TT system bad earth help

[Deleted member 9648]

Out of interest....if somebody was to install an earth rod on a TT and the Ra was bang on 200 ohms, then years later you tested the earth rod at an Ra of say 280 ohms with the install value unknown to you, what would your opinion be of the person that installed it.....?

I would consider the ground conditions at the time,if it was high summer and dry ground,it is unlikely to deterorate further and will probably be at it's worst,if the ground was wet there may be cause for concern. But I would not panic as some appear to on here...considering the prime and preferred earth fault protection device will provide protection up to 1667 ohms and if it has an 100ma up front device as well then up to 500 ohms would meet the requirements.....An Ra of 280 ohms on a properly designed TT system with an S type 100ma main switch and 30ma protection to all circuits would provide exactly the same protection as an Ra of 30 ohms.
Discuss if you want.

 

[Paul505]

I would consider the ground conditions at the time,if it was high summer and dry ground,it is unlikely to deterorate further and will probably be at it's worst,if the ground was wet there may be cause for concern. But I would not panic as some appear to on here...considering the prime and preferred earth fault protection device will provide protection up to 1667 ohms and if it has an 100ma up front device as well then up to 500 ohms would meet the requirements.....An Ra of 280 ohms on a properly designed TT system with an S type 100ma main switch and 30ma protection to all circuits would provide exactly the same protection as an Ra of 30 ohms.
Discuss if you want.

Your right a 30mA RCD will protect right upto 1667 ohms....
Maybe people naturally worry about the stability around the 200 ohm value, due to the fact the regs states the Ra becomes unstable >200 ohms, so they dont want to be close to that 200 value even though there is a buffer upto 1667 ohms....
Good debate anyway :yesnod:

 

[La Poste]

Just one thing I'd like to discuss about extra deep rods and low Ra is the touch voltage in the event of an earth fault.

I mean the deeper the rod and the lower the Ra the lower the touch voltage would be if you were to touch a piece of bonded metalwork during an earth fault.

As far as the calculations go for the exact touch voltage I am not sure and someone with a bit more knowledge than me would probably be able to explain better.

Or am I barking up the wrong tree?

 

[Engineer54]

I agree totally with the single RCD issue,and have stated on here many times that I believe an up front 100ma s type should be a requirement on aTT.
My argument is simply that even at 10 ohms you are still reliant on an RCD....so what is the difference in practice between 10 ohms and 200 ohms?...the RCD will still operate exactly the same at both values. I also dispute the point that only lower Ra's are stable.....having installed maintained and tested many at 100-200 ohms I can assure you that they remain stable in my area regardless of season.
As far as I'm concerned unless you are going for TN values of Ra there is no point...and smugly stating 10-30 ohms is what should be aimed for and anything else is unprofessional is simply snobbery.

Yes i'm aware that you agree with the RCD policy on TT systems!! I'm not going to get into another head banging discussion, it's all been said before ...many times over!!! As for the stable 200 ohm TT systems you talk about, yes they can be stable given the depth they are driven too, and agree there are areas, where it's going to be uneconomical on a domestic situation to bang in more than the 2 X 5/8'' coupled rods because of the poor conductive ground conditions. Though on an Industrial or large commercial installation it wouldn't be uneconomical or acceptable to leave a TT system at 200 ohm Ra levels... I would also confidently state, that the vast majority of 200 ohm UK TT installations are anything like being stable systems...

Contrary to what you believe, there is nothing unprofessional, Smug or Snobbish about trying to obtain the best stable Ra level that you can reasonably achieve. What IS unprofessional, smug, and snobbish, is someone that advises others, to not to even bother aiming for these very achievable lower Ra values.

As a footnote, i gave my own TT system a through testing last Wednesday, and it is now very close to TN values and i can assure you the ground conditions here, are far and away from being anything like good!! My system values has been constantly improving over the years, as will most TT systems as the ground slowly consolidates around the electrodes. It's not rocket science, but knowing what your doing and with attention and determination, it can go a long way in what you can achieve...

 

[SPARTYKUS]

Is that in China E54?

 

[RogertheBodger]

Eng54 and Wirepuller.

You guys, Lol. I do love reading the various installments on this debate between you two, admire both your professionalism and the fact that you keep it polite.

Can't help thinking that you are both on the same page really, maybe one reading from the top and the other from the bottom, anyway, the argument is informative and entertaining.

One thing I always get to thinking about during this debate is how safe were TT systems in the days of rewireable fuses and before RCDs came on the scene.

If it was mostly impossible to get a good enough Ra value, say <10 Ohms, there would not be enough fault current to melt the fuses. At the then nominal voltage of 240v, 24Amps is all you would get, not enough to take out anything above 10A fuse wire? to take out a 30A fuse wire you would need what? 45A. (getting hard to remember now lol) and for that you would need an Ra of just over 5 Ohms.

So it was extremely important to get as low an Ra value as possible, or you would have no earth fault protection, and so it should still be, if the RCD fails at least you still have some measure of protection, if you can't get it down because of ground conditions, then you can't, good job we have Rcds. We do rely on Rcds today, with the laughable figure of 200 Ohms stated in the Regs - we have to.

Soooooo, with respect to the ongoing debate, I think you are both correct.

 

[La Poste]

Let's continue the debate in terms of touch voltage.

I mean the lower the Ra the more current will travel through the ground and the less current will flow through you if you touch a bit of bonded metal work under earth fault conditions.

Surely this is another good argument for as low an Ra as possible.

 

[RogertheBodger]

Let's continue the debate in terms of touch voltage.

I mean the lower the Ra the more current will travel through the ground and the less current will flow through you if you touch a bit of bonded metal work under earth fault conditions.

Surely this is another good argument for as low an Ra as possible.

Yes it's a good argument La Poste, but bear in mind the upper limit of 1667 Ohms Ra.

You would calculate this by using Ohms Law, the value of current is 30mA (RCD) and the touch voltage is 50v..... (your post 33).

So, for example, if you had an Ra of 833Ohms, the voltage would be 25v.

Is this what you are wanting to know?

 

[La Poste]

OK let's say the RCD failed and you touched a piece of live metalwork.

The current you would receive and hence the shock would depend on how much of this fault current was flowing through you and how much was flowing through a low resistance path back to the transformer winding IE through the rod, through the earth, through the transformers own rod and also any other parallel paths that are available.

So you see what I am getting at, if the RCD failed then the amount of current you would receive would be dependent on the value of the resistance of the path from the fault back to the star point of the transformer, your own earth rod forms a part of this route and hence the lower the reading of Ra the lower the amount of current that would flow through you (electricity always takes the easiest low resistance path).

If the fault current has a nice easy route through the ground via a very deep earth rod then it is less likely to want to find a path back to the star point through you and hence you would receive less of a shock in the event of an earth fault.

I am sure with the right maths you could calculate exactly what this shock current would be but I'm just too knackered to try this evening.

 

[Piratepete]

My system values has been constantly improving over the years, as will most TT systems as the ground slowly consolidates around the electrodes.

I have to disagree with the latter part of this statement! All the TT systems I have come across in this area (Warks) have deteriorated over the years. In some cases it's due to the reduction in height of the water table and in others the lack of maintenance of the rod, cabling and connection to the rod.
I've just had one house at 350 ohms PMEd -the rod was buried under concrete!
At another, I found the rod lying horizontally on the flower bed behind the dog kennel - didn't get too good a reading off that. On another you could slide the clip up and down the old piece of waterpipe that had been banged into the ground 60 years ago.
LOL
Pete

 

[RogertheBodger]

OK let's say the RCD failed and you touched a piece of live metalwork.

The current you would receive and hence the shock would depend on how much of this fault current was flowing through you and how much was flowing through a low resistance path back to the transformer winding IE through the rod, through the earth, through the transformers own rod and also any other parallel paths that are available.

So you see what I am getting at, if the RCD failed then the amount of current you would receive would be dependent on the value of the resistance of the path from the fault back to the star point of the transformer, your own earth rod forms a part of this route and hence the lower the reading of Ra the lower the amount of current that would flow through you (electricity always takes the easiest low resistance path).

If the fault current has a nice easy route through the ground via a very deep earth rod then it is less likely to want to find a path back to the star point through you and hence you would receive less of a shock in the event of an earth fault.

I am sure with the right maths you could calculate exactly what this shock current would be but I'm just too knackered to try this evening.

You have a rest while we think about it lol.

TT system, the RCD is not working, if the Ra is not enough to trip the mcb and you have hold of this circuit, then I am afraid you are in trouble.

 

[GLENNSPARK]

just another thing here n all.....you say that the C/U looked in poor condition so you conducted an Ra test.....so if the C/U had been in good condition would you still have conducted an Ra test?....

 

[La Poste]

Then I am afraid you are in trouble.

The story of my life. :8:

I'm just wondering if this discussion about low Ra values is purely about bringing stability to the system or if there is a touch voltage element involved in this as well.

Are we looking at low Ra values to make the system more stable or are we looking at low Ra values to reduce the shock current we might receive in the event of a fault?

 

[Engineer54]

I have to disagree with the latter part of this statement! All the TT systems I have come across in this area (Warks) have deteriorated over the years. In some cases it's due to the reduction in height of the water table and in others the lack of maintenance of the rod, cabling and connection to the rod.
I've just had one house at 350 ohms PMEd -the rod was buried under concrete!
At another, I found the rod lying horizontally on the flower bed behind the dog kennel - didn't get too good a reading off that. On another you could slide the clip up and down the old piece of water pipe that had been banged into the ground 60 years ago.
LOL
Pete

Yes you have a valid point, if the ground conditions on a rod location change for the worse, such as drying out. But under normal conditions, deep driven electrodes will generally always improve over a given time, as the soils consolidate around the bare electrode. Especially when coupled rods are involved, as the coupler has a slightly larger diameter than the electrodes, so any rod(s) after the coupler(s), will not be in solid contact with the surrounding soil directly after installation...

How many electricians maintain the rods that they have installed?? I'd say virtually none, certainly not on domestic installations!! Come to that, how many present day electricians know how to maintain a TT system?? Again, i would have to say Not Many, most wouldn't even consider the need for maintaining a TT system!!!

 

[Engineer54]

Is that in China E54?

No, it's my house in Cyprus. I'd say that around 95% or so, of all installations in Cyprus, are TT systems here. Plus most supplies will be from overhead networks, apart from those in the built-up towns and cities where they have been converting to underground distribution.

Generally speaking, only installations that are directly fed from an in situ distribution TX such as in a commercial building or a large block of apartments will have a TN-S supply...