Exported earth

[a1]

The above post, was in responce to ABC. (post #39).

 

[ABC Technical]

Thankyou for the pointer and the welcome Admin.

 

[SPECIAL LOCATION]

 

[SPECIAL LOCATION]

Hey Biker Mike.....

due reckon that last post was worth about 10 posts...??

I want my scores adjusted REF!!!!!!!!!!!!!! :^O :^O:^O

 

[steptoe]

Im going to add one more thing to this.

BS 7671 is a GUIDANCE

please dont start being stoopid like a lot of forums and just go by it.

I dont really care what anyone does with their earthing arrangements but if I was to test a house with PME/TN-C-S and TT/SPIKE mixed in one installation I would have a CODE 1,

and possibly even a disconnection of supply noticed served.

there is NO way I would/could ever sanction the use of a parallel path to earth for any kind of fault.

I think you will find in EAWR this practice is no longer deemed acceptable except under certain circumstances,

and would doubt a domestic installation would qualify for this status.

if none of you can be bothered to look for the section I'll try dig it out tomorrow sometime.

the bottom line is if you mix 2 systems you are introducing a secondary point of potential.

 

[Nicky Tesla]

Your diagram proves yourself wrong. Cos the voltage exported to cct b from the dis board earth bar is not the same voltage that is present at the fault cos the fault current has had to pass through R2a first which creates a voltage at the earth bar lower than at the fault. So it is not like a load of resistors in parallel is it, but in series like i said.

Also there is a touch voltage that is mentioned which is well known to be at 50v, but you say all your ccts will have a voltage of 230v on the cpc and metalwork, so something is obviously wrong with what you say.

 

[davetheglitz]

Hi Steptoe - I'm fairly new and limited to domestic experience to this so bear with me - but I can't see why you are against the mix of earth types - particularly with a TT-TN mix.

Considering an exported earth to a garage with water supplied via a metal pipe - you would definitely bond the pipe. How is that electrically different to connecting to an earth rod?

By adding an earth rod at the garage end surely you are giving a belt and braces earthing point for the RCD to act on in the event of a failure of one earth or the other.

My other point is how independant can earthing be anyway? Two terraced houses are not going to have completely independant earths - they will be joined outside of the properties boundaries - so why is a parallell earth path such a bad thing if both paths can handle the full pfc? The ultimate parallel earth path is the earth iself!

Could you elaborate please - it's obviously something you feel deeply about and I want to understand.

Cheers

Dave

 

[steptoe]

Hi Steptoe - I'm fairly new and limited to domestic experience to this so bear with me - but I can't see why you are against the mix of earth types - particularly with a TT-TN mix. Considering an exported earth to a garage with water supplied via a metal pipe - you would definitely bond the pipe. How is that electrically different to connecting to an earth rod?

By adding an earth rod at the garage end surely you are giving a belt and braces earthing point for the RCD to act on in the event of a failure of one earth or the other.

My other point is how independant can earthing be anyway? Two terraced houses are not going to have completely independant earths - they will be joined outside of the properties boundaries - so why is a parallell earth path such a bad thing if both paths can handle the full pfc? The ultimate parallel earth path is the earth iself!

Could you elaborate please - it's obviously something you feel deeply about and I want to understand.

Cheers

Dave

unless you are VERY friendly with your neighbour you are not going to be in contact with their earth and yours at the same time as a fault developes and decides to use you a an additional conductor for a parallel path to earth!

also,

if you want to certify the installation,

what are you going to put in the Ze box?

highest reading?

180ohms at your rod?

makes a mockery of having to fail the rest of the house just because you wanted to pass the garage as safe!!!

 

[steptoe]

Hi Steptoe - I'm fairly new and limited to domestic experience to this so bear with me - Considering an exported earth to a garage with water supplied via a metal pipe - you would definitely bond the pipe. How is that electrically different to connecting to an earth rod?

you do NOT export the earth!!!

 

[steptoe]

if you are going to export the earth, and its a very short run,

ie shed 2 m up garden and your earth to it will carry the full fault current(10mm?) then you can bond the metal water pipe.

read your guidance on PME systems, all main bonds must be capable of carrying the FULL fault current for the MAX fault time of the installation, 5s.!

on PME this PFC can be quite LARGE

25mm earth anyone?

 

[SPECIAL LOCATION]

Your diagram proves yourself wrong. Cos the voltage exported to cct b from the dis board earth bar is not the same voltage that is present at the fault cos the fault current has had to pass through R2a first which creates a voltage at the earth bar lower than at the fault. So it is not like a load of resistors in parallel is it, but in series like i said.

Well there you go!

the voltage dropped down the CPC R2a will make a big difference eh?

maybe a volt or two??

If you re-read (numerous posts back now)... I was referring to two earth methods,

incorrectly joined together.. would provide a 'parallel' earth path!!

I don't think I actually said that the voltage would remain the same??

I did explain somewhere the volt drop due to resistance in 'series' would come into play..

{e.g. about a 1ohm cpc & 100ohm rod potential divider}

But either way an incorrectly earthed system could allow a hazardous voltage to become simultaneously accessible..

Unlike your claim that a CPC conductor coming into contact with a live conductor with 230v potential on it will not go to mains voltage during the instant of the fault prior to automatic disconnection!

The NICEIC quotation.....??? not my words...

"Furthermore, the voltage will be exported to the exposed conductive parts of circuit

 

[SPECIAL LOCATION]

you do NOT export the earth!!!

I think that sums up 50+ posts quite nicely Mr Steps!!

 

[steptoe]

look guys,

if you really want to do this, then the only safe and logical way is to do exactly the same as the DNO

make yourself a TN-C submain.

then when your all nice and cosy in your newly wired garage,

add the -S

there you go,

just use your Neutral Combined to the garage, and add the Terra Separate when you get there.

mind the DNO dont catch you tho, cos you'll blow their transformer when it all goes wrong :_| :_| :_|

why oh why does it have to get so complicated NOT to do something???

 

[davetheglitz]

SL and Mr Steps,

Thanks for your replies - I can now see what the electrical issues are and understand the advice of not mixing earths - paricularly when bonding is involved.

It really is good that people will spend the time to help get others to understand why the regs are as they are rather than mindlessly quoting them (as seems to happen on some other forums!).

Cheers

Dave

 

[welshwizard]

SL and Mr Steps,Thanks for your replies - I can now see what the electrical issues are and understand the advice of not mixing earths - paricularly when bonding is involved.

It really is good that people will spend the time to help get others to understand why the regs are as they are rather than mindlessly quoting them (as seems to happen on some other forums!).

Cheers

Dave

I'd second that Dave.

 

[steptoe]

SL and Mr Steps,Thanks for your replies - I can now see what the electrical issues are and understand the advice of not mixing earths - paricularly when bonding is involved.

It really is good that people will spend the time to help get others to understand why the regs are as they are rather than mindlessly quoting them (as seems to happen on some other forums!).

Cheers

Dave

thanks Dave,

although one of the main reasons I tend not to quote the regs is made very clear on one of my other posts.

I dont really care much for them TBH.

but I do use them as a guide, which is all they are.

and as long as you can always satisfy yourself that what you do is safe,

then all is well in this green and pleasant land of ours,

NOW,

your round I think,

 

[Nicky Tesla]

What if an extension is built from the garage to the house then yippee! we can wire it the same as the rest of the house now.

---------------------------------------------

I agree with everything that the niceic have written in this thread and it backs up everything that I say and demolishes your theory not mine.

How on earth can a cpc of 0.1ohms have a voltage across it of 230v in an instant which means that there MUST be 2300A flowing through it for that to be true. And Where did all the rest of the 230v go if you say in your last reply that only "maybe a volt or two" would be dropped across the cpc.

The niceic have it right like me, cos the voltage may RISE to a touch voltage of 50V at the earth bar and be EXPORTED to other cpcs at that potential. according to you all your ccts could have touch voltages of 230v in a fault condition.

I don't meen to be rude, but are you actually reading anything i write, or are you just brushing it aside.

I like to learn from constructive criticism

 

[SPECIAL LOCATION]

your round I think,

Mines a pint please! ]

 

[steptoe]

What if an extension is built from the garage to the house then yippee! we can wire it the same as the rest of the house now.---------------------------------------------

I agree with everything that the niceic have written in this thread and it backs up everything that I say and demolishes your theory not mine.

How on earth can a cpc of 0.1ohms have a voltage across it of 230v in an instant which means that there MUST be 2300A flowing through it for that to be true. And Where did all the rest of the 230v go if you say in your last reply that only "maybe a volt or two" would be dropped across the cpc.

The niceic have it right like me, cos the voltage may RISE to a touch voltage of 50V at the earth bar and be EXPORTED to other cpcs at that potential. according to you all your ccts could have touch voltages of 230v in a fault condition.

I don't meen to be rude, but are you actually reading anything i write, or are you just brushing it aside.

I like to learn from constructive criticism

you really have no concept of earthing have you.?

place a short from PHASE to EARTH on any circuit and instantaneously the cpc will rise to 230v, before the protective device kicks into action.

you dont seem to have grasped the theory of what the 50v touch voltage is measured from

 

[biker mike]

Hey Biker Mike.....due reckon that last post was worth about 10 posts.?^O:^O:^O

dude i think ur digits are on some sort of roids cos dayumn u can lay some letters down cant ya

more like a 12 post old chap ill see the adjudicator and put a word in 4 ya :^O