Zs high on tt installation

[schoe]

Hi all I was just doing a zs test on a TT installation on a ring circuit at home and the figures came back high, 165 ohms, my ze was also very high at 501 ohms, the rod is in damp ground at the moment but the lowest I've had it down to is 275 ohms which is still higher than the 200 ohms max stated.

All circuits on my cu are protedcted by a 30 ma rcd so looking through the regs 411.5 it states the max zs is 1667 ohms, so although my results appear high are they acceptable?

Oh I disconnected bondings to avoid parrell paths.

thanks chris.

 

[topcat]

Hi Chris as I understand your reading are ok. But it would be good to get the ze down so extra rod or extend the one you have to reduce the reading. Also whats the size of your main bond to the spike as if its small you may be able to up the csa to reduce the reading Iam sure some one will come along and confirm my information .

 

[Deleted member 9648]

Your results comply with bs7671.....the 200 ohm max is a recommendation only.
I would agree though that it needs improving,as topcat stated an additional or longer rod should bring it down to below the recommended 200 ohms.
You state all circuits are protected by 'a 30ma rcd'.....is that a single up front rcd? If so bear in mind that if it fails you will have no earth fault protection at all with typical TT values of Ra.
Ideally use an S type 100ma RCD main switch and RCBO's to final circuits.

 

[Guest123]

Additional rods at gretaer depth are required here.

I myself would not be happy leaving it in this current situation.

The 200 ohms in BS 7671 is generous at best. The NIC ask for 100 ohms, IMO it should be less than that.

 

[Amp David]

Get yourself some quality earth rods and get bangin them in.

Where in Derbyshire are the located?

 

[schoe]

Thanks it's a split board with two 30 ma rcds, the main switch is definaltly a god idea.

 

[schoe]

Get yourself some quality earth rods and get bangin them in.

Where in Derbyshire are the located?

Im located in hathersage in the peak district, thanks for the advice

 

[trev]

Big fat rods coupled together and driven deep should have an effect on your reading or if you cant get depth go for additional locations and link them together. Also as TC said up the CSA of the cable to the rod(s)

 

[telectrix]

did you have the bonding connected for Zs reading? you only dis. the bonding to measure Ze.

 

[HandySparks]

Hi Chris as I understand your reading are ok. But it would be good to get the ze down so extra rod or extend the one you have to reduce the reading. Also whats the size of your main bond to the spike as if its small you may be able to up the csa to reduce the reading Iam sure some one will come along and confirm my information .

I'd agree that extending the rod is likely to improve things. But increasing the size of cable between rod and MET won't make much difference unless it's currently a very long length of very thin wire. If you improve the Ze from 501ohms to 500ohms by doing this, you'll be doing well.

 

[schoe]

did you have the bonding connected for Zs reading? you only dis. the bonding to measure Ze.

Yeah the bonding was connected for the zs, just been out a bought some more rods this morning so hopefully the ze will come down

 

[Engineer54]

As a starting point use 2 X 5/8'' extendible rods to give you around 2.4 metre depth and then test as to what sort of value you are have obtained. At the very least, this will/should give you a stable earth electrode which by your description you certainly don't have now with your present set-up!! As stated by Wirepuller, an up-front 100mA S type rcd would be very prudent, especially with an overall low Ra value. As has been previously stated increasing the earth conductor size will make little difference if any, large sized conductors are only for conductor protection as such, in small domestic TT earthing systems. Depending on the final overall Ra value, i wouldn't install anything over 10mm in the UK. It would only make sense to go higher if you achieve below 1 ohm (or TN value) ...lol!!

This sounds like your own property, i know that i wouldn't be happy with leaving your present system in place with those sorts of Ra values. Depending on how deep you can go, (or can't go) there are other forms of electrodes you can install, but will obviously cost more than just driving rods in the ground.

Let us know how you get on with the two coupled together 5/8'' rods first....