Tn-C-S Question

[Dairyspark]

Hi everybody, I'm wide awake at 12:50 and I have a question lol, I seen in a recent post that carrying a tncs Earth out to an outbuilding can be dangerous and that a tncs supply to a transportable building is a no no, I was just wondering why?

Cheers

Bobby

 

[steptoe]

TNCS is earthed at the Tx, which can be some distance away,

and can be potentially substantially different from the general mass of earth, which you are standing on,

so you can have a potential difference from TNCS earth to actual earth,

does that make sense?

 

[Dairyspark]

That does make sence, so I'm assuming by taking it further out of your house/other building your yet again increasing the distance from you to the transformer

 

[steptoe]

do you understand the concept of the faraday cage?

 

[Dairyspark]

Sort of, an equal charge is formed around the cage so there is no difference in potential so no crurrent flows?

 

[ProDave]

Isn't the BIGGER problem that in the (not uncommon) event of failure of the outer core of the concentric incommer you get a loss of neutral.

AND because E and N are one and the same, then the "earth" can rise up to 240V under those circumstances.

THAT is why you never connect it to a (usually metal clad) portable building like a caravan.

 

[apprentice87]

Was just coming to that....

Hi Sparkybob,

As has been explained, with TNCS the neutral conductor is also used as the earth, then, they are separated at the cutout, with neutral being neutral, and earth being earth from then on.

Now, say in a normal house, you were to get the line conductor and short it to one of the radiators, just stuff the line into the thing! Blam, flash and a bang and the MCB trips, because them radiators are, if connected with metal pipes, earthed at some point. Lets say you stuff the line into something else metallic and also part of the electrical installation, same thing will happen, bang flash...

Now, say the DNO neutral is broken..... All the earthed metal work in the installation will now, or at least could be, at mains voltage. You stuff the line into the radiators, now, and nothing will happen, because they will be at substantially the same voltage. You will be ok, because everything in the house will all be at the same voltage [assuming you got proper bonding!], just hope you have not got any bare concrete floors in there......

Now, say the same happened with a transportable thing. Most of them have metal frames, and the thing itself will generally be fairly insulated from the mass of the earth, standing on bricks or whatever. The entire frame of the building will now be live.... Someone walks up outside and catches hold of the metal door handle and then they form the return path for all current that would usually be travelling back down the neutral, not just "your" current mind you, could be half the street.... Bad plan. Draw it out and you will see..

TNCS is a crappy system with nothing really to commend it and was basically cobbled together to save the DNO's from having to maintain the integrity of their earthing system properly..

It is banned from use in petrol stations by the HSE because of some of the problems that can arise with differing earth potentials and "stray currents". Why do you think that bonding conductors and earthing conductors have to be a specified [larger] size?? Because in the event of a fault in the DNO network, YOUR installation can become the neutral return path for half the street.....

Give it a few years, when their new, extremely crap modern CNE cable is all dying, and they will make everyone have TT. Anyone that works for a DNO will tell you that they have FAR more problems with the "modern" stuff, than they ever did with PILC......

john...

 

[Dairyspark]

See this is why I like this forum, you get quality answers and you don't get ridiculed for not knowing thanks

 

[steptoe]

except,

I was going to get you to figure it out for yourself,

and not simply explain it all at once,  :|

 

[Evans Electric]

Stepps has been on a teacher training course you know . :innocent

 

[Dairyspark]

Well steps pretend I didn't read the above posts lol hit me with your teachings lol

 

[Sharpend]

First things first, open pencil case take out bottle of beer!

 

[apprentice87]

Ah... sorry Steps!........

 

[steptoe]

I'm going ,

and,

I'm taking my ball

:coat

 

[kerching]

I Used to go out with a lass from Woolies, she taught me a thing or two

Just saying

 

[Rob_the_rich]

Ah... sorry Steps!........

stepping on steptoes toes

 

[apprentice87]

Nooooooooooo.....

Besides, Steps knows a LOT more than me. Steps has forgotten more than i will ever know....

john..

 

[steptoe]

First things first, open pencil case take out bottle of beer!

I've got a F big pencil case,  

 

[Dairyspark]

Steps!! Come back lol

 

[Sharpend]

I've got a F big pencil case,

Oh.. the pencil crate!