Acceptable Ra values

[Deleted member 9648]

Ok....I'm clearly in a minority on this. So to all the guys on here who install small scale TT systems some questions.
What value of Ra do you accept? What values of Ra do you aim for? How far do you go to achieve those values? If you cant achieve your desired value (ie values comparable to a TN system) would you refuse to connect a TT if your Ra meant you were reliant on an RCD for earth fault protection?

 

[imago]

True enough it wouldn't be necessary most of the time. I just think given the negligable cost involved in strapping steel which has to be there anyway would give a much greater level of flexibility.

The DNO's could use it as the first (or last depending on how you look at it) point of a PME. It's available for emrgency use if faults occur elsewhere. It could be used as a datum for comparisons. The main reason being it's a damn sight easier to put in at build than it is trying to drive rods in afterwards.

Belt and braces really, if the earthing arrangements are way better than they need to be then it allows for redundancy under fault conditions of any other part of the system. Which when you think about it is what earthing is all about in the first place.

 

[Engineer54]

Maybe the ground where you are is easier to get a decent Ra and these measures would be unnecessary. I saw a post recently where a 4 foot rod was giving an Ra of less than 50ohms, we wouldn't get even close to those kind of readings at four or five times that depth here.

It all depends where you are in the UK Marvo, there are very good areas and areas similar to your experience, though for different reasons. I'm sure there must be good areas in South Africa too, especially around salt marshlands, but maybe not in your part of the country...

Still i think it is indeed a cost effective measure on new properties in the UK. Not so much the earth mat's but certainly using the re-bar in a raft type foundations as a Ufer earth. Certainly better than having a short thin rod and a RCD ...lol!!!

 

[Deleted member 9648]

Most of the TT systems we install are remote pump and booster set locations. We don't do any domestic installation work.

Our goal is to obtain a stable Ra of 10ohms or better but we usually have the luxury of being able to install earth mats and re-bar bonding during the construction phase. Because of the nature of the installations there is no RCD protection nor is there any other services that would result in a better Ze so anything much > than 10ohms won't guarantee the effective operation of MCB's under fault conditions.

The UK domestic regs on this subject obviously are written around reliance of the RCD device for user safety. There's several factors that I would find worrying about this, the poor quality of many cheaper brands being one. If there's a 100mA rcd on the incoming plus a 30mA RCD in the CU for plug circuits plus a legal requirement for periodic testing then I wouldn't consider it unsafe. If it was my personal home I would however still be striving for a good stable Ra of 10ohms if it was achievable.

It all depends where you are in the UK Marvo, there are very good areas and areas similar to your experience, though for different reasons. I'm sure there must be good areas in South Africa too, especially around salt marshlands, but maybe not in your part of the country...

Still i think it is indeed a cost effective measure on new properties in the UK. Not so much the earth mat's but certainly using the re-bar in a raft type foundations as a Ufer earth. Certainly better than having a short thin rod and a RCD ...lol!!!

I've stated on here before that I believe a 100ma TD up front RCD and 30ma protection should be a requirement on all TT systems....not a single device as is currently permitted......if that was the case I honestly believe the 'short thin rod' is every bit as efficient as a more elaborate arrangement.

 

[Marvo]

I think there's a surprising amount of consensus emerging.

I would also agree that with new builds there would be almost zero extra cost to install an extra piece of steel in the foundations or a strap to any reference mesh in the slabs to allow any structural steel to assist the earthing using Ufer principals.

If the present UK regs allow for a single RCD then there would be scope for improvement to add a requirement for an upstream 100mA device just in the interests of not having all your eggs in one basket with a TT system.

I'm sure there must be good areas in South Africa too, especially around salt marshlands, but maybe not in your part of the country...

Many of the local areas we work in very sandy ground that drains very quickly. There's almost no salt content or organic material. Around the Cape coast the best and sometimes only way to achieve earth stability is to use rods that reach below the level of the water table which can be anything between 7 and 12 meters. Achieving 10ohms Ra can be a battle of wills using just rods. I can remember one a few years ago in particular where we used more than 45 x 2.3 meter rods on one installation and called it quits at 18 ohms. I can't describe the feeling of sheer despondency when you've spent two whole days already banging in rods by hand and you knock in another 5 rods on top of each other and get a reading of >300 ohms. It's enough to make grown men cry.

 

[GLENNSPARK]

Well certainly on domestic or small commercial you will be limited by the soil resistivity, so in some areas a reading above 200 may be reasonable with cost constraints. Banging in more rod's starts to have less effect and you need to decide as to what if anything you will achieve by doing so.

well i`m sorry chris but just banging in more rods aint worth the effort....you could however keep banging in the rod that you have origionally......deeper and deeper until your Ra comes down to within 200 ohms......oh and by the way....got sick n tired of hearing tony cable bleating on in the background at the elex...going on about 100 ohms max Ra........

 

[GLENNSPARK]

I think there's a surprising amount of consensus emerging.

I would also agree that with new builds there would be almost zero extra cost to install an extra piece of steel in the foundations or a strap to any reference mesh in the slabs to allow any structural steel to assist the earthing using Ufer principals.

If the present UK regs allow for a single RCD then there would be scope for improvement to add a requirement for an upstream 100mA device just in the interests of not having all your eggs in one basket with a TT system.



Many of the local areas we work in very sandy ground that drains very quickly. There's almost no salt content or organic material. Around the Cape coast the best and sometimes only way to achieve earth stability is to use rods that reach below the level of the water table which can be anything between 7 and 12 meters. Achieving 10ohms Ra can be a battle of wills using just rods. I can remember one a few years ago in particular where we used more than 45 x 2.3 meter rods on one installation and called it quits at 18 ohms. I can't describe the feeling of sheer despondency when you've spent two whole days already banging in rods by hand and you knock in another 5 rods on top of each other and get a reading of >300 ohms. It's enough to make grown men cry.

dont bang em in by hand then.....try using a decent rotary hammer on hammer only.......

 

[Marvo]

Some of the areas are remote and without power. It's cheaper here to use manual labour than hire plant and try to haul it up the hills. An unskilled labourer is very happy to go home with ZAR120.00 (<10GBP) per day.

 

[GLENNSPARK]

Some of the areas are remote and without power. It's cheaper here to use manual labour than hire plant and try to haul it up the hills. An unskilled labourer is very happy to go home with ZAR120.00 (<10GBP) per day.

well i suppose its work for someone over there innit but if i had to start brayin in a deep rod then i`m afraid i would reach for the rotary hammer......but maybe i`m just idle...lol....

 

[Chr!s]

Well certainly on domestic or small commercial you will be limited by the soil resistivity, so in some areas a reading above 200 may be reasonable with cost constraints. Banging in more rod's starts to have less effect and you need to decide as to what if anything you will achieve by doing so.

well i`m sorry chris but just banging in more rods aint worth the effort....you could however keep banging in the rod that you have origionally......deeper and deeper until your Ra comes down to within 200 ohms......oh and by the way....got sick n tired of hearing tony cable bleating on in the background at the elex...going on about 100 ohms max Ra........

Not sure which part of my post your referencing too Glenn? If its the part highlighted, i am making a reference to the % effect of parallel rods.

As for banging in rods, it varies, your deep rod may be getting driven into soils with even poorer soil resistivity, so in that instance more shallower rods would give better results. Unless you going to survey the ground its a little trial and error as to what will give the better results.

100, 200 ohms, in my opinion a well designed TT will usually be below 100 though some soil types will be problematic. A stable 200 ohms is no worse than a stable 100 Ohms. It will only be a benefit when the Ra allows some MCB'S to operate, though the use of RCDs in series negates the risk somewhat.