IT Earthing

[EddieB]

Hi and Happy Christmas to you all.

I've a customer with an ITsupply and a metal earth spike outside,I've measured the impedance and it was in excess of 1000 ohms, I cleaned the top of the spike and renewed the clamp this improved the reading to 200 ohms. I was around there the other day and it's risen to over 1600, I called into the neighbour and they have had a newish spike installed in copper and this also was in excess of 1600 ohms. any ideas on how to improve this,and what could the cause of such a high reading?

 

[Amp David]

I'd install a new rod or rods first and measure again

 

[johnboy6083]

you could connect rods together to lower the Ra. thats how they earth substations, the drive loadsa rods down to about 8m ( thats what the blokes were doing on last one i worked on), and they link them together with cable or earth tape

 

[Amp David]

The spacing between rods is supposed to be equal to the depth they are driven, to give the best result. Thinks its in the regs somewhere, but never had to do its myself yet

 

[Guest123]

I've a customer with an ITsupply and a metal earth spike outside,QUOTE]

Thats interesting, where abouts is this customer and is it a public supply or private???

Quote from IET.....

"An IT system is not permitted for a
low voltage public supply in the UK because the​

source is not directly earthed".

 

[johnboy6083]

i thought that lenny, but im guessing its probably a spelling mistake. Isnt an IT system where they put an impedance in the earth path, to limit fault current?

 

[Guest123]

The source of an IT system is either connected to earth through a deliberately introduced
earthing impedance or is isolated completely from earth. All the exposed conductive-parts of the install are then connected to an earth electrode in a TT fashion.​

The OP's Ra is far too high and needs to be brought back to BS 7671 guidelines via additional rods, tapes, or mats etc.​

I very much doubt the install is an IT one as unless the OP has been to the power station and witnessed the connection you just wouldn't know.

Generator connections are more common in IT fashion than public supplies nowadays.​

 

[Deleted member 26818]

If it is IT then surely the OP shoudn't be attempting to lower the impedance?

 

[Guest123]

If it is IT then surely the OP shoudn't be attempting to lower the impedance?

At the source then absolutely not, but at the consumers installation as all exposed conductive parts are connected to an earth electrode the requirementd for ADS in accordance with 41.1.

Figure 2.6 + 411.6.4 refer.

 

[nickblake]

Are we all getting confused here like lenny has said IT supplies are not permitted in the public supplies etc , is this installation a TT ,just a spelling error,with an earth spike does to me to make more sence .or is it just me thats getting confused as my single brain cell is shutting down for xmas

 

[cbw]

I have been having problems recently with previously ok rods now going high, I have put this down to dried out frozen hard ground???

 

[EddieB]

TT it is lads, spelling mistake I'm afraid. happy christmas to you all and regrets for wasting your time

 

[sparks1974]

nickblake im sure he stated that this was a it system , but like u said it is not permisable for it to be a public supply , so i presume it is a typo and it is a tt system

 

[IQ Electrical]

Hi and Happy Christmas to you all.

I've a customer with an ITsupply and a metal earth spike outside,I've measured the impedance and it was in excess of 1000 ohms, I cleaned the top of the spike and renewed the clamp this improved the reading to 200 ohms. I was around there the other day and it's risen to over 1600, I called into the neighbour and they have had a newish spike installed in copper and this also was in excess of 1600 ohms. any ideas on how to improve this,and what could the cause of such a high reading?

If you got the reading down to 200 Ohms (the max recommended in BS7430 is 100 Ohms) and that reading has slipped in the last few days then I'd say frozen ground is the cause. When the soil/substrate freezes, the point(s) of contact on the rod is reduced massively.

The most common methods to improve this situation involve longer electrodes, additional electrodes in parallel or a combination of both.

Where additional electrodes are installed, it is good practice to space the electrodes at not less than the buried depths apart from each other.

 

[Guest123]

I have been having problems recently with previously ok rods now going high, I have put this down to dried out frozen hard ground???

I would hazard a guess that most TT systems at this time of year & in this kind of weather would now exceed acceptable values.

In some instances this could prove a bit of an issue under fault conditions....

 

[telectrix]

I would hazard a guess that most TT systems at this time of year & in this kind of weather would now exceed acceptable values.

In some instances this could prove a bit of an issue under fault conditions....

is this not one of the reasons the regs. stipulate RCD protection on TT systems?.

 

[Guest123]

is this not one of the reasons the regs. stipulate RCD protection on TT systems?.

Well yes but 1600ohms is nearing the limit even for a 30mA RCD let alone any other rating.

 

[Deleted member 26818]

It's well over the top if a 100mA RCD is in use.