TT supply with TNS sub mains

[jmtisme47]

Would like your opinions on the following setup. 300A 3 phase TT incoming supply feeding 15 units with 50A TNS obviously some link somewhere between Neutral and earth but haven't found it yet. I would think that each unit should be TT not TNS thoughts please

 

[westward10]

You have a TT supply and TN-S distribution circuits???

 

[davesparks]

Why do you think the units have a TNS supply? Are you sure you have a TT supply and not a TNS supply with the electrode at the intake?
TT or TNS the supply neutral is connected to earth at the substation, its how the installation is connected to that earth which differs.
If the incoming supply is TT then the whole installation must be, you can't then have a DNO supplied earth appear in the middle of the installation?

 

[jmtisme47]

You have a TT supply and TN-S distribution circuits???

Yeah but not linked at the origin of supply

 

[Strima]

So are the TN-S just distribution circuits? Sounds like all 15 units are relying upon one electrode for earthing.

 

[jmtisme47]

Why do you think the units have a TNS supply? Are you sure you have a TT supply and not a TNS supply with the electrode at the intake?
TT or TNS the supply neutral is connected to earth at the substation, its how the installation is connected to that earth which differs.
If the incoming supply is TT then the whole installation must be, you can't then have a DNO supplied earth appear in the middle of the installation?

I should have pointed out that I am in Africa the whole city is TT it appears that its a common practice to illegally connect neutral and earth in the strangest of places. I have previously found 2.5mm CSA linked between Neutral and earth in a 100A 3 phase DB.

 

[westward10]

Yeah but not linked at the origin of supply

That isn't possible. TN-S would would be derived at the origin. A link between a neutral and earth through the installation will not be TN-S or TN-C-S.

 

[westward10]

This is a UK based forum so it is going to be unlikely we can give you a definitive answer as to whether the set-up contravenes the requirements in place in Africa.

 

[jmtisme47]

That isn't possible. TN-S would would be derived at the origin. A link between a neutral and earth through the installation will not be TN-S or TN-C-S.

What is supplied at the source is TT all units have 5C coming in Ze in the units are between 0.15 and 0.21 Ra at the rod is 3.14 ohms there is a link somewhere turning the TT to TN

 

[westward10]

Is this permitted in Africa?

 

[davesparks]

What is supplied at the source is TT all units have 5C coming in Ze in the units are between 0.15 and 0.21 Ra at the rod is 3.14 ohms there is a link somewhere turning the TT to TN

I'm not seeing what you think a neutral - earth link has been made, Zs can be less than Ra and usually is.

 

[MJPD29]

Sounds like someone is doing some top quality work here

 

[echase]

I should have pointed out that I am in Africa the whole city is TT it appears that its a common practice to illegally connect neutral and earth in the strangest of places. I have previously found 2.5mm CSA linked between Neutral and earth in a 100A 3 phase DB.

Bizarre things happen in Africa. In South Sudan where I have worked the DNO has a good quality new TT system. Yet the DNO often comes along and cuts your wire to your earth rod leaving you no earthing at all. No explanation for this practice is given.

I had one site where the live was accidentally connected to E. Used up loads of electricity and made the earth go to enough volts to give a shock but did not trip the breaker as rod had too high a resistance.

Regarding your measurement of 3.15 ohm for the rod, if a house gets struck by lightning the whole site’s earth wires could be raised to thousands of volts. Except that if you do have a E to N link it’s going to reduce that voltage as some of the lightning surge current flows to the substation. See my post today about Zambia.