Zs on a TT installation

[Rob69]

hi all, i have just changed a couple of socket for a customer and got a Zs of 223 ohms which is good.Then i went to CU and noticed main earth only 4mm and in a bad state so did method 2 and got no reading at all!!!!!!!Also no bonding to water service!!!!!!!!!

How am i getting a Zs with no main earth could it be via the plumbing pipe work or what??????? its confusing me a bit!!!!!!!!

 

[matt.leung]

is the gas bonded?

 

[m4tty]

+1 most prob through the gas. Have you tested ra?

When I tt'd my dads house for assessment ra was 30 ohms but some zs were 0.3 as the water/gas was a much better earth than the rod.

 

[Andy™]

hi all, i have just changed a couple of socket for a customer and got a Zs of 223 ohms which is good.Then i went to CU and noticed main earth only 4mm and in a bad state so did method 2 and got no reading at all!!!!!!!Also no bonding to water service!!!!!!!!!How am i getting a Zs with no main earth could it be via the plumbing pipe work or what??????? its confusing me a bit!!!!!!!!

Zs ia 223 is well within limits of RCD's, however, not something i would consider 'good'. any Ra over 200 should be considered unstable. now either your R1R2 is 23 ohms, or the rod could be unstable.

your Zs includes all parallel paths via any pipework etc, so Zs will usually be lower than Ra

 

[a1spark]

Ditto , as Andy says above. I wouldnt exactly describe Zs [Ra} of 233 ohms as good.

 

[Rob69]

Thanks all,

No , there is no bonding to gas or water!!!!!!!

MY R1R2's varied between 1.23ohms (lights) to 0.16ohms for immersion.

Can someone just confirm how to test for Ra is it not effectively Ze???

Dont forget i have no continuity from CU to earth rod!!!!!!!

Could it pick up an earth through the cylinder immersion via cpc?????

Thanks again Rob

 

[tom1]

Thanks all,No , there is no bonding to gas or water!!!!!!!

MY R1R2's varied between 1.23ohms (lights) to 0.16ohms for immersion.

Can someone just confirm how to test for Ra is it not effectively Ze???

Dont forget i have no continuity from CU to earth rod!!!!!!!

Could it pick up an earth through the cylinder immersion via cpc?????

Thanks again Rob

ze and ra, same test mate,

but it wont be any good if your earth is not connected to the rod.

have you traced were this 4mm earth is going

 

[kme]

"There is no bonding to gas or water"

is not the same as

"There is main equipotential bonding to gas or water."

In reality, there are almost always "paths" back to the pipework through cpcs, immersions etc - this is where your Zs is coming from, I would presume.

The Ra doesn`t exist, if the rod isn`t connected - but you can still test the rod for a reading (method 2, with a wander lead to the rod )

233 with no earth conductor and no main bonding is, IMO unsafe. This should have been spotted, and rectification work agreed, BEFORE changing the sockets, ideally.

KME

 

[tom1]

"There is no bonding to gas or water" is not the same as

"There is main equipotential bonding to gas or water."

In reality, there are almost always "paths" back to the pipework through cpcs, immersions etc - this is where your Zs is coming from, I would presume.

The Ra doesn`t exist, if the rod isn`t connected - but you can still test the rod for a reading (method 2, with a wander lead to the rod )233 with no earth conductor and no main bonding is, IMO unsafe. This should have been spotted, and rectification work agreed, BEFORE changing the sockets, ideally.

KME

i would go with that and if the reading is good get some cable to it.

 

[SPECIAL LOCATION]

Ditto , as Andy says above. I wouldn't exactly describe Zs [Ra} of 233 ohms as good.

As highlighted by Andy, A1spark and others...

Ra >200ohms not considered good...

See second paragraph of 4.8 Earth Electrode

page 31 On Site Guide.

AND

Table 41.5, Note 2:

Page 50 Big Red Book!

233ohms can be acceptable but not good...

best to try and improve Ra less than 100ohms IMHO.

 

[Rob69]

Thanks for all your replies!!!!!!!!!

You have all been a great help!!!!!!!!

Did tell the customer and am going back to fit all bonding and fully test!!!!

Thanks again all.

Rob

 

[la poste]

I am doing a TT system where the water pipe was bonded in the loft by a very old earth wire.

Although there was no bonding at the entry points, the water was bonded further down the line, I got a reading of 15 ohms earth loop impedance at the board.

when I disconnected and tested the earth rod alone I got 500 ohms and coudn't work out why I was still getting 15ohms earth loop impedance at the board with the spike disconnected.

It turned out it was the water pipe being bonded in a very unusual and hard to find place in the loft.

 

[batty]

I am doing a TT system where the water pipe was bonded in the loft by a very old earth wire.Although there was no bonding at the entry points, the water was bonded further down the line, I got a reading of 15 ohms earth loop impedance at the board.

when I disconnected and tested the earth rod alone I got 500 ohms and coudn't work out why I was still getting 15ohms earth loop impedance at the board with the spike disconnected.

It turned out it was the water pipe being bonded in a very unusual and hard to find place in the loft.

Will be a parallel earth path through the water pipe I personally would try getting a better reading on rod by knocking another one in what size is main earth cable. Welcome to the forum.

 

[la poste]

Will be a parallel earth path through the water pipe I personally would try getting a better reading on rod by knocking another one in what size is main earth cable.

The earth rod has been installed in a hurry and not very well by a previous electrician and is not even on the clients property, it's in next doors garden.

I will dig a new pit and fit a new rod complete with inspection cover.

The cable is 10 mil so I will need to up that to sixteen.

Welcome to the forum.

Thanks.