TT system testing

[Timmydoc10]

Hi everyone. A recent install I've done on a tt system is giving me a reading of 93.3ohms as a Ze reading. When I then test from my circuits inside I'm getting readings like 106.3 for a socket circuit supplied from 32 amp type b mcb (which should be no higher than 1.10 ohms in bs7671) anyone help me out as to why my meter is giving me these readings ?

 

[buzzlightyear]

Has above. Meaning a 1 meter add one more on top.

 

[Timmydoc10]

Has above. Meaning a 1 meter add one more on top.

Yea I would have but I had enough bother getting the 1 meter in so I moved 15m to better ground for the 2nd spike

 

[buzzlightyear]

We're you hitting soild ground or the rod would not drive in?

 

[Timmydoc10]

We're you hitting soild ground or the rod would not drive in?

Hitting something solid like stone. That's why Ioved 15m to a garden which had topsoil then gravel I guess below

 

[Mike Johnson]

Sorry Mike dont have the readings on hand. But spike 1 is below the pirelli box. Spike 2 is about 15 meters away and then spike 3 is 10 meters away from spike 2. All spikes are 1m in length

Well they are far enough from each other to be outside the zone of influence, plus your rod reading is good enough, but I would be interested in a reading on the cable from the rods to the DB disconnected from both?

 

[davesparks]

Hi, my Ze was 93.3 ohms and the R1+R2 was 0.92. My measured Zs was 105.3 for that circuit.

All my Zs are within the region of 105.3 to 108.5 ohms

Then something ain't right, 93.3 + 0.92 = 94.22 so your measured value of 105.3 shows a discrepancy you need to find the source of.

 

[Timmydoc10]

Then something ain't right, 93.3 + 0.92 = 94.22 so your measured value of 105.3 shows a discrepancy you need to find the source of.

Where would you start to find the problem ?

 

[davesparks]

All spikes are 1m in length

Well that's no good, it's not deep enough to be stable throughout the seasons. You want to be at least twice that depth.

 

[Timmydoc10]

Well that's no good, it's not deep enough to be stable throughout the seasons. You want to be at least twice that depth.

So why do they make a rod that length if it's not good enough ?

 

[davesparks]

Where would you start to find the problem ?

I would start with logic, if the continuity reading is good but the Zs is not then logically it is going to be something that is unique to the Zs test that's causing the problem.
So that rules out the circuit itself, so that leaves the connections between the CU earth bar and the means of earthing, the connections of the live meter tail, the tester settings, the tester connections.
Start ruling these things out in a logical order, check all of your connections, retake the measurement after checking all test lead connections and the settings of the tester.

Check that you aren't using an 'anti trip' or low current test unnecessarily. If you do need to use an 'anti trip' test then this may account for the discrepancy, test a couple of circuits using the alternative test method and a high current test to see if this clears it up
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So why do they make a rod that length if it's not good enough ?

They make them because there is a demand for the product and its easier to transport and use an earth rod in 4' sections than to try and install an 8' rod or longer in one hit.

Also earth rods aren't only manufactured for permanent install as part of an electrical installation, they have other applications in temporary and permanent installations.
The really thin non-extendable rods are good for electric fence installs, temporary generator supplies and many other applications but they are completely useless for permanent installations.

Manufacturers aren't required to produce products for your specific application, they aren't concerned with BS7671 or best practice, they just make whatever product they can sell.
It is up to you to ensure that you select the correct materials for a job and install them in accordance with best practice.

 

[Mike Johnson]

It is quite normal to have multiple 1m spikes in a TT system in rocky areas, hence the use of copper tape surrounding the building in some locations or laid in a trench it's hard enough to get even a trench dug deep enough especially in mountainous regions. As I asked earlier check the cable between the earth spike and the DB disconnected from both ends, at least then we can rule that out.

 

[buzzlightyear]

I did tell the op the rods need to go deeper then 1mtr.

 

[Timmydoc10]

I would start with logic, if the continuity reading is good but the Zs is not then logically it is going to be something that is unique to the Zs test that's causing the problem.
So that rules out the circuit itself, so that leaves the connections between the CU earth bar and the means of earthing, the connections of the live meter tail, the tester settings, the tester connections.
Start ruling these things out in a logical order, check all of your connections, retake the measurement after checking all test lead connections and the settings of the tester.

Check that you aren't using an 'anti trip' or low current test unnecessarily. If you do need to use an 'anti trip' test then this may account for the discrepancy, test a couple of circuits using the alternative test method and a high current test to see if this clears it up

Cheers mate

I did tell the op the rods need to go deeper then 1mtr.

The rods were hard enough to get 1m in never mind any further

 

[davesparks]

Cheers mate

The rods were hard enough to get 1m in never mind any further

'hard enough' is very subjective and hard to advise on the solution without further information, is this particularly rocky ground or is it just a thick clay or something like that?
How did you install the rods? Small hammer, sledge hammer, kango with rod adaptor, drilled hole, dig and backfill?

Often you will find a hard layer that, once you have got through it, will give way to softer ground and make the second metre much easier.

If sinking an earth rod deeper is impossible I would personally be looking at a trench and horizontal electrode arrangement.

 

[Mike Johnson]

There is an echo in here.