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?

 

[VoltzElectrical]

go by the book. if it's under 200 ohms, connect it up. max value is 1667 ohms for Zs. Usually a lot lower when bonding connected anyway.

 

[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.

 

[Deleted member 9648]

Pretty much my way of thinking,despite the scorn from those who dont accept such values. Presumably those who think this way would refuse to connect a TT system with values exceeding TN levels as they dont believe an RCD can safely provide earth fault protection. So an Ra of say 10 ohms....pretty low by most standards should still not be acceptable to them as it would result in a pefc of 23a on a 230v system,which will not operate most OCPD's within a reasonable time,if at all.
in effect you still rely on the RCD...so whats the difference between an Ra of 10 ohms against an Ra of 200 ohms when it comes to meeting disconnection times?
This is why I have an issue with those who poor scorn on the Bs 7671 acceptable values of Ra but seem to think going to great legnths to achieve an Ra of say 10 ohms is worthwhile....it doesnt stack up. .....Ok an Ra of <1 ohm....brilliant....but I'll wager most of the multiple rod brigade are not getting anywhere near this and are still reliant on the RCD despite all their efforts.

 

[telectrix]

and just to muddy the water, when you connect bonding (gas/water) to a TT system that may have an Ra of 100+ ohms, this may well bring the effective Ze down to below 1 ohm.

 

[Chr!s]

The Key is to have a system which is stable and sound, whether it is at 10 ohms or 300 ohms.

 

[Engineer54]

Yes your absolutely correct, i do pour scorn on the stated 200 ohm max Ra as stated in BS7671. Apart from being totally useless, it gives a false impression that this is all you need to aim for, which is NOT what anyone should be aiming for on any TT system. The whole section needs a complete overhaul!! When i first came into this industry the figure every electrician aimed for was indeed 10 ohms. But because in those days the rods were between 2.3 m and 3m, by coupling 2 rods, far better values were often achievable. Now electricians are fooled into thinking that a cure-all RCD is all that is needed, instead of creating the best possible TT installation within the confines of what is presented to them... It doesn't even occur to them that the TT system they provide, should have a minimum of a 30 year lifespan and often longer...

My objection to the OP, is his advise to newbies of NOT even TRYING to get decent Ra levels, which will basically leave any UK TT system without any stability. The common practice of just bunging in a 1.2m 3/8'' rod willy nilly is NOT by any stretch of the imagination bona fide TT system, at best it's a spike....

Of course you don't want to be spending too much money and time, on a domestic TT system. Then again, I don't think coupling a minimum of 2 5/8'' rods together is excessive by anyone's standards, which is basically what i've been stating in most cases here. But not even TRYING, is most certainly not what we should be promoting on this forum...

 

[Deleted member 9648]

You miss the point.....what use is 10 ohms?...as far as I can see in terms of disconnection times it's no better than 200 ohms. It's not a case of not trying....I will only try (as you put it) when I can see a reason. One TT I did last year on temporary site buildings will serve as an example.
The install was halfway up the south downs....chalk rock. One standard rod produced an Ra of around 300 ohms...the ground was bone dry in an exceptionally dry spring. Clearly one rod was not going to serve so an additional long 5/8" rod was driven in and linked and brought the Ra down to around 120 ohms (still useless by your reckoning). I returned twice over the next 9 months to carry out PIR's...(consruction site) and the Ra stayed at around 120 ohms each time. What advantage would there have been in achieving 10 ohms?.....none as far as I can see.
It's got nothing to do with NOT TRYING.....I make a judgement on the Ra and it's compliance with Bs 7671 and stand by that.
On another thread you stated it's just a case of 'banging in a rod and hoping for the best'.....how can installing a rod in compliance with 7671 and measuring it's resistance in compliance with 7671 be 'hoping for the best'?

 

[Engineer54]

Your only reading into my posts what you want to read...

I've always made a point of saying that everything depends very much on what ground type your dealing with. In some circumstances it's going to be an expensive exercise to get a decent value at a domestic installation, in those circumstances, where it's going to be totally uneconomic to continue, then it WILL be a case of relying on RCDs, and i say RCDs, because i would then install an additional up front S type RCD...

Correct, I don't know how many times i have to say this, but the section covering TT systems/installations in BS7671 is a crock of crap!!! I don't know of any other international National code or regulation that Set's it's max Ra level so high, or is so wishy washy in it's requirements!! It needs a complete overhaul end of....

As for your previous advice to newbies concerning ''Not Bothering To Even Try'' ...what you are saying above, was Not the impression you were giving, in those posts!! In one case i remember, a guy that got a reading of 40 ohms on a single 5/8'' rod, (I think that was a farm installation) and when i advised to couple a further rod, with a more than better chance of considerably lowering his final Ra value and importantly adding to the stability of the system, you piped up with your usual don't bother statement it complies with your magical 200 ohms rubbish....

Now, if you consider that i'm wrong in encouraging those out there, that have little to no experience in creating a TT system, to achieve the best they can in there circumstances. Then i'm sorry, but i'm not going stop trying to give the best advice i can...

As for your other argument, At the very least a 10 ohm Ra that is a ''Stable'' 10 ohms Ra, is going to ALWAYS be better than a 140/200 ohms or more rod that is going to be ''Unstable....''

One last point here, your always trying to make out that achieving a near TN earthing value is close to being impossible without extensive amounts of copper in the ground, that is again nowhere near always the case... Impossible in some areas of the UK i agree, but certainly achievable in many other areas within the UK!!

 

[needasparks]

I think it is quite hard to change the way of thinking for a TT as we rely on all voltage going safely the earth quickly enough for a fuse to break on TN systems where as with TT we only need enough for the rcd to operate which is as we all know a leap of faith will some kit. Just reverse thinking i suppose, dunno

 

[Deleted member 9648]

I've already given an example of adverse ground in which an Ra considerably higher than 10 ohms proved perfectly stable.

 

[Marvo]

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.

 

[Engineer54]

I've already given an example of adverse ground in which an Ra considerably higher than 10 ohms proved perfectly stable.

Yes, but you used 2 rods and the depth gave you the stability whereas one rod would probably not have!! And as you stated, the ground conditions were going to prove difficult to better...

 

[imago]

we usually have the luxury of being able to install earth mats and re-bar bonding during the construction phase.

I don't know why that hasn't been introduced years ago here as part of the building regs to be honest. The additional costs in materials and time are such a tiny percentage of the build costs. It would be easy enough to include it when any steel framing goes up for extensions, or when the BRC is put in for raft floors etc etc.

 

[Marvo]

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.