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 ?

 

[marcuswareham]

93.3 ohm Ze + the resistance of r1+r2 for the circuit being tested

if you do a loop impedance test between L & N on the circuit you can work out the PSCC which will hopefully be high enough to instantly trip MCB on a dead short between L & N

Due to high impedance earth electrode on TT systems you are reliant on RCDs to meet earth fault disconnection times and MCBs for overload

That is my understanding anyway

 

[buzzlightyear]

is type 1.37 B bs 60898 ,where did you get that number above .

 

[Timmydoc10]

is type 1.37 B bs 60898 ,where did you get that number above .

I was working with the 80% values on the test sheets
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93.3 ohm Ze + the resistance of r1+r2 for the circuit being tested

if you do a loop impedance test between L & N on the circuit you can work out the PSCC which will hopefully be high enough to instantly trip MCB on a dead short between L & N

Due to high impedance earth electrode on TT systems you are reliant on RCDs to meet earth fault disconnection times and MCBs for overload

That is my understanding anyway

Thanks. When filling out your test sheets for maximum Zs do you fill in the 1667 ohms (given in table 41.5) or would you fill in 200 ohms? (which is the recommended value that the earth electrode does not exceed)

 

[pc1966]

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

That is too big a difference to be just the R1+R2 cable unless something is very wrong. However, if you measured them a while apart in time then changing soil moisture levels could explain it.

I don't do this professionally, but I would prefer the 200 ohm value given as maximum acceptable from a stability of earth rod point of view in the regs book. Some of the experienced sparks on this site might have a better guidance.

 

[Timmydoc10]

That is too big a difference to be just the R1+R2 cable unless something is very wrong. However, if you measured them a while apart in time then changing soil moisture levels could explain it.

I don't do this professionally, but I would prefer the 200 ohm value given as maximum acceptable from a stability of earth rod point of view in the regs book. Some of the experienced sparks on this site might have a better guidance.

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

 

[buzzlightyear]

View: https://youtu.be/HsWWKzjVUxE?t=549
WATCH .

 

[bigspark17]

That is too big a difference to be just the R1+R2 cable unless something is very wrong. However, if you measured them a while apart in time then changing soil moisture levels could explain it.

I don't do this professionally, but I would prefer the 200 ohm value given as maximum acceptable from a stability of earth rod point of view in the regs book. Some of the experienced sparks on this site might have a better guidance.

more likely testing Ze on high and zs @ socket on low due to rcd protection.

 

[pc1966]

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

That is R1+R2 = 12 ohms, for 2.5 T&E the length would be about half a kilometre!

Even allowing 0.5 ohm or so for "RCD uplift" on the tester it is still unexplained. Presumably you measured R1+R2 during dead testing (or ring loop values)?

Most likely if a change in rod Ra with time/rain

 

[Pretty Mouth]

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

Hi Timmy. You may be experiencing 'RCD uplift'. This is when your test meter shows an unexpectedly high earth loop impedance reading when testing a circuit protected by an RCD, using the 'no trip' setting. There should be a few threads about it somewhere on the forum, have a search for more info.

To get around the problem, you can bypass the rcd while testing. To do this, isolate the circuit at the RCD, link L-E at the furthest point of the circuit, then carry out a 2 lead test across the RCD.

Let us know how you get on.

 

[Timmydoc10]

That is R1+R2 = 12 ohms, for 2.5 T&E the length would be about half a kilometre!

Even allowing 0.5 ohm or so for "RCD uplift" on the tester it is still unexplained. Presumably you measured R1+R2 during dead testing (or ring loop values)?

Most likely if a change in rod Ra with time/rain

Both tests were done within 20 mins

 

[pc1966]

Both tests were done within 20 mins

OK that makes weather changes unlikely! The only fast change would be a drop in Ze due to rain fall, drying out is always slower.

Did you do Ze with rod separate as only path?

If you do a Zs test at the CU do you get the same result as socket?

Just wondering if rod not reconnected properly and you are seeing a water pipe or similar as it is higher R and part of bonding of extraneous stuff?

 

[Timmydoc10]

OK that makes weather changes unlikely! The only fast change would be a drop in Zs due to rain fall, drying out is always slower.

Did you do Ze with rod separate as only path?

If you do a Zs test at the CU do you get the same as socket?

Just wondering if rod not reconnected and you are seeing a water pipe or similar as it is higher?

Definitely not a water pipe or anything like that. I did the Ze with the earth disconnected yes.

 

[pc1966]

What make/model of MFT are you using (out of curiosity)?

Zs going up when rod is reconnected is pretty difficult to explain, but i guess tomorrow is another day to repeat some tests and see if something obvious (or non-obvious) comes out of double-checking stuff.

 

[Timmydoc10]

What make/model of MFT are you using (out of curiosity)?

Zs going up when rod is reconnected is pretty difficult to explain, but i guess tomorrow is another day to repeat some tests and see if something obvious (or non-obvious) comes out of double-checking stuff.

Thanks for your help. I honestly cant see anything wrong as it's a simple install with no other possible paths to earth ?

 

[buzzlightyear]

Definitely not a water pipe or anything like that. I did the Ze with the earth disconnected yes.

possible the earth rod is either not deep or lose connection .

 

[Timmydoc10]

possible the earth rod is either not deep or lose connection .

I would think that if my earth reading wasnt 93.3 ohms. I personally think that's good for a tt system. Just wondering why the Zs readings jump to 107.3 ohms and they are all in and around that

 

[buzzlightyear]

I honestly cant see anything wrong as it's a simple install

did you install the rod or was it still present .

 

[pc1966]

It is probably something simple like a bad connection, wobbly rod being kicked by someone, or similar.

Do a few dead tests using a wander lead to confirm the R2 for rod-MET-CU and CU-socket, same for phase to socket L. Poke thing (not while live!) to see if loose or something changes.

 

[Timmydoc10]

did you install the rod or was it still present .

I installed 3 new rods to get the reading to that

 

[Mike Johnson]

Did you take the rod reading at the DB or at the rod, assuming you did it at the rod, have you checked the reading of the connecting cable disconnected from the rod at the DB?
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I installed 3 new rods to get the reading to that

Hope that's in one continuous rod or each outside the zone of influence of the other.

 

[Timmydoc10]

Did you take the rod reading at the DB or at the rod, assuming you did it at the rod, have you checked the reading of the connecting cable disconnected from the rod at the DB?
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Hope that's in one continuous rod or each outside the zone of influence of the other.

That's with all rods connected

 

[Mike Johnson]

Sorry don't quite understand your response.

From your reply I assume three separate rods connected together, can you tell us the length of the rods, the distance between them, readings from each rod, readings from the rod to the DB and the reading of the cable to the DB disconnected at both ends?

Bed calls.

 

[Timmydoc10]

Sorry don't quite understand your response.

From your reply I assume three separate rods connected together, can you tell us the length of the rods, the distance between them, readings from each rod, readings from the rod to the DB and the reading of the cable to the DB disconnected at both ends?

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

 

[buzzlightyear]

Why did you put 3 in?

 

[Timmydoc10]

Why did you put 3 in?

Couldn't get the readings down

 

[buzzlightyear]

Couldn't get the readings

What type of soil? Clay

 

[Timmydoc10]

What type of soil?

Bit of everything. Mostly gravel

 

[buzzlightyear]

Bit of everything. Mostly gravel

I would have put another rod in with the first one and see what results are.

 

[Timmydoc10]

I would have put another rod in the first one and see what results are.

What do you mean by another rod in first one

 

[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.