Zs testing on TT

[Ben Butcher]

Hello all,

Many Thanks to everyone on previous questions, I am new to forums generally but it sometimes feels like there's a big bus full of keen knowledgeable people outside that can answer any question!

So todays question; Whilst doing some work at a house yesterday I found myself wondering if some results I got could be correct. A TT installation, and firstly I measured Ze as you would elsewhere, I imagine the result I got of 205 ohms would be expected? and is this recorded as Ze or Ra? Secondly whilst doing Zs at the far point I got 83 ohms, this obviously would not comply with the tabulated maximums or are they for TN systems only? Also how can the Zs be lower than the Ze, is it just because of paralell paths and water main etc?

Any guidance would be gratefully received, Ben.

 

[moddeydhoo]

not a spark again. so many on here. it says at the top electricians forum.ra is the earth rod on its own legal limit 200 ohms niceic limit 100 ohms, far point never heard of this, this is realy basic ****

 

[Ben Butcher]

Hello, sorry to offend, only a question, I did see someone on here say that a TT dosen't have a Ze and another saying that do a normal Ze reading and record it as Ra, just trying to clear it up. The far point is just that I do it there as you would an r1 r2 reading, just a habit, sorry!

 

[Engineer54]

First point it is Ra that you are measuring not Ze on a TT system. If the rod's Ra value is high then your Zs is going to be high, that should stand to reason. The reason your getting lower a Zs reading is because of parallel earth paths via your bonding cables etc.... RCD's are basically classed as your primary earth fault protection and not additional protection as on a TN installation. That is why 30mA RCD'sor RCBO's are an absolute must on a TT system.


200 or 100 ohms are both pretty pointless values, and will also probably indicate an unstable earth electrode rod. These sort of values tend to be seen when short, thin twig so-called earth rods have been used.

 

[SPARTYKUS]

not a spark again. so many on here. it says at the top electricians forum.ra is the earth rod on its own legal limit 200 ohms niceic limit 100 ohms, far point never heard of this, this is realy basic ****

Crikey has someone upset you today?

I think you'll find anyone can, will, and should register on this forum!

If you don't like the basic questions maybe it would be better to just not post rather than assume you know everything about a posts originator and what their perceived level of knowledge should be!

I'm quite a laid back chap but i find your berating Ben Butcher slightly offensive and no need for it really old boy. Peace.....

 

[ackbarthestar]

it is Ra that you are measuring not Ze on a TT system

I've always been baffled by this statement since by definition:
Ra is the sum of the resistances of the earth electrode and protective conductors connecting it to the exposed conductive parts
also defined by
the resistances of the earthing arrangement of the exposed conductive parts of the equipment of the low voltage installation

I would like to know what has happened to the supply phase conductor in this arrangement since it is not possible to measure the Ra with an earth loop impedance meter. Better to call it Zs(e) or Ze(Ra) as opposed to just Ra.

Just thought I'd throw that one into the mix......

 

[Ben Butcher]

Thanks Spartykus for the backup, I was feeling a little shaken down! If I had 40 years experience I probably wouldn't be asking the question... I would rather try to do it right than guess.

 

[Engineer54]

I've always been baffled by this statement since by definition:
Ra is the sum of the resistances of the earth electrode and protective conductors connecting it to the exposed conductive parts
also defined by
the resistances of the earthing arrangement of the exposed conductive parts of the equipment of the low voltage installation

I would like to know what has happened to the supply phase conductor in this arrangement since it is not possible to measure the Ra with an earth loop impedance meter. Better to call it Zs(e) or Ze(Ra) as opposed to just Ra.

Just thought I'd throw that one into the mix......

It's Ra because there is no direct path back through a made conductor to the distribution supply TX as there would be with TN earthing systems. You can get a reasonable assessment of an earth electrodes Ra value via an ELI test. It may not be as accurate and be influenced by noise and other interference than a purpose made earth tester, but it's still going to give you a pretty good indication of a rod's Ra value...
The resistance/impedance of the connecting conductor between the earth electrode and the point of testing at the service head/CU position is going to be negligible, and not worth consideration. We have to take it on assumption on this point that the connection point at the rod has been checked and found sound!!

 

[moddeydhoo]

you could define it as you like, ze zs depends on where in the system you are, at origin, one sub main is a zs of another, which becomes a ze of another, on most paper work ra is the earth rod with the main earthing conductor, on its own. legal limit 200ohms. But i know in many places this can not be achived, most rcd's guarantee, they will operate at 1000 ohms

 

[Engineer54]

you could define it as you like, ze zs depends on where in the system you are, at origin, one sub main is a zs of another, which becomes a ze of another, on most paper work ra is the earth rod with the main earthing conductor, on its own. legal limit 200ohms. But i know in many places this can not be achived, most rcd's guarantee, they will operate at 1000 ohms

No answer to that statement!! lol!!!

 

[ackbarthestar]

It's Ra because there is no direct path back through a made conductor to the distribution supply TX as there would be with TN earthing systems. You can get a reasonable assessment of an earth electrodes Ra value via an ELI test. It may not be as accurate and be influenced by noise and other interference than a purpose made earth tester, but it's still going to give you a pretty good indication of a rod's Ra value...

Ra measurements are then only applicable when using proprietory earth resistance measuring equipment (resistance of the soil/electrode contact resistance once the soil resistance is established).
Its not a Ze measurement (without including either the phase or neutral conductor) and neither IMO is Ra. even though its assumed to be the standard external loop resistance.

 

[ackbarthestar]

you could define it as you like, ze zs depends on where in the system you are, at origin, one sub main is a zs of another, which becomes a ze of another, on most paper work ra is the earth rod with the main earthing conductor, on its own. legal limit 200ohms. But i know in many places this can not be achived, most rcd's guarantee, they will operate at 1000 ohms

This is how I read it...

Ze = External earth loop for TN systems
Zs = Both Ze and R1+R2 of the complete loop from supply to point of utilization
Zdb = Sum of the external loop at an internal distribution board within the installation with all parallel paths in place
Zs(e) = External loop of TT including Ra
Ra = earth path resistances only

 

[Richard Burns]

Hello all,

Many Thanks to everyone on previous questions, I am new to forums generally but it sometimes feels like there's a big bus full of keen knowledgeable people outside that can answer any question!

So todays question; Whilst doing some work at a house yesterday I found myself wondering if some results I got could be correct. A TT installation, and firstly I measured Ze as you would elsewhere, I imagine the result I got of 205 ohms would be expected? and is this recorded as Ze or Ra? Secondly whilst doing Zs at the far point I got 83 ohms, this obviously would not comply with the tabulated maximums or are they for TN systems only? Also how can the Zs be lower than the Ze, is it just because of paralell paths and water main etc?

Any guidance would be gratefully received, Ben.

205 ohms is a bit high for stability and really should be better, but can take a lot of work and if the installation is old and the earth rod connections are not corroded it is probably stable.
The value you measure is an approximate value of Ra (because precision is not needed for this situation) but since you have only Ze on the paperwork I would record it as Ze, not a precise definition but covering the same intent.
The 83 ohms would be from parallel paths through alternative routes rather than all directly through the earth rod.
The maximum value for EFLI on a TT is 1667 ohms to ensure the RCD will trip in time, (but never accept that high a value).

 

[polo1]

Thanks Spartykus for the backup, I was feeling a little shaken down! If I had 40 years experience I probably wouldn't be asking the question... I would rather try to do it right than guess.

Unfortunately, there are some on here who only like the "sound" of their own post and would be far better finding another site to use! Ask away - we are all on a learning curve in this industry (though some have a problem admitting it). Better to ask and be safe/provide safe, compliant installations for your customer. Don't be put off by the occassional daftie.

I attend very few sites that are TT so am happy to be exposed to the views/experience of others on this particular subject.
Regards

 

[Engineer54]

Ra measurements are then only applicable when using proprietory earth resistance measuring equipment (resistance of the soil/electrode contact resistance once the soil resistance is established).
Its not a Ze measurement (without including either the phase or neutral conductor) and neither IMO is Ra. even though its assumed to be the standard external loop resistance.

I'm afraid the powers that be, don't agree with you. As far as they are concerned any test reading that includes any type of TT system electrode will be valued as Ra... Storm in a tea cup really, i think everyone knows what OP's mean, when they state testing Ze on a TT system. ...lol!!!