Earthing Arrangement- confused

[batty]

The reason you cannot use a metallic pipe as your main earth is it may get changed to a plastic one and you will no longer have an effective earth.

 

[steptoe]

O.K., just wanted to make sure we were talking about the same thing which started this thread. So you believe that adding an extra earth rod to a TN-S or TN-C-S system is dangerous (to a code 1 extent) because it creates a parallel earth path - Correct?

YES for TNCS, TNS systems work slightly different, but in general, rods should NOT be used in addition

That's what I'm getting from your comments in that other long thread, but I can't find anywhere in that debate - or on other threads I've found so far - in which you explain why you believe that to be a problem.

have you fully understood the implications of imported faults? that is the main reason not to have parallel earths

You've also stated clearly in the thread about bonding that a metallic water system should be bonded, regardless of what the current diluted edition of BS7671 says. On that issue, I agree with you fully.

thanks

If the supply pipe is plastic, then there are likely to be a few stray paths of relatively high-resistance to earth by way of pipework fixed to the building structure etc. Do you consider these parallel earth paths to be a danger?

in the same manner as metal back boxes, without being pedantic I think under general circumstances these can be disregarded

If the supply pipe is metallic, then despite the long-standing Wiring Regs. prohibition on using such as the earthing means, it's likely to be a much more effective earth electrode than the typical 5/8-inch 4-ft. long earth rods used today.

By bonding such a water system to the TN-S or TN-C-S earth, you have therefore introduced a parallel earth path which is every bit as effective, if not more so, than the parallel path obtained by connecting to an earth rod. Do you consider this parallel path to be a danger?

it is as you say a concern, I do consider it a major issue when deciding as to what I should do, but as is the norm, most of these type of supplies are in the country where it is also mostly TT install anyway. as for actually having a TNCS with a metal incoming water, then I thing you would probably have your <0.05 ohms anyway .

me in red

I hope this helps make it clearer, from my perspective anyway,

Im not here to run down anyone, just give my opinions, although sometimes that is/can be a lot over the top sometimes.

 

[PBC_1966]

The reason you cannot use a metallic pipe as your main earth is it may get changed to a plastic one and you will no longer have an effective earth.

That possibility was noted way back in an amendment to the 13th edition, before the absolute "must not be used" came into force with the 14th, but it still isn't going to stop a buried metallic water pipe acting as a quite effective earth electrode.

me in redSo you believe that adding an extra earth rod to a TN-S or TN-C-S system is dangerous (to a code 1 extent) because it creates a parallel earth path - Correct?

YES for TNCS, TNS systems work slightly different, but in general, rods should NOT be used in addition

But weren't you arguing near the start of this thread that the rod used in conjunction with the TN-S supply is dangerous as well? Or was that only for the argument about how the DNO might see a rod and just decide to disconnect their TN-S earth?

Whatever you feel about this though, as far as a code 1 is concerned, how would you justify that on a report?

That's what I'm getting from your comments in that other long thread, but I can't find anywhere in that debate - or on other threads I've found so far - in which you explain why you believe that to be a problem.have you fully understood the implications of imported faults? that is the main reason not to have parallel earths

Where is an imported fault going to come from just because you have an extra earth electrode connected to your installation's TN-S or TN-C-S based earthing system?

in the same manner as metal back boxes, without being pedantic I think under general circumstances these can be disregarded

O.K., so you're only concerned about parallel paths of significantly low resistance.

it is as you say a concern, I do consider it a major issue when deciding as to what I should do, but as is the norm, most of these type of supplies are in the country where it is also mostly TT install anyway. as for actually having a TNCS with a metal incoming water, then I thing you would probably have your <0.05 anyway .

TN-C-S is becoming more common around this rural area with refurbishments, and there are certainly still plenty of buried metallic water supplies.

Would you code 1 such an installation as you said you would do for a parallel earth rod? Again, if so, what justification would you give for that?

 

[steptoe]

IMO as I see it, if you purposely insert another earthing method (ie, a rod) then you are responsible for it,

not only as a DNO issue, but as a parralell path to picking up a fault from your neighbour who may be using a full TT system, whereby his fault can be pulled into your install to go to star point on your cable sheath,(path of least resistance), this is why PME needs such large earth relevant to Neutral.

I see your point about the metal water pipes, but you cant code 1 something that isnt part of the electrical install, like you cant code 1 the fact NO smoke detectors are fitted, but you can make a note on the cert.

 

[Andy™]

IMO as I see it, if you purposely insert another earthing method (ie, a rod) then you are responsible for it,not only as a DNO issue, but as a parralell path to picking up a fault from your neighbour who may be using a full TT system, whereby his fault can be pulled into your install to go to star point on your cable sheath,(path of least resistance), this is why PME needs such large earth relevant to Neutral.

I see your point about the metal water pipes, but you cant code 1 something that isnt part of the electrical install, like you cant code 1 the fact NO smoke detectors are fitted, but you can make a note on the cert.

i see your point about a neighbours TT fault current using your rod back to TNCS, but to give this a code 1, yet no code for exactly the same with a metallic water/gas pipe, just doesnt make sense, especially since the fault current through the metallic services could be much higher than that of the rod

 

[kme]

Although I understand and appreciate where Albert is coming from on this, I cannot envisage a scenario whereby a code 1 (immediate danger) would exist. Apart from owt else, I think you`d be somewhat remiss as an electrician if you left a property with a code 1; even on a PIR, without doing summat!

If the cover was missing from the CU, and the live busbar was exposed - THAT is a code 1. Would you mark that down, and leave it; or would you put something there to provide a (temporary?) barrier? I know I would.

KME

 

[Andy™]

Although I understand and appreciate where Albert is coming from on this, I cannot envisage a scenario whereby a code 1 (immediate danger) would exist. Apart from owt else, I think you`d be somewhat remiss as an electrician if you left a property with a code 1; even on a PIR, without doing summat!If the cover was missing from the CU, and the live busbar was exposed - THAT is a code 1. Would you mark that down, and leave it; or would you put something there to provide a (temporary?) barrier? I know I would.

KME

you mean leave the job like this?!

 

[steptoe]

[quote name='Andy

 

[Andy™]

an earth rod, by its nature, wont have a very high current able to flow....

i.e even if you get a very good reading of 23 ohms, thats still a max of 10A that will flow. hardly going to melt that bit 6*mm thats its wired with

*even 1.5 would most likely cope with the load

 

[steptoe]

[quote name='Andy

 

[Andy™]

even a good reading of 9 ohm is still 25.5A fault current. if you got it down to 5 ohm thats still 46A. not a massive amount

chances are though, if the good reading is through a local metallic pipe to next doors PME, then your fault current would mostly flow through your bonding (which could be hundreds of amps) and metallic pipe than the rod

 

[steptoe]

but the point is,

you can be pulling someone elses fault into your TN system by adding an extra un needed rod to the situation.

this was all discussed when we talked about why exporting was a bad idea,

and the same applies still for having 2 different systems in one installation/building.

everyone can have their own choice,

personally, I think the dangers far outweigh any theoretical gains,(not that I can understand any).

 

[batty]

but the point is,you can be pulling someone elses fault into your TN system by adding an extra un needed rod to the situation.

this was all discussed when we talked about why exporting was a bad idea,

and the same applies still for having 2 different systems in one installation/building.

everyone can have their own choice,

personally, I think the dangers far outweigh any theoretical gains,(not that I can understand any).

I would agree with not putting unnecessary earth rods in but there again I would rather use a pme earth than have a rod.

 

[spinlondon]

but the point is,you can be pulling someone elses fault into your TN system by adding an extra un needed rod to the situation.

this was all discussed when we talked about why exporting was a bad idea,

and the same applies still for having 2 different systems in one installation/building.

everyone can have their own choice,

personally, I think the dangers far outweigh any theoretical gains,(not that I can understand any).

Could you explain why an earth fault would travel from someone else through the TN earth and then to your rod?

Would not the same thing happen if your earthy services were bonded to your TN earth?

Surely the fault would follow the lower resistance path back to the TN supply earth?

To my mind the problems associated with exporting an earth would be if your resistance to the massive earth at your location was less than that to the TN earth, unlikely as the TN earth should have a resistance no greater than 0.35ohms.

Or in the case of PME, where there is a supply neutral fault and you provide the least resitance path to earth. Having a rod at the installation would hopefully provide a lower resistance path to earth than a person.

 

[SteveH37]

Has anybody got any veras?

Laaaaavleeeeeeeeeee

 

[Andy™]

Could you explain why an earth fault would travel from someone else through the TN earth and then to your rod? Would not the same thing happen if your earthy services were bonded to your TN earth?

easily. lets next door has TT with a dead short to earth (no RCD). fault current uses shortest path to your rod (or you common metallic utilities), through bonding and to neutral at head

 

[spinlondon]

[quote name='Andy

 

[Andy™]

it doesnt miss their rod. the current goes from their rod to ground and back to neutral/star point. which will partly be through your earth rod and neutral at head (path of least resistance). or in case of shared metallic services, through that, your bonding to neutral

 

[spinlondon]

[quote name='Andy

 

[Andy™]

i didnt say it was... that would be Steptoe