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 ??

 

[Octopus]

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.

he's been looking at the picky in the OSG!

 

[Pondy]

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?

Hi Alan,
thanks for the reply, the isolater is one of the old fashioned ones with the arm which only allows you to open the door when you have turned it off, and yes inside it has a bs88 63a fuse. The main earthing terminal is at the isolater. Why, what you thinking? It's a real install by the way, not a homework question. Thanks again.

 

[D Skelton]

TNCS requires a minimum of 25mm tails and 16mm earth conductor.

Absolute nonsense

 

[Des 56]

Hi Alan,
thanks for the reply, the isolater is one of the old fashioned ones with the arm which only allows you to open the door when you have turned it off, and yes inside it has a bs88 63a fuse. The main earthing terminal is at the isolater. Why, what you thinking? It's a real install by the way, not a homework question. Thanks again.

Interlock Bill was his name

 

[Engineer54]

Absolute nonsense

And i Absolutely agree with you!! lol!!
Yet another that thinks, or takes a picture in an OSG is a rule set in stone rather than just for indicative purposes!!

 

[alanl]

If the MET is at the isolator, and the earthing conductor, and the main protective bonding are up to current standards, connected at the MET, (10mm + according to tails etc), then (without doing the calcs), the 16mm T+E will be fine supplying the CU, as that will be a sub-main.
Work out if the CPC of the 16mm T+E is enough by testing the PEFC and PSCC, then doing the adiabatic.
From the info given, I would say yes, but check yourself.
Then you have the problem of the higher Zs at the DB.
6 metres of 16mm T+E cable will add around 0.05 ohms to the Zs, so there is either a bad connection somewhere, or the incoming supply has a high Ze, which is not uncommon, especially if the supply is at the end of its run in a road etc.

 

[Pondy]

If the MET is at the isolator, and the earthing conductor, and the main protective bonding are up to current standards, connected at the MET, (10mm + according to tails etc), then (without doing the calcs), the 16mm T+E will be fine supplying the CU, as that will be a sub-main.
Work out if the CPC of the 16mm T+E is enough by testing the PEFC and PSCC, then doing the adiabatic.
From the info given, I would say yes, but check yourself.
Then you have the problem of the higher Zs at the DB.
6 metres of 16mm T+E cable will add around 0.05 ohms to the Zs, so there is either a bad connection somewhere, or the incoming supply has a high Ze, which is not uncommon, especially if the supply is at the end of its run in a road etc.

ive looked at a couple of these and they all seem to have highish ze, it is a rural location and I'm guessing the transformer is quite a long way away, my main concern was that I was right to say that the main earth needs upgrading. Thanks to all that have replied, it's nice to get other people's opinion, everyday is a learning day.

 

[Buzz-light-bulb]

Am I right in saying aslong as your ze at the origin of the installation ( before the isolator) is less than 0.35 ohms.
The Ze at the sub board should not be greater than the max zs of the 63a fuse at the fused isolator.

 

[jamesBoater]

From the sub board you would be measuring Zdb as Ze is origin only, the Zdb would need to be low enough for max disconnection times of the 63amp fuse to be met , 5 seconds as its a distribution circuit.

 

[Flanders]

From the sub board you would be measuring Zdb as Ze is origin only, the Zdb would need to be low enough for max disconnection times of the 63amp fuse to be met , 5 seconds as its a distribution circuit.

I agree the Ze is the external earth loop reading so should be taken at the origin in this case the main isolator, the reading at the CU will be the Zs.

 

[SPARTYKUS]

I suspect the "63a isolator" is not an overload or short circuit device, maybe a 63A RCCB?

in which case the ZDb does not need to match this value, it just, as has been stated by a n other, needs to be low enough for subsequent PDs to operate in time.

 

[Buzz-light-bulb]

If its got an isolator between the db and the primary fuse wouldn't it be classed as a submain, therefore your zdb would not be classed as your ze because your ze has to be less than 0.35, whereas your zdb would have to be less than the max zs of the 63a fuse in the isolator, that's how I see it.

 

[jamesBoater]

Pretty sure the op mentioned the isolator has a BS88 63A inside.

 

[Buzz-light-bulb]

Pretty sure the op mentioned the isolator has a BS88 63A inside.

???? This would mean what exactly

 

[jamesBoater]

In relation to SPARTYKUS post27

 

[SPARTYKUS]

Ahh yes OP did comfirm later that there is a 63 Amp fuse in a container. Damn this Stella.