High ze

[davesparks]

If you look at these pages from the ukpn network design guide you can see that 0.35 is a typical value for an existing 100A tncs supply.
0.35 is their maximum value for a new 100A service connected to an existing network but for a new service from a new network it is 0.25.

Other DNOs will provide an 80A service with a higher Ze.

 

[HandySparks]

...but on a tnc-s system my ze was 69 ohms ... his reply was its what we do until we install a new supply cable...

Hang on there a minute. If the fault is in the DNO's cable then, since this is TN-C-S, the line impedance will also be 69 Ohms. A line impedance of 69 Ohms won't let you pull more than a few amps out of the supply before you run out of volts. So maybe the fault is just a loose connection in the head between the earthing conductor and the neutral terminal.

Did you measure the line impedance?

 

[Farmelectrics]

If I'm honest hands park you've lost me a little there could you explain in more detail don't quite get what you mean lol

 

[Farmelectrics]

Sorry handy spark it ment to say

 

[malcolmsanford]

Handysparks is relating to the Neutral and Earth are tied together and at the header the reading for Line to Neutral and Line to Earth should be the same.

I think he is asking are you geting the same on the Line to Neutral

 

[Richard Burns]

Where is the 69Ω resistance generated?
If it is in the supply cable on TNCS then the RCD will only protect against an earth fault and a short circuit will go unnoticed (but would also be fairly irrelevant as only 3A will flow, but might be nice to know!).
If the fault is with the earth connection to the neutral conductor then the line to neutral result will be low and the fault will not be with the supply cable.

 

[LankyWill]

Quoting that document is all well and good dave but as electricians we dont follow the DNOs guidelines on MAX Ze we Follow the bs7671 guidelines on max Ze readings. So as suggested by myself and others it does not comply with the max 0.35 ohm reading. I also agree with Lee if it was slightly over +0.5 on either TNS or TNC-S it wouldnt be a big deal, but it is marginally over that reading.

 

[Deleted member 9648]

Quoting that document is all well and good dave but as electricians we dont follow the DNOs guidelines on MAX Ze we Follow the bs7671 guidelines on max Ze readings. So as suggested by myself and others it does not comply with the max 0.35 ohm reading. I also agree with Lee if it was slightly over +0.5 on either TNS or TNC-S it wouldnt be a big deal, but it is marginally over that reading.

A rather contradictory post, you cant have it both ways. If you believe in following the max figure suggested in bs7671 you cant be accepting a slightly over reading either.

 

[LankyWill]

A rather contradictory post, you cant have it both ways. If you believe in following the max figure suggested in bs7671 you cant be accepting a slightly over reading either.

I see how it looks but at least im governing myself to the BS 7671. Daves saying just because the DNO allow it he can follow their guidelines willy nilly

 

[HandySparks]

Handysparks is relating to the Neutral and Earth are tied together and at the header the reading for Line to Neutral and Line to Earth should be the same.

I think he is asking are you geting the same on the Line to Neutral

Where is the 69Ω resistance generated?
If it is in the supply cable on TNCS then the RCD will only protect against an earth fault and a short circuit will go unnoticed (but would also be fairly irrelevant as only 3A will flow, but might be nice to know!).
If the fault is with the earth connection to the neutral conductor then the line to neutral result will be low and the fault will not be with the supply cable.
View attachment 31293

Exactly chaps.

If this is really a TN-C-S supply and the house is still functioning normally, ie a kettle still boils, lights not dimming, etc., then the fault isn't in the supply cable.

farmelectrics: How do you know it's TN-C-S? Do you have a photo of the head, labelling, etc?

 

[davesparks]

Quoting that document is all well and good dave but as electricians we dont follow the DNOs guidelines on MAX Ze we Follow the bs7671 guidelines on max Ze readings. So as suggested by myself and others it does not comply with the max 0.35 ohm reading. I also agree with Lee if it was slightly over +0.5 on either TNS or TNC-S it wouldnt be a big deal, but it is marginally over that reading.

Bs7671 doesn't specify max Ze readings, it just mentions contacting the dno to find out iirc.
It is obviously a fault, but the 0.35 is a nominal value not a maximum by anyone's rules for an existing supply

 

[Farmelectrics]

Sorry chaps been busy the weekend not been back. Yes it is a tnc s and the reading between live and earth was 69ohms. The house I done was ok because of the dual rcd board but in the head inside was a looped cable supplying next door were I've been told they had a wylex rewirable fuse board Sunday morning the dno turned up with a digger ready to install a new cable

 

[davesparks]

Sorry chaps been busy the weekend not been back. Yes it is a tnc s and the reading between live and earth was 69ohms. The house I done was ok because of the dual rcd board but in the head inside was a looped cable supplying next door were I've been told they had a wylex rewirable fuse board Sunday morning the dno turned up with a digger ready to install a new cable

The point being made is that a in a TNCS the live-earth loop is the live-neutral loop so at 69ohms the installation must have been experiencing some serious problems

 

[Farmelectrics]

Well no one has reported any Dave. What serious problems are you suggesting

 

[ImpededLoop]

Well no one has reported any Dave. What serious problems are you suggesting

Bearing in mind the supply will be constructed of relatively large conductors, why do you think that an impedance as large as 69 ohms has been measured?

 

[Farmelectrics]

Because there's a fault hence why the dno replaced the supply cable Sunday

 

[davesparks]

Well no one has reported any Dave. What serious problems are you suggesting

Seriously? The voltage drop on a 69ohm supply will be massive!
The lights would be pretty dim!

 

[Farmelectrics]

Ok that's a fair example Dave I'm not aware of any such complaints though and it has been renewed so should all be ok

 

[davesparks]

Ok that's a fair example Dave I'm not aware of any such complaints though and it has been renewed so should all be ok

Which makes it pretty hard to believe!
The kettle must have made a pretty poor showing too!
If a 3KW 240V kettle has an impedance of 19ohms and the supply is 69ohms that gives you an 88ohm series circuit. The current flow would then be 240/88=2.7A with the PD across the kettle being around 50volts. That's going to take forever to boil!

 

[HandySparks]

I'm betting it was TN-S. (May not be the same now.)