T.T earthing *bonding*

[captaincargo]

Hi guys...im a new one to these parts and im hoping to pick your brains...
First off ...
Question...
It is made clear we should not use copper service pipes (water/gas) as a main connection to earth but when i came across an old installation the water pipe was used as the main bond to earth and i measured the Ze and it showed 5.5 ohms to earth, i then installed an earth electrode and measured in the region of 90ohms to earth (which in the regs is a pass).

Now ...when i install the main equipotential bonding by the book and say in the future there is a fault to earth....which route will the fault take?

The path of least resistance is a safe bet and this will surely be the water pipe so why cant we just use it in the first place?

 

[bernardgreen]

The problem with using the water pipe as an earth is that the water company come along some time in the future and replace the metal pipe with plastic and thus effectively disconnect the earth from ground.

 

[Spark123]

Yeah, a majority of it will take the path of least resistance, but that path is not guarenteed to not be replaced with plastic next week.

Bah, too slow. At least we are both singing from the same sheet!

 

[Taylortwocities]

Because the utility companies say you can't!

Please do not confuse the main earth (your electrode) with main earth bonding. The process of connecting the gas, water and the earth electrode together is to ensure that they are at the same potential.

Yes, if there is an earth fault the path will be the path of least resistance (multiple parallel paths) but there's no danger if all services are connected together. In a TT system the maximum earth fault current will be a nominal 100mA for not very long because that is when the RCD (incomer) will trip.

Edit, damn this slow keyboard!

 

[captaincargo]

Superb response...thanks fellas

 

[brookwood]

the main reason being the water pipe isnt yours so you cant legally use it, and as others suggest rely on it.

 

[securespark]

Yes, as others have said, the current would take the path of least resistance - I've had a rod at 1900 odd before now, but the ZS was 0.98 because connections to gas & water are fantastically good.

In the above case, of course the fault current would travel down the service pipework. But that is why it is absolutely imperative to have an RCD incomer on a TT supply.

Using the water pipe as an earth electrode was outlawed in the 14th edition, 1966.

It has NEVER been allowable to use the gas pipe as an earth electrode.

 

[captaincargo]

Cheers again Guys...

Secure...
As its a MUST to install a 100mA RCD to protect the whole installation is the method for protection then not taken as EEBADS anymore?
Or is it still EEBADS due to the RCD still causing the automatic disconection of supply?

 

[Spark123]

It is still EEBADS, earthed equipotential bonding and automatic disconnection of supply. The protection against indirect contact is being provided by an RCD which is the preferred option for a TT system in the regs where RAI/\n < 50v for a normal location. (RA is the sum of the resistance of the electrode and the protective conductor connecting it to the exposed conductive parts, I/\n is RCD operating current) Do you have a copy of the regs? See p49 of the brown cover version.

 

[securespark]

Gottle of Geer..... I said all that without moving my fingers on the keyboard....

 

[ricicle]

Think of the poor sod from the water board.Comes to replace/maintain the water pipe outside your property, has it disconnected near to your property, poor conduction to earth on the bit still in the ground.You get an earth fault in your property and he cops one!!!!