Earth Loop Impedance on a TT system

[roukel01]

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.

 

[Baker1988]

Can you connect another rod on top and put it deeper that way or maby get a thicker rod

People say that any thing under 1667 ohms is good enough as the 30mA RCD will still operate in time (I don't really agree with this but that is what I am told)

 

[roukel01]

Can you connect another rod on top and put it deeper that way or maby get a thicker rod

People say that any thing under 1667 ohms is good enough as the 30mA RCD will still operate in time (I don't really agree with this but that is what I am told)

I thought the result had to be under 200 ohms or it wasn't deemed a stable result?

I was planning on putting a thicker/longer rod in, I just wanted to make sure this would make enough difference to get the result down a bit

 

[Baker1988]

Well even if it was 180 ohms That would only make a fault current of 1.3 ohms give or take (230/180 = 1.27777) so that fault current would not trip any MCB any way so 200 is a bad reading it should be as low as possible about 21 ohms but even that wouldn't trip much

But as I said if you have a RCD then that will provide your fault protection protection I'm not saying that you should only rely on the RCD but its better then nothing.

I would try a longer rod and put it deeper

 

[roukel01]

Well even if it was 180 ohms That would only make a fault current of 1.3 ohms give or take (230/180 = 1.27777) so that fault current would not trip any MCB any way so 200 is a bad reading it should be as low as possible about 21 ohms max.

But as I said if you have a RCD then that will provide your fault protection protection I'm not saying that you should only rely on the RCD but its better then nothing.

I would try a longer rod and put it deeper

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.

 

[Baker1988]

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.

lol ok but i have worked on a few tt systems and a longer rod buryied deeper usualy will bring the resistance down, i cant think of any other way that will

 

[Baker1988]

i know 200 is the max i just think its a bad reading and as far as fault currents go 200 ohms is not going to do any thing to the overcurrent device

 

[roukel01]

Its a TT system, so all circuits are protected by RCD for fault protection.

I can't do alot about the result! I'm just trying to reduce it below the stated 200 ohms as I think once the ground dries up a bit, the result will increase quite a bit

 

[Engineer54]

There are plenty of threads here on this question, and no matter what i'd advise you to do, there will be the same people that will tell you not to bother doing anything!! The choice is yours, make the effort to do things properly to improve this existing TT installations stability and Ra values, or throw the hat in ring before you start and rely on the existing RCD!!

 

[Baker1988]

well i would definatly try a longer rod and bury it deeper as that is what has worked for me in the past

 

[roukel01]

There are plenty of threads here on this question, and no matter what i'd advise you to do, there will be the same people that will tell you not to bother doing anything!! The choice is yours, make the effort to do things properly to improve this existing TT installations stability and Ra values, or throw the hat in ring before you start and rely on the existing RCD!!

This is what I'm worried about, if a longer rod is only going to reduce the Ze result by an ohm or two then I'm not going to bother, I just wondered what peoples experiences were on replacing existing rods with longer ones.

 

[jaydee81]

definately install longer rod 30ma rcd all circuits and 100ma time delay rcd as main switch

 

[Baker1988]

This is what I'm worried about, if a longer rod is only going to reduce the Ze result by an ohm or two then I'm not going to bother, I just wondered what peoples experiences were on replacing existing rods with longer ones.

a longer rod should lower the value significantly the last one i did lowered the value by about 80

 

[Fast&Rough]

We usualy just pish on it till the reading is acceptable job done

 

[roukel01]

We usualy just pish on it till the reading is acceptable job done

Thats the trouble, ground is already wet through!!