Meter tails and main earth

[Pondy]

Hi guys,
Been to a one bedroom retirement bungalow today, supply is tncs into a cupboard just outside the front door, it goes into a 63a isolater then it goes from there into the roof void and into the consumer unit which is approx 6 meters from the isolater. The board is supplied via 16mm t+e with a 6mm (or perhaps 4mm) main earth. The ze was .50 which is obviously too high, I have now installed a 16mm main earth and the ze is now .38 which I am aware is still a fraction too high. My question to you all, is I am right in saying that the old earth was too small as I'm sure that it should be at least half the size of the meter tails.
They had another more experienced electrician do a pir a couple of weeks ago and he didn't mention it.

Im sure I'm correct in what I'm thinking, but you know what it's like, doubt starts to kick in and I would love to hear what you think.

 

[Benjamiin]

Me thinks you be right! I always run a 10mm earth with 16mm t&e and then use the earth in the twin too! Hope this helps

 

[SPARTYKUS]

Hi Pondy,

what was the Ze at source?

 

[paul_h]

As above, I'd imagine the ze at source to be highish..... Plus another 6meters, tipping it over.

id agree that the 4mm was easily too low, 10mm minimum IMO....

 

[Pondy]

Hi Pondy,

what was the Ze at source?

Hi,
didnt take a reading, wish I had, and unlikely to be going back there in the near future, what a ---.

 

[telectrix]

don't be sad. ---- are useful.

 

[Pondy]

So is your opinion!

 

[7029 dave]

At the main supply in the z value should be .5 max (ug feed)

 

[SPARTYKUS]

It's all academic anyway when you look at the tolerances of meters. Easy to loose a few thousanths of an Ohm

 

[Captain Custard]

TNCS minimum Ze is 0.35ohms. If it's outside this you need to inform the supply company to investigate.
TNCS requires a minimum of 25mm tails and 16mm earth conductor.

 

[jamesBoater]

Surely the adiabatic equation will tell you if you have a sufficient sized Earthing conductor.

 

[Clive P]

Hi,

Sorry Captain but I disagree with both your statements.

Regards.

 

[jamesBoater]

He must mean max Ze 0.35. As for 25mm Tails, there must be thousands of PME systems out there on 16mm tails and 10mm Earth.

 

[alanl]

Right, you've got a Zdb of 0.38.
That equates, (roughly, depending on actual voltage) as a PFC of 460A
What is the cut-out fuse size?
Is it a 63A as you stated, or is that an isolator?
So, assuming it is a 63A BS88-3, it will trip in one second - is this a homework question, as 460A is right there on page 296.
Do the adiabatic, and you get a 4mm earth conductor.
But then you need to cross-reference to 544.1.1 which states that a 10mm earth conductor is the minimum for a TN-C-S (PME) system.
So 10mm is required.
However, is the main earthing terminal in the CU, or at the isolator?

 

[Octopus]

TNCS minimum Ze is 0.35ohms. If it's outside this you need to inform the supply company to investigate.
TNCS requires a minimum of 25mm tails and 16mm earth conductor.

On new installs and rewires yes. existing ??