TT to TNC-S

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gazza96

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Hi Guys,

Got a job coming up (full rewire) where the customer is having an extension built, the Project Manager is getting the supply company in to move the exisiting supply (its overhead, is currently TT & it is in the way!). The supply company are going to put it down the pole under the path and into a new style white box on the side of the new extension.

I take it they will be able to give me a TNC-S earth connection ?? (On several previous jobs i've done they have given me a TNC-S but they were all from a TNS systems, this is my first TT).

Could anybody verify this for me, Thanks in advance

Gazza

 

DonkeyDong

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Hi there,

The only people who can verify what you will be getting will the DNO/REC themselves (the guys who will move it).

It all depends if TNCS is available in the area. If its not, you'll be left with TT.

Give them a call, (or have a butchers around at the local poles/transformer, see if they are PME).

D.

 

steptoe

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tbh,

and its only a big preference of mine,

I would rather stick to TT.

plus I cant really understand how they can properly change a prev TT system to a tncs just by spiking the last metre or so, dont make sense,

have a google of a proper tncs setup and see how it should work, and what can (and DOES) go wrong with PME.

 

gazza96

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Thanks for the replies, having had a brief look at the area when i was leaving i noticed most houses had TT systems, I always thought that TNC-S was the better earthing system to have (0.35 ohms, very low resistance, better than 21 ohms for TT)

Could someone clarify some abbreviations,

DNO (or as i call them 'the supply guys'!)

HED (where fuse 1361 is, and earth conductor goes into side of?)

I will contact the local supply company and ask about whether TNC-S is available in the area

Thanks

Gazza

 

Admin1

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tbh,and its only a big preference of mine,

I would rather stick to TT.

plus I cant really understand how they can properly change a prev TT system to a tncs just by spiking the last metre or so, dont make sense,

have a google of a proper tncs setup and see how it should work, and what can (and DOES) go wrong with PME.
Would you be kind enough as to post a link please Albert? :D

 

DonkeyDong

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Howdy,

Its not just about spiking at/near the client end. The supply/transformer needs the correct earthing. They will also earth along the supply at various points.

Agreed, TNCS can present issues which aren't present with TT, but TT can have its own drawbacks, earth paths, double pole isolation, etc.

Mr BigThingy

 

steptoe

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Would you be kind enough as to post a link please Albert? :D
I'll have a look in a bit ADMIN

in short,

a proper TN-C-S requires to be earthed(spiked) in numerous places throughout its length,

hence - protective MULTIPLE earth

also as has been said earth faults picked up from neighbouring properties can find a back feed up your (lone) spike just outside your house, or through your watermain etc.

BUT,

I hear you cry, wont TT pick up the same faults?

in short, very unlikely, due to the higher impedance to earth, the fault will take the easiest path,(brings us back to the exporting PME supplies).

I'll try find some examples for you.

 

cheggers

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The trouble with TT you are relying on the rcd for disconnection. I know about what if you lose the neutral.

But in my experience I have never come across this but I have come across loads of faulty rcd's. So I would suggest pme/tncs or tns so you can rely on the mcb and the rcd to do the job of disconnection in the case of an earth fault

 

steptoe

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The trouble with TT you are relying on the rcd for disconnection. I know about what if you lose the neutral.But in my experience I have never come across this but I have come across loads of faulty rcd's. So I would suggest pme/tncs or tns so you can rely on the mcb and the rcd to do the job of disconnection in the case of an earth fault
wrong,

you never rely on the RCD for disconnection,

unless you have a very poor system of wiring,

nobody should ever EVER rely on an RCD,

grab the line and N at the same time and see how long it takes an RCD to trip out,,,,,,,,,

still waiting,,,,,,,,

stil waiting,,,,,,,,,

thought so, it wont ever happen until you go overcurrent, then your MCB provides the protection, all down to your R1 R2 and proving a good touch voltage.

oh, and the 21 ohm figure, I have no idea where that came from, some DIY book? readers digest?

its laughable to think you can get that, Im happy witj anything below 180.

nothing to do with your earthing conductor solely, but more to do with a 50v touch, which you will easily comply with if you have a good earth.

TOO MANY people read TOO many books and take TOO much as GOSPEL without the relevant knowledge to discriminate and judge by experience and common sense as to what is safe and what is just a published opinion.

as mine is too. MHO.

 

cheggers

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If you read my post properly I said to earth again you jump in with both feet.

if an appliance had a earth fault on a TT system with a Ra of lets say 100 ohms the fault current would be not be high enough to take out the mcb so you are relying solely on the rcd correct or not.

 

steptoe

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If you read my post properly I said to earth again you jump in with both feet.if an appliance had a earth fault on a TT system with a Ra of lets say 100 ohms the fault current would be not be high enough to take out the mcb so you are relying solely on the rcd correct or not.
well give us the full theoretical fault then and we will try the touch voltage test and see if it passes.

I think you may be misunderstanding the difference between Ze and Zs.

you NEVER as I said EVER rely on the RCD, unless you are a fool.

you rely on your disconnection time as per MCB disconnection curve.

maybe you still have a voltage trip in your house.

or can you tell me one single benefit of a TN system over a TT system.?

(apart from being simple for an installation but more complicated for proper earthing) just ask anyone with a recording studio or serious ham enthusiast.

this is one of the most serious aspects of electrical installations and most people in UK/England just accept what DNO/OSG/BS or whatever say without question or education.

I shall await your reply with enlightenment.

Albert.

 

steptoe

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The trouble with TT you are relying on the rcd for disconnection. I know about what if you lose the neutral.But in my experience I have never come across this but I have come across loads of faulty rcd's. So I would suggest pme/tncs or tns so you can rely on the mcb and the rcd to do the job of disconnection in the case of an earth fault
isn't that another form of TT, in one of its many incarnations?

as most (if not ALL) domestic installations in the UK are TNS on the consumer side.

 

gazza96

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Hi Guys,Got a job coming up (full rewire) where the customer is having an extension built, the Project Manager is getting the supply company in to move the exisiting supply (its overhead, is currently TT & it is in the way!). The supply company are going to put it down the pole under the path and into a new style white box on the side of the new extension.

I take it they will be able to give me a TNC-S earth connection ?? (On several previous jobs i've done they have given me a TNC-S but they were all from a TNS systems, this is my first TT).

Could anybody verify this for me, Thanks in advance

Gazza
They have given me a TNC-S (0.1 ohms, not bad!), so it can be done but i wasn't there when they did it to get more info.

Gazza

 

steptoe

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They have given me a TNC-S (0.1 ohms, not bad!), so it can be done but i wasn't there when they did it to get more info.Gazza
thats good mate, is that with or without your bonds in place?

its always gonna be good anyway, its your Neutral cable your measuring, NOT the earth impedance.

 

sirsparkabit

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The most important part of any installation EARTHING i love it:8}

By the way albert,Do you remember my post and the missing earth?

Well the earth had been hidden by whoever had built the cuboard and when i took out the shower c/u what do you think i found ,that lovely little PME sticker.Why the last spark thought it was good to put the c/u on top of the label i will never know.

Still living and learning every day cheers SSAB.

 

steptoe

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jeez SSAB, just goes to show some people really dont have any common sense at all, anyways, at least it solved that mystery for you.

I know that lots of people disagree with me on some things(MOST things probably, :eek: :O ) but like my sig says, its only my opinion, and maybe Im too stuck in my ways sometimes and just like things the way they are/were/should be(according to the world of Albert!).

anyhowz folks, always remember that I only give advice as I know it and due to my own experiences and (limited) knowledge of such things.

its not a definitive answer I have for anything.

Albert.

 
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