Earth Loop Impedance on a TT system

[PEM]

I work on TT a lot in the depths of Suffolk/Norfolk. I'd recommend a longer rod in addition to the existing. I've even used 3 to bring Readings down to acceptable levels.
Regards

 

[Lee Marshall]

either put a longer rod in or another one, or two, or three lol

 

[Buckfast]

The problem you have encountered is a common scenario and sticking rods in deeper and deeper is generally not the answer. As you have stated the reading is at this value when ground saturated and will rise when ground dries (Summer). A 30mA rcd will work upto a Zs of 1666.66 ohms but obviously a value below 100 ohms would be what i would be expecting. Look at Furse who are the earthing mat specialists. They make a copper matt approx 1 meter square which when buried to a depth of a meter (dependant on ground) will drop earth reading. Generally i can obtain between 33 and 80 ohms in poor conditions.

 

[Engineer54]

The problem you have encountered is a common scenario and sticking rods in deeper and deeper is generally not the answer. As you have stated the reading is at this value when ground saturated and will rise when ground dries (Summer). A 30mA rcd will work upto a Zs of 1666.66 ohms but obviously a value below 100 ohms would be what i would be expecting. Look at Furse who are the earthing mat specialists. They make a copper matt approx 1 meter square which when buried to a depth of a meter (dependant on ground) will drop earth reading. Generally i can obtain between 33 and 80 ohms in poor conditions.

So you think a 1m square latice mat at a depth of 1m will be better than a deep driven rod do you, ....somehow I don't think so!!

He's more than probably got a thin twig as his earth electrode Which are the next best thing to being totally useless and more likely than not, positioned right next to an external wall of the house where all the old building rubbles is going to be!!

He's best bet, is to use at least 2 X 5/8'' rods coupled together for stability and a much improved Ra value!! Positioned at least 1.5m away from any external house walls. If the ground has a poor conductive value then there are soil conditioners that can be applied, that can/will also lower Ra values....

 

[Knobhead]

21 ohms is the value of the DISTRIBUTORS earth electrode at the supply transformer , 200ohms is the maximum value of the CONSUMERS earth electrode (which is the one I'm talking about)

As my first post said, I'm asking folks who have experience in this, if a longer rod would bring the result down enough, or if another method my be needed.

Where did you get that figure for supply transformers from?

 

[Richard Burns]

Where did you get that figure for supply transformers from?

It is given in the introduction of the OSG as the maximum stated resistance!

 

[Knobhead]

That's not worth the paper it's written on. The supply companies don't read the OSG. Greater than one ohm would cause problems with MV/LV earthing arrangements.

 

[Deleted member 9648]

Afternoon all,

I did a standard Ze test on a TT installation yesterday, and got a value of 215 ohms. The ground is really wet at the moment and so I'm assuming its about as good as the rod is going to get.

Would simply replacing the rod with a longer type improve the Ze result? and if not, what methods are best to reduce the result?

Many Thanks.

The problem you have encountered is a common scenario and sticking rods in deeper and deeper is generally not the answer. As you have stated the reading is at this value when ground saturated and will rise when ground dries (Summer). A 30mA rcd will work upto a Zs of 1666.66 ohms but obviously a value below 100 ohms would be what i would be expecting. Look at Furse who are the earthing mat specialists. They make a copper matt approx 1 meter square which when buried to a depth of a meter (dependant on ground) will drop earth reading. Generally i can obtain between 33 and 80 ohms in poor conditions.

Interesting that once again people seem to think that 'between 33 and 80 ohms' is the dogs danglies....whats 33-80 ohms going to do that 215 wont?
Anyway....:nonod:
.......Regarding the OP.....I had a rod on an EICR recently on a separated earth on a small sewage plant with an Ra of 700+ ohms. Two coupled 4ft rods and it went down to around 13.....so there's your answer.
(that said though whats 13 ohms going to do that 700 wont in practice....a 30ma rcd will trip just the same on both readings,the OCPD will not trip on either reading)

 

[Buckfast]

So you think a 1m square latice mat at a depth of 1m will be better than a deep driven rod do you, ....somehow I don't think so!!

He's more than probably got a thin twig as his earth electrode Which are the next best thing to being totally useless and more likely than not, positioned right next to an external wall of the house where all the old building rubbles is going to be!!

He's best bet, is to use at least 2 X 5/8'' rods coupled together for stability and a much improved Ra value!! Positioned at least 1.5m away from any external house walls. If the ground has a poor conductive value then there are soil conditioners that can be applied, that can/will also lower Ra values....

As i am sure you are well aware 2*5/8ths rods no doubt will reduce the earth loop impedance value but think you are grasping at straws. A 'Furse' copper lattice mat will have at least 20 to 30 times the conductive surface area to ground than rods. This is what is required to lower resistivity. Sorry but you are mistaken in this matter.

 

[Buckfast]

Interesting that once again people seem to think that 'between 33 and 80 ohms' is the dogs danglies....whats 33-80 ohms going to do that 215 wont?
Anyway....:nonod:
.......Regarding the OP.....I had a rod on an EICR recently on a separated earth on a small sewage plant with an Ra of 700+ ohms. Two coupled 4ft rods and it went down to around 13.....so there's your answer.
(that said though whats 13 ohms going to do that 700 wont in practice....a 30ma rcd will trip just the same on both readings,the OCPD will not trip on either reading)

I fully agree that the Rcd will trip on both 33 to 80 ohms and also 215 ohms but do not understand the arrogant remark ' dogs danglies'. Maybe better to give this a little further thought. Why is 33 to 80 better than 215 ohms ?. Maybe this will give an increased assurance that electrode will not rise above acceptable levels. Do not wish to get in to slagging match over this. Designing and installing these on a regular basis. Earth lattice matt over earth electrode will win hands down every time.

 

[Engineer54]

Interesting that once again people seem to think that 'between 33 and 80 ohms' is the dogs danglies....whats 33-80 ohms going to do that 215 wont?
Anyway....:nonod:
.......Regarding the OP.....I had a rod on an EICR recently on a separated earth on a small sewage plant with an Ra of 700+ ohms. Two coupled 4ft rods and it went down to around 13.....so there's your answer.
(that said though whats 13 ohms going to do that 700 wont in practice....a 30ma rcd will trip just the same on both readings,the OCPD will not trip on either reading)

And there you have it, as i said earlier, the DO NOTHING brigade is here!! lol!!

 

[Engineer54]

I fully agree that the Rcd will trip on both 33 to 80 ohms and also 215 ohms but do not understand the arrogant remark ' dogs danglies'. Maybe better to give this a little further thought. Why is 33 to 80 better than 215 ohms ?. Maybe this will give an increased assurance that electrode will not rise above acceptable levels. Do not wish to get in to slagging match over this. Designing and installing these on a regular basis. Earth lattice matt over earth electrode will win hands down every time.

Believe me, you couldn't be more wrong if you tried!! When talking about TT earth electrodes, Depth always Rules supreme!! lol!! So unless your contemplating burying a 1m X 1m lattice mat at at 2.4m / 3.2m (your welcome to try!! lol!!) forget it!!

 

[telectrix]

my old man used to grow lattice for salads. they were always above ground, never buried. :stupid:

 

[Sharpend]

Only you can determine the condition of the ground and therefore make best judgement as to whether going deeper is better or more rods will be as effective. If the ground is poor conductivity for three metres and your are only putting rods to a depth of two meters then I doubt the reading will improve vastly, but put an additional Rod in slightly further afield then you may have much better quality ground therefore better earth reading.
The general rule is to get the largest surface area in as good ground as possible.

 

[telectrix]

The general rule is to get the largest surface area in as good ground as possible.



thought that was fred west's forte.

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