures the pipe is at the same potential as the main electrical earth for the building, and it has a low resistance path to earth.
This means that if (example) a live wire touched a water pipe, the voltage on the exposed pipe is very low, reducing or eliminating the risk of electric shock. In most cases, it would also cause the circuit breaker to trip.
Without this, the pipe could remain live, as the current might not be enough to trip the circuit breaker. Anyone touching the pipe would get an electric shock.

Not strictly true. Main protective bonding is to ensure that all extraneous metal work entering the building rises to the same potential as the earthing system in the building in the event of a line to earth fault on any equipment. Thus anyone simultaneously touching an exposed conductive part and extraneous conductive part under fault conditions will not receive a significant shock.

 

[Napa2010]

Thanks guy's, I get the Earthing, just not the Bonding.

 

[ricicle]

In a nutshell - earthing limits duration of touch voltages, bonding limits the value of touch voltages

 

[ericmark]

If there is a fault to earth then the earth rod or other item used to earth would raise in voltage in respect to true earth. So long as all metal items are raised to same voltage one will not get a shock so all metal is bonded together.

 

[NotHimAgain]

To those who believe that bonding creates an equipotential zone please look at the definitions in the 17th Edition - you will find that the term 'Earthed Equipotential Zone' has been omitted - why - because there is no such thing in an electrical installation under fault conditions.

Bonding acts to reduce touch voltage by eliminating the voltage dropped across the impedance of a portion of the earth fault loop path that is outside the installation - namely, the supply cable earth conductor in TN-S, or the neutral in TN-C-S.

It is only completely effective if all extraneous-conductive-parts are included. This would require that 'earthy' floors and the like are bonded. This could be achieved by grids under the building or, less effectively, by a radial conductor buried in a trench around the perimeter.

The reason that a 0.4 second disconnection time is used in a TN system is that it provides a reasonable chance of survival even if the bonding is not effective. This is also the reason why 0.2 of a second is now used for TT installations. The lower limit being necessary as the touch voltage is now that dropped across the installation earth electrode - rather than the supply earth or neutral conductors - this voltage approaches the supply voltage whereas in TN systems it is nearer half the supply voltage.

You will note that the term earthed equipotential bonding has also been dropped from the protection acronym EEBAD (or EEBADOS). This is now simply ADS - this is not an accident.

Now will bonding survive in the 18th .

 

[Napa2010]

Thanks guys, I had to do a three plate lighting system today and I nailed it, which means i did the earthing right, which means I understood it

Thanks again.
Joe.