TT System Questions

[alarm man]

I theory perhaps, but not in reality, not in what the general consensus of what gets installed these days in the UK that professes to be a TT system!! Most are just a thin short spike, banged into the ground, more often than not in the worst locations possible!! lol!! Most of the year, it's virtually doing absolutly nothing!!! Not that 200 ohms will be doing much anyway...lol!!!

not necessarily i done one for a family member,2x 5/8 rods which is 8ft of decent copper into the ground with a mash which isnt easy work,conduit run for earth cable, set in a nice earth pit.just a rod in the ground makes it sound wrong..i wasnt allowed to test it because ive not done a di course,and i dont have the test equipment anyway..but it passed at 76ohm

 

[clicon357]

I wonder if many here knows what a NER is, and if they do know, would they know what it is actually doing, let alone where you would most commonly see them in use??

Thou this has nothing to do with TT systems.... lol!!

NER - something to do with ac generating kit from memory and reducing fault current through the star point............?

 

[Engineer54]

NER - something to do with ac generating kit from memory and reducing fault current through the star point............?

Yes, ...That's one of the most commonly seen uses for NER. Normally used on the larger permanently placed stand-by and prime power generator installations...

 

[Engineer54]

not necessarily i done one for a family member,2x 5/8 rods which is 8ft of decent copper into the ground with a mash which isnt easy work,conduit run for earth cable, set in a nice earth pit.just a rod in the ground makes it sound wrong..i wasnt allowed to test it because ive not done a di course,and i dont have the test equipment anyway..but it passed at 76ohm

I'm not saying ''ALL'' UK TT systems are crap, i'm sure there are many decent TT systems in place, created by decent electricians. But from what i have been reading on this and other UK forums, the general practice these days are to bang in a thin short earth rod (if indeed you can call them an earth rod) in the easiest and worst locations you can place a rod. If it's Ra is around 200 ohms, it's a good-un and then happily totally rely on the RCD(s) in the CU for earth fault protection. They probably don't even know that the first metre of that rod will be rendered useless when the ground freezes in winter, or dries out in the summer...

So which system would fail first in these conditions, ...haha, the TT system, every time!!!

 

[alarm man]

I'm not saying ''ALL'' UK TT systems are crap, i'm sure there are many decent TT systems in place, created by decent electricians. But from what i have been reading on this and other UK forums, the general practice these days are to bang in a thin short earth rod (if indeed you can call them an earth rod) in the easiest and worst locations you can place a rod. If it's Ra is around 200 ohms, it's a good-un and then happily totally rely on the RCD(s) in the CU for earth fault protection. They probably don't even know that the first metre of that rod will be rendered useless when the ground freezes in winter, or dries out in the summer...

So which system would fail first in these conditions, ...haha, the TT system, every time!!!

its either that or by the grace of god,a decent rod length should ensure protection all year round,ive never seen any problems in summer or winter,and i know of some farms where the water main freezes under ground so they leave a tap running all day and night,which is a pain when your metered..

 

[telectrix]

should let that tap run over the earth rod then.

 

[alarm man]

should let that tap run over the earth rod then.

it will freeze at the clamp,or in the hose.

 

[Engineer54]

its either that or by the grace of god,a decent rod length should ensure protection all year round,ive never seen any problems in summer or winter,and i know of some farms where the water main freezes under ground so they leave a tap running all day and night,which is a pain when your metered..

There is no problem with rods of any decent depth/length. But these thin short rods that you can't even extend are useless. They should be taken off the market, as there not fit for purpose, full stop!! lol!!

 

[Elmocash]

so ,can I fit an RCD incomer to a localy earthed (rod and structural steel) metalclad board that is fed by an SWA cable, supplied from a bussbar chamber, via a (metalclad) switch/fuse, all SWA, no 'tails' ?

 

[ackbarthestar]

There is no problem with rods of any decent depth/length. But these thin short rods that you can't even extend are useless. They should be taken off the market, as there not fit for purpose, full stop!! lol!!

There are several methods of creating a reliable earth path, assuming that the surrounding resisitvity of the soil is reasonably low.
One method as used by lightning engineers is to use a conductive cement and increase the csa of the earth rod. No need to use anything more than 15mm2 rods with a length of 1.2m.

 

[UKMeterman]

I did see TWO NERs in the 33 - 11 kV sub at the university...

 

[Engineer54]

We perhaps older guy's know there are several methods of creating a decent TT system!! The problem is, the younger sparks, ...that just seem to be playing at creating TT systems, because they know no different.

....What are you and other lecturers teaching these new students of today at collage, about providing a functional TT system?? As far as i can see, there being taught to totally rely on a RCD device for there earth fault protection, and only need one of these pointless 1-2m 3/8'' rods, that can't be extended even if you wanted
too.

The problem with your suggestion, is that you would need to drill a 50 to 75mm bore, that you can fill with the conductive cement, or bentonite mortar. Not particually cost effective on a domestic installation, as it would involve the hire off the drill and rig. Personally, i would consider 3m as the minimum depth, but 2 X 1.2m rods is i suppose better than One rod!!

 

[Chr!s]

I think the opinion is that at depths beyond 1.5m are not prone to seasonal variation, Certainly in this country.

Bentonite is a gel/slurry, create a pool and drive in your rod, it will pull the bentonite down, it will help a little but not much. Time is the best, the soil will consolidate over time reducing the Ra the most.

Marconite is an aggregate added to cement to create a conductive cement, costly as E54 has mention, cheaper to add several rods. Though in hard ground it does have its place.

I'd just couple 2 5/8 rods 1.2 or 1.5 long. This will put a minimum of 1 m out of the reach of seasonal variation. But as a minimum add at least one other, i tend to try and connect an electrode at every DB, along with extraneous conductive parts this soon reduces the Ra.

Just to add, a rule of thumb for parallel rod resistance percentage improvement.

Second rod 60%
Third rod 45%
Fourth rod 35%

Regards Chris

 

[Engineer54]

I think the opinion is that at depths beyond 1.5m are not prone to seasonal variation, Certainly in this country.

Bentonite is a gel/slurry, create a pool and drive in your rod, it will pull the bentonite down, it will help a little but not much. Time is the best, the soil will consolidate over time reducing the Ra the most.

Your description of Bentonite is completely wrong Chris, Firstly you do not just create a pool and have the earth rod take the slurry with it.... that would be a complete waste of time and of the bentonite. Who on earth taught you that method??

Secondly Bentonite is a recognised throughout the industry as a working enhancement of earthing electrodes and as such a proven system. Your description of '' it will help a little but not much'' is totally incorrect. Maybe in the installation method that you propose, but certainly not, when used correctly!! ie, ...in a bore or in a excavated trench etc!!
Used correctly Bentonite will fulfill it's function of substanually reducing the resistivity of the surrounding soil for probably the life of the installation, as it will continually absorb water/moisture and hold that moisture, it's also in it's own right, a conductive medium.

As for the other points you brought up in your post, ...Well there certainly better than most of the TT systems that you'll find in the UK, ...that's for sure!! lol!!

Depth equals stability first and reduction of Ra values secondly. Both are important factors in creating a ''Functional'' TT system....

 

[Chr!s]

Your description of Bentonite is completely wrong Chris, Firstly you do not just create a pool and have the earth rod take the slurry with it.... that would be a complete waste of time and of the bentonite. Who on earth taught you that method??

Well, it's a method i've used, suggested by several chartered engineers who work for two of the largest Earthing solutions company's in the UK. where drilling and back filling with Bentonite would be to costly/time consuming. As i said create a pool and drive in the rod keep pouring the mixture and this will be drawn down with the rod, this will fill most voids around the rod and lower its resistance. Jobs a good un

Secondly Bentonite is a recognised throughout the industry as a working enhancement of earthing electrodes and as such a proven system. Your description of '' it will help a little but not much'' is totally incorrect. Maybe in the installation method that you propose, but certainly not, when used correctly!! ie, ...in a bore or in a excavated trench etc!!
Used correctly Bentonite will fulfill it's function of substanually reducing the resistivity of the surrounding soil for probably the life of the installation, as it will continually absorb water/moisture and hold that moisture, it's also in it's own right, a conductive medium.

Not disagreeing if used as a back fill, though there is a limit om the radius and its effectiveness. Though certainly in many domestic situations i don't see many drilling holes LOL. The method i mention is relatively effective without to much hassle. Though i would probably use Marconite if i was trenching, it doesn't rely on water, Bentonite can dry out.

As for the other points you brought up in your post, ...Well there certainly better than most of the TT systems that you'll find in the UK, ...that's for sure!! lol!!

Depth equals stability first and reduction of Ra values secondly. Both are important factors in creating a ''Functional'' TT system....

Regards Chris