Earth Fault Loop fot TT

[orbital]

Could someone please explain the Earth Fault Loop for a TT system.

Many Thanks

 

[PC Electrics]

That question is very wide open, Orb! We could write a book to answer it as put, so could you be more specific about what areas are causing confusion or where you'd like help. It'd help us to help you if you could.

 

[spinlondon]

From the Transformer, to the installation CU/DB along the line conductor to the fault.

Then back down the CPC to the MET, along the earth conductor to the earth electrode, through the ground/terra to the transformer earth electrode, then to the star point on the transformer.

It's basically the same as with TN-S or TN-C-S, except that the ground/terra becomes part of the path.

 

[orbital]

thanks.

Thats what I thought, but I find it hard to grasp he idea that the fault current could travel through the ground back to the transformer.

I put my first rod in the other day and got a good reading of about 24 ohms. So that distance to the transformer, which I guess could be 100's of meters away only has a resistance of a few ohms, but a millimeter of PVC has a resistance of millions of ohms. Makes my mind boggle.

Does the fault current not get confused by other metallic things under ground

 

[m4tty]

I done a tt at my dads house and the ra was 30ohms but when bonding connected the zdb was 0.30 ohms so all the fault current must travel through water/gas pipes through to next doors tns/tncs and back to the supply transformer that way rather than through the earth as it's much lower resistance.

If someone was standing bare foot on the ground when fault current was coming past would they get a shock?

 

[Andy™]

thanks.Thats what I thought, but I find it hard to grasp he idea that the fault current could travel through the ground back to the transformer.

I put my first rod in the other day and got a good reading of about 24 ohms. So that distance to the transformer, which I guess could be 100's of meters away only has a resistance of a few ohms, but a millimeter of PVC has a resistance of millions of ohms. Makes my mind boggle.

Does the fault current not get confused by other metallic things under ground

soil (/earth/concrete etc) is a conductor. not an insulator. obviously not as good at conducting as copper. which is why you get high readings (compared to TN)

and yes, it can get 'confused' by metallic things under ground. i.e it may travel through a metallic service pipe to another property with TN, and take its route back to star that way, along with some that flows through the ground all the way back to substation

 

[batty]

I think some people may be getting confused here the current will never flow the same as in a TN systems that is why you have an RCD to break the fault and disconnect the circuit.

 

[monkey5]

do you measure the ze the same way on a TT, use the earth cable attached to the rod?

ive got a reading of 184omhs for ze and 74.4ohms for zs on socket circuit.

all rcd protected and bonding to gas and water fine.

have suggested to install an xtra rod to try and bring reading down, have i done right?

cheers wayne

 

[Andy™]

depending on location of rod, its often best to test it as soon as its hit into the ground, incase reading is too high and needs moved

at 184 ohms, it complies, and with Zs being 75, i wouldnt bother trying to get it lower

 

[batty]

do you measure the ze the same way on a TT, use the earth cable attached to the rod?ive got a reading of 184omhs for ze and 74.4ohms for zs on socket circuit.

all rcd protected and bonding to gas and water fine.

have suggested to install an xtra rod to try and bring reading down, have i done right?

cheers wayne

184ohms Is below the 220ohms so is ok. A lot depends on what ground is made off. If you are getting zs of 74.4ohms that is probably through one of the service pipes. I would be suprised if you could bring it down much more with another rod.

 

[monkey5]

[quote name='Andy

 

[spinlondon]

BS7671 suggests that anything over 100ohms could be unstable.

 

[m4tty]

BS7671 suggests that anything over 100ohms could be unstable.

I thought that it was 200 ohms and the 100 ohms was one of nic's special rules. Or is this another myth lol

 

[Chr!s]

I thought that it was 200 and the 100 was one of nic's special rules. Or is this another myth lol

Depends which standard your working to, spin probably meant BS 7430 which states 100 ohms

Ive heard the 100 ohms will be introduced into BS 7671

 

[Andy™]

I thought that it was 200 ohms and the 100 ohms was one of nic's special rules. Or is this another myth lol

7671 does say 200. NIC says 100, but then NIC dont make the rules

 

[Sidewinder]

Not checked (yet) but as has been suggested it may be that 7430 requires 100 ohms.

You'd have a job justifying not complying really IF it came to SERIOUS issues!

 

[steptoe]

or you could just prove it comes under 1667, and still be legal.

just prove it is safe.!

that is what the law requires,

everything else is just an aside to help with your proof.