Exported earth

[SLIPSHOD & SLAPDASH]

Thats a very good explanation. I could not have been bothered to type all that out .

 

[SPECIAL LOCATION]

Thats a very good explanation. I could not have been bothered to type all that out .

Cheers M8

but you make me sound all like I am all sad and lonely...

just tryin to help the young uns get a few basics around there heads...

As you implied in your earlier post...

I don't think all of these courses necessarily have the right motives..

I fear the keeping up the colleges pass rate statistics can become more important that checking that the student actual understand the topic that is supposed to be taught!. or that they have the requisite skills to understand the course in the first place.

I mean in a lecture I guess it would take less than 5 mins to explain that basic concept to someone asking?

whether it is part of the specific course or not!! ?:| :|

but there you go!!

 

[SparkyMarky]

Thanks Special Location. That would've taken me a couple of days to type! That was exactly what I wanted to know - the "why" not the "how". Cheers!

 

[SPECIAL LOCATION]

Thanks Special Location. That would've taken me a couple of days to type! That was exactly what I wanted to know - the "why" not the "how". Cheers!

No prob M8!

Still don't know why the guy at college couldn't just run through it verbally with you??? ?:| ? :|

However... that we will never know?

Main thing is matter sorted.

problem with all forums is sometimes can skip read a post..

miss a small bit of info..

and go running off in the wrong direction!! :_| :_|

then its takes a bit of stoppin to turn it back on course!

 

[SLIPSHOD & SLAPDASH]

I do remember the phrase 'that is outside the scope of this course' being used often when i sat the 2381. It must be either the same guy or the get out clause you they taught at lecturers college ; \

 

[SPECIAL LOCATION]

I do remember the phrase 'that is outside the scope of this course' being used often when i sat the 2381. It must be either the same guy or the get out clause you they taught at lecturers college ; \

Many a true word spoken in jest Slip's

as they say!!! ; \ ; \

 

[davetheglitz]

SL - Thanks for that explanation - brilliant!

WW - is it possible to have a section for FAQ - this would be a prime candidate and a damn good explanation of why rather than what!

One thing I'm not sure about in the reasoning is how an earth fault finds its way to the remote earth rod, as this pre-supposes that:-

a) The fault is not large enough to trip the shower MCB

B) There is a fault in the main earth.

The Ze on most installations would be well below 1ohm, and Ze for the earth rod - around 100 ohm (if it's a goodun!) - so less than 100th of the PFC will be going down the CPC. Max fault current down CPC in the event of an internal house shower fault would be 250/100 - 2.5A - so not enough to blow any fuse in the event of a main earth failure.

Are we really saying that:-

a) By TTing the system we can ensure that there is no difference between 'garage earth' and real earth - but we are then reliant purely on the earth rod and RCD.

B) If we do have a fault in the house - the worst case scenario is that anyone touching real earth and exported earth at the same time is 250v touch voltage for the time it takes the fuse to blow - regardless of RCD and earthing arrangements in the garage.

c) If we use an exported earth and touch non bonded metal in the garage we risk a difference in earth potential. Assuming a Ze for the non bonded meatalwork of 100 ohms, even if there was a 50A earth fault, the potential difference between the earths would be 50v max - and this could be improved with an earth rod.

On balance I would suggest that the risk of relying on a mechanical component to switch the supply off is more than balanced by the small risk of someone being in contact with two different earth potentials for the small time that a max pfc is flowing. In general my vote would be to export the earth and add an earth rod!

Am I being too simplistic here?

 

[SPECIAL LOCATION]

Golden rule 1]:

The wiring regs are guidance...

the designer, installer & tester uses their skill & knowledge to ensure the fundamental principals of chapter 13 BS7671 will protect the safety of "People Property & Livestock"!!!

Golden rule 2]:

Electric shock currents...

1 - 2ma barely perceptible

5 - 10ma throw off, painful

10 - 15ma muscular contraction can't let go

20 - 30ma impaired breathing

50ma & above ventricular fibrillation & death!!!

(I have a sheet with this info printed on it...

cant remember if I scanned it out of one of my books

or if I downloaded it from somewhere )

Pictures

http://i291.photobucket.com/albums/ll306/SPECIALLOCATION/ELECTRICSHOCK1.jpg

Writing

http://i291.photobucket.com/albums/ll306/SPECIALLOCATION/ELECTRICSHOCK2.jpg

One thing I'm not sure about in the reasoning is how an earth fault finds its way to the remote earth rod, as this pre-supposes that:-

a) The fault is not large enough to trip the shower MCB

B) There is a fault in the main earth.

ah well spotted sir davetheglitz a shower maybe wasn't the best example...

cuz its own RCD should trip anyway!!

But remember

Golden Rule 3:

Resistors in series.. current same voltage divided.

Resistors in parallel.. voltage the same, current divided.

No matter how large the resistance is, a small portion would travel down the high res side of the parallel earth path..

Lets change the rules a bit..

Fault on the house lighting circuit.. NO rcd.... Permissible trip time 5 secs!

But the possible exported earth fault current goes outside!

need 0.4 sec max disconnect time..

The Ze on most installations would be well below 1ohm, and Ze for the earth rod - around 100 ohm (if it's a goodun!) - so less than 100th of the PFC will be going down the CPC. Max fault current down CPC in the event of an internal house shower fault would be 250/100 - 2.5A - so not enough to blow any fuse in the event of a main earth failure.

50ma enough for death...

6amp lighting cirtcuit...

100th of 6amps = 60ma which exceeds 50ma ?

increase current above 6a during PFC.. short phase to earth!!! say 1.5kA.???

Are we really saying that:-

a) By TTing the system we can ensure that there is no difference between 'garage earth' and real earth - but we are then reliant purely on the earth rod and RCD.

garage earth rod and any garage structural metal work all sit on the same bit of physical ground are near equivalent potential.

A main house TT supply relies OK on RCD to cut off supply in the event of fault.

B) If we do have a fault in the house - the worst case scenario is that anyone touching real earth and exported earth at the same time is 250v touch voltage for the time it takes the fuse to blow - regardless of RCD and earthing arrangements in the garage.

c) If we use an exported earth and touch non bonded metal in the garage we risk a difference in earth potential. Assuming a Ze for the non bonded meatalwork of 100 ohms, even if there was a 50A earth fault, the potential difference between the earths would be 50v max - and this could be improved with an earth rod.

On balance I would suggest that the risk of relying on a mechanical component to switch the supply off is more than balanced by the small risk of someone being in contact with two different earth potentials for the small time that a max pfc is flowing. In general my vote would be to export the earth and add an earth rod!

However you understand (or don't understand) Reg 542.1.8 specifically says that IF earthing arrangements are common then all CPC must be capable of carrying max PFC. OR be earthed ONLY at one installation.

Am I being too simplistic here?

Don't forget.. that these guidelines are NOT aimed specifically at you average domestic property where a remote garage is actually very local to the main building.

consider a more rural application ..

or agricultural environment..

remote building may well be genuinely remote..

& potential difference can be more significant.

 

[SPECIAL LOCATION]

Electric shock currents...

1 - 2ma barely perceptible

5 - 10ma throw off, painful

10 - 15ma muscular contraction can't let go

20 - 30ma impaired breathing

50ma & above ventricular fibrillation & death!!!

Morning chaps (& chapesses!).

I remember where this info was from..

one of them electric books by "Brian Scaddan" et al.

which go over the electric science & theory sort of stuff..

and works though example C&G questions.

Anyway those currents are the recognised perceived risks for a mains voltage shock.

i.e. 50ma @ 230v can cause death, dependant upon the health of the persons grabbing the live bits!

Which is why we either..

reduce voltage. (transformers)

restrict duration exposed to harmful current. (automatic disconnection)

prevent access to simultaneous exposed parts that could be at different potentials. (bonding / insulation etc..)

So back to our earth fault situation...

taking a short circuit live to cpc fault in the house...

for the initial instant of the fault before disconnection by fuse blowing..

the earth CPC becomes a "Live" conductor passing 230v down to earth!

Supply ->

Phase ->

Faulty connection ->

CPC ->

Mass of earth (load that dissipates voltage) ->

Back to Supply.

We try and control the CPC potential from the house by fuse blowing,

but cant control the potential at remote building...

unless it is all bonded e.g. with a 10mm as mentioned in Slipshods IEE article.

which every way you chose..

either prevent dual contact with two earth potentials,

or bond them both with big enough CPC to carry any fault currents going either way.

This is what the circuit designer takes into account when accessing the perceived risks to people property & livestock - Vs - costs of materials etc..



Any way..

goto go know wire a conservatory..

whats the bettin it rains when I have to put the external lawn mower socket on!!!!!

 

[davetheglitz]

Blimey SL - do you ever sleep!!!

(This reply has been heavily edited - as I think that putting it down here helps to clarify the mind - so sorry if it appears to ramble a bit!)

Great reply - and I think your last one puts it all into perspective - bond with big enough CPC to allow for fault currents either way.

Just one clarification

You are permitted in the wiring regs to join the TT rod & MET together.. BUT if you do.. "All of the CPC's must be capable of carrying the max PFC for any part of the installation" because for example.. a fault in the shower on the house.. could make a path down to the earth rod in the garage!

Correct me if I'm wrong - but the max current on the cable to the garage due to a house fault on a TT system with a rod Ze of 100 ohms is 2.4A.

The max current on that cable from a garage fault is the full fault current based on the Zs at the garage CU - which will blow the fuse back at the house.

This means that the cpc to the garage with the additional earth rod does not have to carry the full PFC for the installation - just the garage. As a general rule of thumb if you select the size of SWA cable based on the fuse rating for the garage and the appropriate volt drop - and use three core with the armouring as an additional cpc you will be on the safe side as far as exporting the earth!

I think my problem is that I've got a particular installation in my mind - and in this case there is no exteraneous metalwork.

When we start to think of a water pipe becoming live at the garage we would need the SWA to be able to sink the current to the MET. In this case the protection would need to be the full fault current based on Ze at the house. (by George I think he's got it!) The moral of this is to do all your pipework in plastic!!!!

As an aside - we are using more and more RCDs as a means of protection - but it only takes a N-E fault to make an RCD ineffective on a TT system (due to very little current in the earth path) This in effect makes the system a DIY TNCS system - so the reliance then is on the MCB tripping out - assuming the fault can handle the current. The fault could be on any circuit - so R1 + R2 under fault conditions cannot be ascertained - so there is no way of knowing if it will trip out in the required time. For that reason I would prefer to export the earth rather than rely on a mechanical device alone - which can be compromised by a circuit/appliance fault - and for which there appear to be no published failure rates. Perhaps we should go back to rewireable fuses - 'cos we know how they work!!!

 

[SPECIAL LOCATION]

. (by George I think he's got it!)

Just to prove I CAN do short posts as-well

 

[SLIPSHOD & SLAPDASH]

Dave, i am sure ( once again i will have to look this up later) you are not allowed to have 2 forms of earthing on one system, ie No earth rod allowed on a TNCS. I think this is ridicules as with TNCS you have to use a min 10mm bond to all extraneous metal work which is the same as having a earth rod ???

 

[davetheglitz]

Hi Slipshod - I understand where you're coming from with regard to TNCS - but apart from a cryptic mention in Caravan Parks (reg 708.411.4) - which says TNCS can't be used for caravans feeds - I can't back find any reference.

I've only got the 17th Edition version of the reg - but 542.1.8 says "Where a number of installations have separate earthing arrangements, any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow through them or be earthed within one installation only and insulated from the earthing arrangement in the other ...."

So TNCS can have a remote earth rod - providing the Protective Conductor is up to taking the max PFC for the system. The cross sectional area can be worked by the adiabatic equation or selected by selection from table 54.7.

Must admit I was surprised how small the conductor can be when applying the equation!

 

[Nicky Tesla]

[

But remember

Golden Rule 3:

Resistors in series.. current same voltage divided.

Resistors in parallel.. voltage the same, current divided.

No matter how large the resistance is, a small portion would travel down the high res side of the parallel earth path..

The Ze on most installations would be well below 1ohm, and Ze for the earth rod - around 100 ohm (if it's a goodun!) - so less than 100th of the PFC will be going down the CPC. Max fault current down CPC in the event of an internal house shower fault would be 250/100 - 2.5A - so not enough to blow any fuse in the event of a main earth failure.

50ma enough for death...

6amp lighting cirtcuit...

100th of 6amps = 60ma which exceeds 50ma ?

The resistors would not be in parallel though would they!

It would be 1 resistor ie the the lighting cpc having the fault current flowing down it. Then you have to consider the said 6a flowing through Ze resistance eg 0.1a which gives V=6ax0.1; v=0.6V. So the voltage across the earth rod to earth will not be 250v at all, no-where near, but just 0.6v in this case. Now the calculation for the earth rod will be I=v/R ; Amps=0.6/100 ; =0.6ma

YOUR ALL MISSING THE POINT. THE PROBLEM IS THAT IT IS A PME SYSTEM AND THE PROBLEMS OF BROKEN NEUTRALS.

 

[Nicky Tesla]

[

The resistors would not be in parallel though would they!

It would be 1 resistor ie the the lighting cpc having the fault current flowing down it. Then you have to consider the said 6a flowing through Ze resistance eg 0.1a which gives V=6ax0.1; v=0.6V. So the voltage across the earth rod to earth will not be 250v at all, no-where near, but just 0.6v in this case. Now the calculation for the earth rod will be I=v/R ; Amps=0.6/100 ; =0.6ma

YOUR ALL MISSING THE POINT. THE PROBLEM IS THAT IT IS A PME SYSTEM AND THE PROBLEMS OF BROKEN NEUTRALS.

 

[SPECIAL LOCATION]

[The resistors would not be in parallel though would they!

It would be 1 resistor ie the the lighting cpc having the fault current flowing down it. Then you have to consider the said 6a flowing through Ze resistance eg 0.1a which gives V=6ax0.1; v=0.6V. So the voltage across the earth rod to earth will not be 250v at all, no-where near, but just 0.6v in this case. Now the calculation for the earth rod will be I=v/R ; Amps=0.6/100 ; =0.6ma

YOUR ALL MISSING THE POINT. THE PROBLEM IS THAT IT IS A PME SYSTEM AND THE PROBLEMS OF BROKEN NEUTRALS.

Hi Tesla

welcome to the forum

great to see another person postin, discussin & debating various topics.

I know this thread has been a bit long in parts, just to clarify a couple of your points...

the initial discussion was the concept of 'Exported Earth'

which brought us into various points of discussion options of mixed earth type

on a site with remote buildings etc..

& whether combined or separate earths & the possible dangers etc..

REG 542.1.8. is under general earth requiements... it is neither specific to PME(TNCS) TNS or TT.

the illustration of the light fault was to help with understanding fault currents and either having large enough CPC or keeping dual earth types separate.

I need to address a couple of your assumptions..

the 6a is the MCB rating not the fault current.

In a short circuit fault at a light fitting the CPC & phase conductors are in direct contact, (no load in between them)

so the full supply voltage is now on the CPC.

Hope you take this the way it is ment...

but some of you calculations appear a bit out? :|

_______________________________________

resistance eg 0.1a which gives V=6ax0.1; v=0.6V.

________________________________________

Resistance is ohms not 'a' V=IxR so I=V/R

The voltage is fixed by the supply company so taking your figures

230v/0.1ohm = 2300amp (2.3kA) which is your PFC (which in this case is the bit that is going to blow the fuse!).

If earth and phase are combined at the instant of the fault 230v will be on the CPC. (cuz its touching the 230v live!)

If the CPC has two paths (no matter what resistance) they are in parallel!

(a TNS & a ROD) and the 230v will be across both of those earth paths.

i.e. go and connect as many resistors, (various values), as you like across a 12v battery.

The potential of the battery will be across all of the resistors i.e. 12v.

but the current flowing through them is different...

The voltage will only be different with resistors in series..

so say the CPC was 1ohm and rod was 100ohm

230v across 101ohm = 2.3V droped across CPC and 227.7 droped across 100ohm rod!

_______________________________________

Now the calculation for the earth rod will be I=v/R ; Amps=0.6/100 ; =0.6ma ???

________________________________________

0.6/100 cannot be 0.6ma it would need to be divided by 1000.

(tho the formula's incorrect anyway.)

unless someone can correct me???

I do get bits wrong..

so it is good to bat them about!

I fink im right wiv me calc values????

now you have started postin ..

keep it up M8

any probs and WW & the DON boys will sort things out

R U spark? traning? yung un? mature change jober??

 

[biker mike]

Specloc when it comes to posts You da man

 

[Nicky Tesla]

Hi Teslathe 6a is the MCB rating not the fault current.

In a short circuit fault at a light fitting the CPC & phase conductors are in direct contact, (no load in between them)

so the full supply voltage is now on the CPC.

______________________________________

resistance eg 0.1a which gives V=6ax0.1; v=0.6V.

________________________________________

Resistance is ohms not 'a' V=IxR so I=V/R

The voltage is fixed by the supply company so taking your figures

230v/0.1ohm = 2300amp (2.3kA) which is your PFC (which in this case is the bit that is going to blow the fuse!).

If earth and phase are combined at the instant of the fault 230v will be on the CPC. (cuz its touching the 230v live!)

If the CPC has two paths (no matter what resistance) they are in parallel!

(a TNS & a ROD) and the 230v will be across both of those earth paths.

i.e. go and connect as many resistors, (various values), as you like across a 12v battery.

The potential of the battery will be across all of the resistors i.e. 12v.

but the current flowing through them is different...

230v across 101ohm = 2.3V droped across CPC and 227.7 droped across 100ohm rod!

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

Nice polite reply their SP, but your still completely wrong.

230v will not actually by magic appear at the cpc at the fault, i know it may seem to, but it doesn't....We come back to ohms law again, V=ixr

The cpc has a resistance itself which is added to the Ze eg 0.2r and Ze 0.1r is a resistance of 0.3r ( i put r in place of the ohms symbol)

Now when the current begins to flow in the cpc a voltage appears across this resistance growing as quick as the current grows, which is in fragments of a second and a graph needs to be observed to show this.

But what also happens is current is also being drawn along the live conductor and again using ohms law and a resistance to the fault of 0.2r( if the conductors are of the same csa) both conductors live and cpc and Ze will have a voltage across them also. If we forget about Ze for a minute the max voltage at the fault on the cpc will eventually become half of 230v, which is why lights go dim when a fault occurs, but a fuse or mcb will have blown long

before that.

So no 230v does not suddenly appear at the cpc

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

My next point is how on earth does 230v appear at the earth rod from a fault on a lighting cct in the house.

Using your battery senario, it would be like having a number of resistors side by side with one end connected to the + and the othe end connected to the -. But these are not the cpc resistances but would represent the LOAD resistances in parralel.

Each cpc would be represented by a seperate resistor for each load joined together at the - end but o/c near to + end next to each load.

Being that none of the cpcs are joined together at the + end even if one does in your scenario it will not effect the other cpcs ( only the Ze can create a rise in voltage when a large current is drawn)

So to sum up, the only voltage that will appear across the earth rod during this fault, will be V=IxR, The current drawn from the fault (B type =5x rating and C type = 7 Times rating) X Ze

6x5=30a X 0.1r= 3v

Therefore current going down the earth rod will be I=V/r, 3v/100r =30mA

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

Answer to the other bit their SP, Im a sparks but have also got a HND in electronics......looking forward to see your reply

 

[ABC Technical]

I believe that was the reply OP was looking for Albert. Try not to get worked up Albert - Watch your blood pressure. OP was unsure - so he asked - That's what we are here for Sir... To help each other out. That where everyones experience comes in.

I am on the C&G course - although we had used swa, we haven't been told of the earthing requirements on long runs - So I can sympethise with the OP. I have picked up a heck of a lot of knowledge through buying books and studying, going to college, Prcticing in my workshop etc and more so from Guys like YOU, whom I respect.

I don't like forums that give carp replies, hence my reason for starting this one, Sir.

Let's keep it insult free (ish) & word it better. And that goes for us all - not just you Sir.

Now...Who's for a free Promo Sticker. :^O

I am off for a minute to order them.

Ooh ooh ooh, if Im not too late, can I have one please?

 

[a1]

Yes Mate, Just PM *welsh.wizard* with your name, address and preference of interior or exterior sticker and he will send you one Mate.

PS - Welcome to the forum.