earth bonding

[iamelectric]

can someone explain earth bonding to me , i get that pipes and metal work has to be earthed incase a fault makes them live but what does " a conductive part such as a metal pipe, liable to introduce earth potential into the building ." mean and if pipe work jumps from plastic to copper all over a building does every bit of copper pipe have to be earthed

 

[telectrix]

it's not so simple. basically, any metalwork entering the building will be at earth potential. i.e. 0 volts. now , if an exposed conductive part , say the metal case of an appliance, were to become live due to a fault , and you were in contact with that live part and at the same time, in contact with metal pipework, you would possibly receive a fatal shock. the pipework is bonded to earth terminal and so it's potential will rise to the supply voltage for the duration of the fault ( i.e. until the fuse/MCB trips ). that is also why we strive to ensure that the disconnection time is ( generally, on most circuit) <0.4 secs.

 

[iamelectric]

thanks , 1 more question if the incoming water supply is plastic and it jumps to copper for some parts is it best to leave it unearthed?

 

[44ALLAN]

And earthing and bonding are two different things

 

[telectrix]

thank s , 1 more question if the incoming water supply is plastic and it jumps to copper for some parts is it best to leave it unearthed?

IR test from MET to the copper pipe. if it reads <22KΩ , then bond it. if it's >22KΩ , then don't.

 

[iamelectric]

thanks but what does this mean and why , sorry for all the questions but theres no point knowing this if i dont understand it

 

[telectrix]

ohms law. if the resistance to earth is 23kΩ, the the maximum current through your body is 10mA, which is not likely to be fatal ( 230V/23000Ω). (sorry, i used 22k earlier, instead of 23k)

 

[Engineer54]

Do you not bother to ask your college lecturer(s) the things your covering at the time, but don't fully understand??

 

[telectrix]

half the time, the lectrurers won't understand, let alone be able to explain.

 

[brucelee]

lol yeah and half the time they cant be bothered to answer as they jsut follow handouts

My whole lv3 was mainly done by them printing a hand out and then reading through that as far as they were concerned that was the subject covered

Nothing like the old apprenticeships were you had to do related electrical sciences, Physics, Maths etc etc and loads of practical

now its jsut a print out that is very basic with very little practical assessments

I found it easier to teach myself by printing all the lv3 of in 1 go and study at home with related electrical books
These college courses arent all they are cracked up to be mate nowhere near as good as the old ones and nowhere near as in depth

 

[iamelectric]

yes telectrix is correct i am currently doing the 2394 course and the teacher is not that great one time i asked him about N & E combined as i couldnt get my head around it and his answer was neither could he .

 

[brucelee]

its combined together in the cut out using a link between the earth terminal and neutral and from there all the way back to the star point at the supply transformer i believe using concentric cable which is basically a Line conductor surrounded by multiple combined protective earthing and Neutral conductors and there are also multiple Earth rods at intervals back to the transformer (PME) protective multiple earthing seperate in the consumer side and combined in supply side Ie TN-C-S Terre , Neutral- combined - seperate

but others such as engineer may be able to explain better

 

[IQ Electrical]

ohms law. if the resistance to earth is 23kΩ, the the maximum current through your body is 10mA, which is not likely to be fatal ( 230V/23000Ω). (sorry, i used 22k earlier, instead of 23k)

You were right with the 22kΩ Tel-there's another 1000Ω allowed in the equation for the resistance of a human body

 

[SirKit Breaker]

half the time, the lectrurers won't understand, let alone be able to explain.

I agree, but i can and do. Any student on any of my courses can ask me what they want as many times as they want, and i will explain until the penny drops:sunny:

 

[SirKit Breaker]

yes telectrix is correct i am currently doing the 2394 course and the teacher is not that great one time i asked him about N & E combined as i couldnt get my head around it and his answer was neither could he .

Its easy. TNS, Earth Neutral Separated. TNCS, Earth Neutral Combined and Seperated.

The first thing that you need to know and understand, is that the Neutral and Earth are NEVER combined in the installation and it is not allowed. If you look in the OSG or GN3 you will see from the illustrations that TT, TNS and TNCS are identical on the consumer side of the illustration.

Therefore when we refer to Neutral and Earth combined it is on the SUPPLY side only.

TNS, this system provides a two core supply cable to the cutout fuse (Supply head), and the main earthing conductor is connected to the metal sheath of the supply cable, so you have 3 seperate connections back at the supply transformer, Line Neutral and Earth.

TNCS, this system provides a split concentric cable (it looks like a single core SWA cable if you cut through it..dont!) to the supply head, The core is used as the Line, the steel armouring is connected to the Neutral at the supply head, and the main earthing conductor goes into and connects to the Neutral in the supply head from either an adjacent earth block or the earth bar in the CU. Therefore there is only one combined connection for the Earth and Neutral from the supply head to the supply transformer. In some cases the distributor will fit additional earth rods to the split concentric cable to ensure that the Ze stays in spec. TNCS is sometimes referred to as "PME" protected multiple earthing.

Cheers..............Howard