Bringing down Ze?

[Ross1]

Hi,

Im getting a Ze reading of 580 ohms on a TT installation, the existing earth to rod is either a 2.5 or a 4mm and im not sure of the route.

My question is if I run a 10mm earth (approx 35m) how much will it bring down the Ze?

 

[Octopus]

Silly question but why is the cable 35m long?

 

[Deleted member 26818]

Not by much.
You would do better installing a longer rod, or even extra rods.

 

[Ross1]

Silly question but why is the cable 35m long?

Because of the route Id have to take

 

[Ross1]

Not by much.
You would do better installing a longer rod, or even extra rods.

the existing rod is under a carpet in a conservatory, ican barely access the top of rod

 

[Ross1]

Does the Ze have to be under 200 ohms on a TT guys?

 

[IQ Electrical]

Does the Ze have to be under 200 ohms on a TT guys?

No but it has to be stable and that is generally accepted to be around 200 Ohms, meaning that degradation due to drying and freezing of the soil will struggle to push the value to an unacceptable level.

I always say this but give me a deep rod at 400 Ohms any day rather than a shallow one at 200 Ohms, depth is king!

If you work out the resistance of 35M of 2.5 6491-x you'll see that 99.9 % of your reading is the rod itself so that's where you need to concentrate your efforts.

 

[malcolmsanford]

I think you have what is called in the trade a right pig of a job .......................

Without seeing the install you have hobson choice really, go deeper with the one in the conservatory, which is unbelievable that you would build a conservatory around an earth rod, which might explain why you have such a poor reading, or find another location and try to fit another electrode(s)

 

[JLD]

Hi,

Im getting a Ze reading of 580 ohms on a TT installation, the existing earth to rod is either a 2.5 or a 4mm and im not sure of the route.

My question is if I run a 10mm earth (approx 35m) how much will it bring down the Ze?

One question, why are you running a 10mm2 earth conductor?

 

[malcolmsanford]

One question, why are you running a 10mm2 earth conductor?

Think it was in hope the bigger the CSA of the main earthing conductor, the lower the Ra would be.

 

[Sparky Ninja]

I'd try similar to Malcolm and opt for installing a different type of electrode.

 

[malcolmsanford]

Also may be worth a call to the DNO and see if there is a PME available somewhere.

 

[kenny7askew]

No but it has to be stable and that is generally accepted to be around 200 Ohms, meaning that degradation due to drying and freezing of the soil will struggle to push the value to an unacceptable level.

I always say this but give me a deep rod at 400 Ohms any day rather than a shallow one at 200 Ohms, depth is king!

If you work out the resistance of 35M of 2.5 6491-x you'll see that 99.9 % of your reading is the rod itself so that's where you need to concentrate your efforts.

my wife keeps telling me its the depth of the rod that counts lol

 

[IQ Electrical]

my wife keeps telling me its the depth of the rod that counts lol

Ha, I nearly put an (ooh matron) in there when I read it back!

 

[Engineer54]

Also may be worth a call to the DNO and see if there is a PME available somewhere.

That would be the best solution possible, if it's available and well worth a shot, that's for sure!! lol!!


Unless your installing a bare copper cable at least a metre deep, then forget it, it ain't gonna do a thing for you. The BRB says a 200 ohm Ra is good, ...I do not!! You need to go for as low a value as you can. The deeper you go the better your Ra is going to be, and will be stable. Remember, that first metre is going to be prone to seasonal weather conditions, so when frozen or bone dry, that first metre of rod may as well not be there.

Forget this existing rod in the greenhouse and and install/drive-in 2 or 3 extendable rods (min 5/8'' rods) Nothing to stop you connecting accross to the existing rod, but it being at over 500 ohms pretty pointless!! ... Test them, and if necessary use chemical ground rod enhancement, to increase the ground conductivity.

Unfortunately there's a bit more involved in creating a TT installation, than bunging in 1.2 m 3/8'' rod willy nilly and expecting it to provide a stable decent Ra value.