Max Zs

[matt.leung]

what is the max Ze for a TNCS system, as most of the earthing systems that i come across are TNS.

many thanks

 

[Zeespark]

0.35 hms   maximum.

Andy Guinness

 

[matt.leung]

cos the measurement i took today was 0.88 hms   (TNCS)  with 16mm G/Y. The MET is only 300m away from the main fuse so no long run. all of the connections are tight as well....

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[betty swollocks]

300m away??? :-/

 

[batty]

That's a long way in my book.

 

[Elmer BeFuddled]

That's a long way in my book.

Can you hear me Mother?

 

[apprentice87]

The impedance of the 16mm on its own would be .345 so what do you expect.... Besides, it would not be Ze you were measuring anyway, as Ze is external to the installation, which 300m of earthing conductor most certainly is not.... add R1 + the .345 + the Ze, and you will get figures about like those you are measuring

john

 

[alpha spark]

John I think he means 300mm

 

[steptoe]

there is NO max Ze for TNCS

0.35 is merely an assumed max figure that calculations are based on,

only how I understand it, although there may be something in ESQCR to make me wrong, theres usually something, somewhere.

 

[batty]

Dno are not supposed to give TNCS earth if its over .35 ohm but I have seen it a few times but generally as a rule of thumb its a lot less I find.

 

[Sidewinder]

The 0.35 figure is not set in stone.

In rural areas you may get higher.

However, the DNO need to ensure that their system complies with ESQCR & their rules.

With a value in excess of 0.35 for even a 60A supply you could be approaching a voltage rise of 20+ volts, extrapolate this up and you'll begin to see the problems, and that is without the associated volt drop that you would experience on the supply side.

 

[tom1]

I would just  ring dno and let them deal with it. Had them out a few times for high ze, they come out and fix it. 

 

[Sidewinder]

Thing is Tom, how high is high?...

This is the question that needs answering...

OK, yes say 90+ Ohms on a TN-C-S is too high, where do we draw the line with the two systems?

Discuss?...

 

[tom1]

up to now if its above 0.35 tncs

0.8 tns then i have rang them.

The lowest  i got was 0.40 on a tncs, which was out in the sticks but i rang them anyway, they sent someon out to test it, they went up the pole and said its fine. which is what i thought they would do.

The highest i got was about 12ohm on a tns. for this they came out made a few phone calls temp tncs connection, i got all my work done while they decided wether or not to start digging the road up, which i dont think they did in the end.

if its above the stated values then i ring them, it is theres and its up to them what they do about it.

 

[wozz]

I've been to a site a few times with a 0.40 Ze and therefore a 0.40 l-n loop. And the voltage problems are horrendous unfortunately ukpn are not interested as one of the engineers said to solve it would be a horrendous amount of money so they will keep not being interested.

Another engineer said ' what do you expect with a house that size on the end of those lines!'

They report back voltage is fine on lines and its customers problem job gets closed. Fed up of arguing.

 

[batty]

I have had DNO out twice recently both TNS systems one overhead and the other underground. One Ze was 35 ohm and the other 15 ohms. Both had been fine but over a few months had failed. One the voltage was 210 to 216 volts so this really needed sorting. They came out both times and sorted it and this was UKPN both times. If an earth is provided it has to be maintained.  

 

[matt.leung]

yes i meant 300mm away...

 

[apprentice87]

I had a new install with exactly the same problem, the DNO numpty's had forgotten to do the screws up in the cutout... My Ze was something like .52 on one phase, and after they did the screws up it dropped to .28.

Get the DNO out, they might claim that "there is no max figure for Ze" but there is for voltdrop though, and as it is TNCS so you are effectively measuring L/N impedance, and so you will find that with the figures you gave, you will only be able to draw about 16 A and the voltage will drop below that allowed by the ESQCR regs..

That will prod them into action...

john