Supply Characteristics Advise needed

[s1m0n]

Hi,
I am currently installing a ring final in a domestic house for the Kitchen, got past the building controlpart no problem,
The advise I need is on the Characteristics of the Supply for the Certificate.,

I'm not a complete novice, but at the stage of working in the real world after 3 years at college.

My problem is, the supply is a TNS earthing system ( main earthing cable going to the sheath of the incoming supply) which is an underground supply.
But .... there is another earth cable going to an earth rod just the otherside of the exterior wall, about 2 feet away.

what do I put down for the supply TNS and make a note of the additional earth rod.

I have taken Ze measurement on both in islation,
as TNS 64.9 ohms, TT 501 ohms,

all advise welcome, trying to go by the book, but seems to be different on each ocasion

thanks simon

 

[bugsy]

70 ohms for a TN-S supply is not right , is your test gear recently calibrated?
If it is correct contact DNO spike has proberbly been installed to supplement this - contact your DNO
State supply as TN-S

 

[electro]

[FONT=Times New Roman, serif]Hi[/FONT]

These figures are very high and the installation is not suitable to be upgrading with an additional ring main, until the earthing is sorted out.

regs figures

TNS 0.8 [FONT=Times New Roman, serif]Ω[/FONT]
TT 200 [FONT=Times New Roman, serif]Ω[/FONT] max (should be as low as possible)

 

[s1m0n]

Thanks for the replies,

The owner only bought the house in Noveber so no history to go by.

test equipment less than a year old, Kewtech range.

out of curiosity I took a Ze reading with all the parralel paths in place

using 3 lead method, main MET, and incoming Phase and Nuetral,

and got a reading of 9.94 ohms

also too a Zs reading, changed a Light accessory, and got 4 ohms (well within acceptability)
there is a 80A 30mA RCD between supply and Consumer Unit,

Do you think this was put in by the DNO,
I was thinking of contacting them to get advise.

Thanks.

 

[steamboatwilly]

Something's wrong.

Ze doesn't count with parallel paths (in theory). Definitely check with the DNO and see if they have any records for the address/installation. If they record it as TN then report the problem to them. If they have advised a TT rod to back the TN up then they should have a record of it.

Besides, 501 Ohms is also not much good.

 

[s1m0n]

Thanks for the advise,

It didn't fel right when I saw it,
I will contact DNO to check it out before I connect up the Ring Final circuit.

Thanks again
Simon


"who said live was straight forward"

 

[s1m0n]

Hi all, just to give you an update,

DNO came out to the house and comfirmed that their earth was not adequate, problem seemed to be with looping off next doors, but wont be upgrading it. They said to go with the Earth Rod and 80A 30mA RCD protecting the whole installation.

I added another earth rod, and got the reading below 59 ohms,

So feel a lot happier now,

Thanks Simon

 

[pevvers]

They said to go with the Earth Rod and 80A 30mA RCD protecting the whole installation.

They said that? Were they facing away from you and talking from their bottoms at the time as that's not the way to protect a TT system at all!

Minimise inconvenience in the event of a fault springs to mind....

Either normal main switch and two RCD's to protect the installation, or 100mA S type RCD main switch and a variety of MCBs, 30mA RCBOs and RCDs depending on what the installation methods and circuit types are.

 

[Deleted member 9648]

They said that? Were they facing away from you and talking from their bottoms at the time as that's not the way to protect a TT system at all!

Minimise inconvenience in the event of a fault springs to mind....

Either normal main switch and two RCD's to protect the installation, or 100mA S type RCD main switch and a variety of MCBs, 30mA RCBOs and RCDs depending on what the installation methods and circuit types are.

Agreed but no doubt the installation was wired long before the 17th edition and you cant force the client to spend their money.With the earth sorted and 30ma rcd protecting it is at least safe. Maybe when they've had a few rcd trips halfway through eastenders on a january night they will see the sense of a multi rcd board.

 

[pevvers]

Yes, but should the DNO be giving information that's incorrect?

 

[s1m0n]

Hi again,

just to add that the present setup had been there before current owner bought it last november.
I have noted on the certificate to this effect, also what was said by the DNO,

I have advise an upgrade of the consumer unit, but as you say , you cant force them.

 

[acat]

No they shoulnt but as we all know they are a law onto themselves and havent heard of the 1st edition never mind the 17th. Rant over


Chris

 

[pevvers]

No they shoulnt but as we all know they are a law onto themselves and havent heard of the 1st edition never mind the 17th. Rant over


Chris

Exactly, if there's a problem with the supply I will tell someone to contact the DNO because I know little about the ESQCR (probably got that abbreviation wrong) regs. Surely the DNO should take the same line and tell the customer to consult a qualified electrician, and not give guidance which resorts in a non-compliant installation. Oops, just realised I had my rose tinted glasses on again!

 

[WayneL]

Hi all, just to give you an update,

DNO came out to the house and comfirmed that their earth was not adequate, problem seemed to be with looping off next doors, but wont be upgrading it. They said to go with the Earth Rod and 80A 30mA RCD protecting the whole installation.

I added another earth rod, and got the reading below 59 ohms,

So feel a lot happier now,

Thanks Simon

The reason they will have said this is probably because of your 501ohm Ze - too high for a 100mA RCD for earth fault protection.
Segregation of circuits would have to come second to providing earth fault protection for the installation.

To be honest, any TT installation goes against the regs with regards the 'fault on one circuit not affecting other circuits' rule

Now that you've got it down to 59 ohms you would definitely get away with a 100mA main switch and possibly even a 300mA mainswitch.

That will then give your discrimination for your RCDs for additional protection.

 

[pevvers]

The reason they will have said this is probably because of your 501ohm Ze - too high for a 100mA RCD for earth fault protection.

Again, completely against the regs to even have it that high.

Segregation of circuits would have to come second to providing earth fault protection for the installation.

I agree, but still against the regs.

To be honest, any TT installation goes against the regs with regards the 'fault on one circuit not affecting other circuits' rule

Not if you have a 100mA selective RCD on the front end and 30mA RCBO's on the rest....

I think the term is actually "minimise inconvenience in the event of a fault" which is why you have 2 RCD's in TN systems, which would minimise it. Not perfect, but RCBO's aren't cheap.