TT earth systems versus TN earth systems

[park1]

See, now I have a problem with those who think they know more than others, if you are going to talk about a poly phase system for instance it is proven that there are a multitude of problems with this type of supply. The idea behind a poly phase is for motors, and the main problem we encounter are larger voltage drops on the neutral which knocks the phases out of true 90 degrees.I have been in the business for more years than I care to remember, and yet each day I can and do learn something new.

Having a go at a moderator, and posting in the like of, just shows a lack of respect. I am willing to pass on any information that I can so that people can benefit from all the experience I have gained, and equally I learn from the very good debates given on this forum.

I do know for a fact that if you had posted so on another forum you would have been given a red card, that is the difference with this forum, we are willing to put up with some level of abuse, and moderators are not above any member and use the powers they do have very carefully.

Mr Smith is not an electrician, but a very good judge of character and does not like any childish behaviour, his posts are stylist that's how he has always been, Doc Hudson is from an electrical background and from memory has a long list of qualifications, some of which even I have not achieved. We all can from time to time be mistaken, we can from time to time take things the wrong way, but I like to think we have a very good forum here where we can all learn something without being one up on everybody else.

Manator, I don't claim to know it all, I don't and far from. I am learning all the time also, that said I'm no slouch either.

As for poly phase circuits, a three phase circuit is also a poly phase circuit. And yes the two phase ac phases at 90 degrees ment more current in the neutral as compared to a 120 degree shift.

A forum is only as good as it's members.

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Sidewinder

I do not doubt for one minute your intellect, I wasn't pointing a finger at you. Im sure your points are perfectly valid, I felt Nick was confused and a simplistic approach was required. After all the adiabatic is an approximation and errs on the side of caution.

---------- Post Auto-Merged at 22:42 ---------- Previous post was made at 22:35 ----------

Sooooooooooo when do we get back to the learning bit?

Where do you want to start?

 

[kme]

Just out of curiosity, how often do you guys come across TT systems ? Can't say I've worked on more than a handful, and when I have, i have my regs at hand all the time. I'm not new to this either before anyone opens that chestnut, I do mainly commercial, a bit of light industrial and domestic when I have to. I know it's unlikely to get a TT system in a commercial or industrial application, I just don't see many houses with one either, more of a rural thing I guess ?

Must admit that I get at least as many TT as not - maybe slightly more.

They`re not witchcraft (I got married for that) - but they do present a slightly different set of issues than a TN.

Generically speaking, TT installs aren`t near enough to a transformer to have significant PSC / PFC currents, by TN standards......but the stability of the Ra reading; and the extra protection required to ensure disconnection can make them a little more of a challenge. I tend to think of `em like most other things....the more you do something, the more proficient and comfortable with it you are.

 

[Mr Smith]

mister phased

mr smiths intervention may be

unwelcome and unwarranted

to some poor souls

other members may approve

mr smith neither knows

nor cares

mr smith is asked to provide a moderating influence to the forum

which is attempted to the best of his ability

mr smith is reminded of a quote

you can please some people all the time

and all the people some of the time

but you cannot please all of the people all of the time

mr smith believes that

mimicry is the best form of flattery

therefore the post from mr phased

although a poor copy

is nevertheless accepted by mr smith as flattery

thankyou

mr smith is well versed in the human psyche

and a very good understanding of a person

from the way they say certain things

ergo

mr smith has evinced certain characteristic traits of mr phased

that are somewhat at odds with his written statements

mr smith is not questioning mr phased or his honesty

simply making an observation

mr smith hopes this discord will be laid to rest at this point

as further escalation does not bode well

mr smith

 

[NozSpark]

Just out of curiosity, how often do you guys come across TT systems ? Can't say I've worked on more than a handful, and when I have, i have my regs at hand all the time. I'm not new to this either before anyone opens that chestnut, I do mainly commercial, a bit of light industrial and domestic when I have to. I know it's unlikely to get a TT system in a commercial or industrial application, I just don't see many houses with one either, more of a rural thing I guess ?

I replaced a board on a TT install yesterday,,, previously it didn't have a rod or RCD protection

 

[unphased]

I encounter TT systems regularly. I have also encountered several TN systems where the expected Ze has been high and pushed Zs readings over max limits. On reporting said high Ze to the DNO I have had mixed response. In a couple of instances the DNO treated it as an emergency and corrected the fault in the street by digging up the road to acces the cable. One of these was a bookies which had been rewired and the high Ze was ignored. Tis affected a flat Ze above the bookies which I rewired and discovered 5 years after the bookies had been rewired. In other cases I was told "your test meter is faulty", in other cases although the reading was high the DNO saw no problem. So it is apparent to me that the DNO cannot be relied upon to remedy potentially unsafe earths. This leaves us open to criticism in the event of an accident on the property concerned as DNOs are notoriously infalable. In another instance on a new build extension the DNO connected up a PME and the tails were reverse polarity. This turned out to be my fault because although the DNO connected the tails and revisited to reverse them I hadn't checked so ultimately they blamed me.

Anyway, TT systems can alsom take several forms. A basic TT is the single earth rod. This is the one I encounter the most. It is my understanding that Ra less than 100ohms will be stable and between 100ohms and 200ohms could be unstable but over 200ohms is unstable. In the vast majority of cases the TT systems I have tested have been in the range up to 100ohms but several have been higher. This presents a need to correct and additional rods or new rods may be required or attention to the clamp on the rod may sort it.

TN systems are as good as the DNO make them. Zero influence. TT systems are as good as the spark makes them. Total influence. Where TT systems are employed RCD protection MUST be used and in domestic environments I cannot see a problem with total RCD circuit protection. RCBO being MY preferred choice.

I think there is a strong case for ditching TN systems serving domestic instalations and adopting TT. Where new builds are concerned the builder can install mats in the ground where multiple connections from properties AND an individual rod per property for belt and braces could be installed. This could be far safer than the dangers lurking with TBN systems with broken neutrals.

TT for me. Guinness

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The administrator has removed my editing privelges and limited it to 3 minutes so my attempt to improve the post was taken away.

 

[soulman]

Hi, I'm a little confused TN-C-S & P.M.E, I finished college just over a year ago now. I was taught at college three main earthing systems, TT, TN-S and TN-C-S. & touched on another two. Firstly we were taught that TN-C-S is the new name for P.M.E or C.N.E. When ever the E.F.L. path was drawn for TN-C-S, (p.m.e. & c.n.e) were in brackets underneath. I was taught that the TN-C-S uses the neutral as the earth return path therefore saving money, however if the neutral is cut, the dno have installed earth spikes at set distances along the route back to the transformer, so that in the event of a fault there was still an earth path this is known as protective multiple earthing. I was also taught never to perform a ze on a tncs without clamping the earthing conductor first checking for current, i was also taught that if the values on a TN system exceeded 0.35 for tncs & 0.80 for tns that the dno should be contacted for advice. This was the basics we were taught never any difference between tncs & pme. Can someone please enlighten me. Cheers

 

[Steve3948]

You are always going to get the odd poor reading on TNS, but it is generally reliable and the reading will almost always be better than a Rod or Mat.

I did over 500 heating controls for local council and only came across 1 poor Ze, which was higher than 0.8 on TNS system but better than Rod.

I phoned it in, it was a Split Load Board, they turned up and fitted 100A/100mA after the meter.

Who wants all the hassle of mats, rods, tapes, inspection boxes, inspection pits, conduits, ducts - I don't.

 

[sb_89]

Anyone else in the same boat as me then? Who doesn't come across them very often at all. I've done no more than 3 during and after my apprenticeship .... So that's in 6 years. Granted not that long then !

 

[Steve3948]

Anyone else in the same boat as me then? Who doesn't come across them very often at all. I've done no more than 3 during and after my apprenticeship .... So that's in 6 years. Granted not that long then !

Move to the Cotswolds, you will get plenty :consoling

 

[unphased]

You are always going to get the odd poor reading on TNS, but it is generally reliable and the reading will almost always be better than a Rod or Mat.I did over 500 heating controls for local council and only came across 1 poor Ze, which was higher than 0.8 on TNS system but better than Rod.

I phoned it in, it was a Split Load Board, they turned up and fitted 100A/100mA after the meter.

Who wants all the hassle of mats, rods, tapes, inspection boxes, inspection pits, conduits, ducts - I don't.

Hi Steve

The TN-S system, in many cases, tests at Ze much less than 0.8 ohms and is, in my experience, similar in test readings to PME. However, when you get Ze testing out at much above 0.5 ohms you are in actual fact pushing Zs on the circuits close to or even over max permitted. It is not so bad for the lower rated mcbs 20A and lower, but 32A and higher where max Zs is getting close to 1 ohm (again not interested in actual values for purposes of the point) a few ohms on the Ze can easily push the Zs over limit. So even if you do get a TN-S at 0.8 ohms I would still be concerned and you may find that Zs tests are over at that level of Ze. This is a reason why it is not just a matter of keeping a close eye on the typical max values for TN-S and PME it is more important to correlate the test reading with the Zs of final circuits and see whethr the readings are withinn tabulated values. I would suggest that a TN-S at the 0.8 ohm level is dangerous for this reason. I don't think some sparks realise this and are more than happy if the Ze is at or below 0.8ohms. Sparks who understand the earth fault loop concept fully will take little notice of the 0.35 and 0.8 ohms values and be more concerned with the Zs. Again, in my experience, reporting PME test readings higher than 0.35 to the DNO brings little concern from them. They turn out, test it themselves and say yeah its okay.

TT systems do not allow disconnection times to be met for earth faults they far exceed the tabulated values. So the humble RCD in its many guises is the device chosen for that job of disconnecting an earth fault.

That is why I raised the discussion. To see the advantages of using a simple earth system with the correct devices versus a potentially dangerous TN system where broken neutrals present a big problem coupled with inconsistent responses from DNOs when high earth Ze is reported.

 

[Steve3948]

I cannot remember the last time I had a reading above 0.5ohms, that's how long ago it was, so in the main everything would fine.

TT can be as bad, last week I attended to a lighting fault at an housing association complex, while there the House Manager asked if I could look at a cable floating about in flower bed and bushes.

It was a 16mm

 

[unphased]

Thats a good point, Steve. if the earth conductor to the rod gets broken.... ray

 

[soulman]

Steve i see that quite often. no earthing conductor attached to the rod.

 

[steptoe]

Thats a good point, Steve. if the earth conductor to the rod gets broken.... ray

or if the N gets broken on a TNCS. :|

 

[Sidewinder]

I could add a lot to this thread, but going on the way I have been treated recently why should I bother, I'll just get shot down in flames again.

There is more to this than has been posted and there seems to be a lot of teaching missing in modern courses, which is not the fault of the students.

 

[Steve3948]

Sidewinder, providing you don't start talking Antibiotics like $park and Nicky, then I'm keen to hear your views.

 

[OnOff]

Hi, I'm a little confused TN-C-S & P.M.E, I finished college just over a year ago now. I was taught at college three main earthing systems, TT, TN-S and TN-C-S. & touched on another two. Firstly we were taught that TN-C-S is the new name for P.M.E or C.N.E. When ever the E.F.L. path was drawn for TN-C-S, (p.m.e. & c.n.e) were in brackets underneath. I was taught that the TN-C-S uses the neutral as the earth return path therefore saving money, however if the neutral is cut, the dno have installed earth spikes at set distances along the route back to the transformer, so that in the event of a fault there was still an earth path this is known as protective multiple earthing. I was also taught never to perform a ze on a tncs without clamping the earthing conductor first checking for current, i was also taught that if the values on a TN system exceeded 0.35 for tncs & 0.80 for tns that the dno should be contacted for advice. This was the basics we were taught never any difference between tncs & pme. Can someone please enlighten me. Cheers

+1

I finished Level 3 2330 this time last year having done the 17th during the second year. Before all that I'd done a 35wk evening EAL Part P college course. So 4 years of college (2 separate colleges) plus the 17th. In ALL that time not ONE lecturer mentioned the difference between PME and TN-C-S, in fact according to them it was one and the same. I had to find out / read up on the differences myself. When it came to TT none of them really wanted to know, saying pretty much "Oh, that's mainly done in rural areas!". Got the feeling a lot of them were coasting and lecturing was easy street for them. Coming on this forum and I constantly learn something though admittedly I'm often shy of asking what might come across as a numpty question! Believe me I've got plenty!

 

[apprentice87]

Well, i would be more than happy to hear Pauls views.... I have met Paul, and take it from me, apart from being one extremely clever chap, you would have to go a very long way to meet someone more willing to help others too..

Anyone that would try to shoot Paul down, must be either extremely brave or plain stupid.....

john...

 

[steptoe]

or wanting some bread&butter! :slap

 

[philbas]

Where TT systems are employed RCD protection MUST be used

While I agree it's unlikely in the real world, reg 411.5.2 makes it clear this is not the case.Any student giving this answer would fail.