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
[automerge]1589830049[/automerge]

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 ?