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Discuss Why do we have TT Supplies?? in the Commercial Electrical Advice area at ElectriciansForums.net

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TonyM58

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Ok, been thinking about this for a while, and its gonna probably seem like a really dumb question, but why do we have TT supplies?

Given that the supply transformer is star connected, with the Neutral coming from the star point, which is also strapped to earth, then the TNCS system is perfectly logical, just splitting the Neutral when it comes in to the house and strapping the house MET to it.

Whats bugging me is why that itsnt just done every time. In a TT system surely the connections are the same at the transformer, so why dont we just strap a 16mm earth to the neutral when it comes into the house? Surely this would MAKE it into a TNCS automatically? In fact, why do bother with TNS either? Surely the Neutral and Earth are always electrically the same point at the transformer, so why would we EVER use the higher resistance and less reliable TT earth spike as our return when we have perfectly good earthed neutral coming into the house? There hundreds of old terraced streets in my area which are almost all TT systems, surely they could all be made infintely safer with a couple of foot of 16mm earth cable in each?

Am I missing something fundamental here? Feel free to shout " Doh" at me in a Homer Simpson stylie!
 
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Cirrus

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  • #2
Now you have mentioned it I am thinking about it and it is bugging me as there really doesn't seem to be any need for it as you say. Strange. I will now have to speak to some wise and old timer sparky friends to see if anyone has got any light they can shed on this.
 
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Minky

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  • #3
I would bring the subject of out of balance Neutral current to your attention .If one of the modified PME systems develops a break then whoever has the best connection carries all the out of balance current,and bearing in mind fault current also.I wouldnt want to own the installation who had the glowing or melted neutral.
 
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SOPHIE4OLLIE

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  • #4
I hate to be the one to mention this, but i believe out of experience that the TT supply was one of the original types of supplies, and invented for the purpose of rural areas! For farming, and for expense of extra conductors.

I do also see what you guys are saying though, because if you have a decent neutral conductor that is usual around 50mm or so (The supply authority side) then surely a good earth can be made possible to the consumer throught a few meters worth of 16mm2 cable from the Neutral in the service head????

I have seen when driving a Overhead system being converted froma TT to a TNCS by just adding a few earth rods (and mats etc) along the route. Which to me is the most sensable answer to a TT.

I suppose the only reason for a TT is because the magical TNCS we all love wasn't invented until 1960, and not commonly used until 1970!!
 
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TonyM58

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  • #5
My question was actually a retorical one - what i am really saying is why do you we still use TT supplies now that we have TNCS (thanks for the history lesson Sophie!)

I have suspected that the actual answer lies down the PME route. To be really safe
( certainly in more rural areas) the Neutral can be staked down to deck as often as every third or fourth pole.

so i guess theoretically it can be done, just a matter of cost

one of the reasons this came up was a (non-electrician) mate reconed he has a "PME system with an earth spike"

Intrigued, I had a look, and his very old terraced house HAS got an earth electrode, which IS connected to the MET as the sole means of earthing. The incoming cable has been renewed some time in the past by the DNO and they have fiited a pme break out box to the incoming neutral with "pme" written on it in big letters (exactly as would be expected on a tncs system) although there is no earth connected to it

now if i or most of you had seen the earth cable connecting the MET to the PME box on the neutral we would have said its a tncs.

maybe it is now MEANT to be a TNCS, but the DNO is leaving the householder to convert his own premises (through a sparks) from a TT to TNCS?

There is no remit on them to supply an earth, so i suppose they have no remit to 'make' householders use their system as PME rather than TT?

Intriguing!!
 
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Minky

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  • #6
Enjoying the discussion re TNCS-TT supplies,and what strikes me as being the most overlooked situation is from the DNO point of view.My point being that the DNO can provide a potentially dangerous supply 230-415v AC into a property but without any means of safeguarding both persons and property.
The TNS system used where conductors are pulled through metal conduit is a prime example,solely relying on continuity of conduit installed approaching 50 years ago,as a path to earth.
I have recently converted this type of system solely on the belief and measurement from TNS to TT,when doing a consumer unit change.
Therefore feel there is still need for the trusty old TT system after all.
 
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TonyM58

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  • #7
Minky, i have spent years (actually decades) working with mobile and static generators. we essentially used a TNS system in that we run a pysicall three core (or 5 core for three phase) cable to our distribution boxes.

the gennys were star connected with the neutral strapped to the generator frame. We then used to conect the generator frame to earth with an earth spike.

The point is, the Zs path for any earth fault would have been from phase and back down the physical earth cable to the star point.

The earth spike was used (for want of a better term) as equipotential bonding between the generator frame and the earth, preventing a shock for anone touching the generator frame under fault conditions.

The spike served a double puropse in that it guaranteed an earth path (like a TT) in the event of the physical earth cable connection being lost (I have worked in some pretty harsh environments)

The point i am getting to is: on the system you have just converted from TNS to TT, theoretically if you had left the TNS system connected, and put a TT spike as a 'back up' (like my genny) would that have worked.

Wonder if it says you CANT do it in the regs!

like i said, this interesting.......

regards

Tony
 
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Minky

  • Thread Starter Thread Starter
  • #8
Tony:-
Don't think there is a reason in the regs why the TNS cannot be made into a TT,only thing that springs to mind is resistance paths to earth.Providing the two dont overlap,which in my previous scenario would be highly unlikely then this would be fine.
Not sure how the DNO would react,but that's their problem.
I felt it necessary to safeguard the installation in this manner,and therefore my ass was duly covered.Allow me to reiterate my original post,in that the DNO can provide a potentially deadly force to an installation without a path to earth.Kind of like driving a rally car without brakes!!!
 
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Andy M

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  • #9
i was at a job today where there is a TT supply, EDF called to see if PME available, it was but the reading was 0.56 !! and the adjacent pole has an earth at it.
 
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alan

  • Thread Starter Thread Starter
  • #10
As TT is used usually for buildings off the beaten track, a 2 core cable supply would be cheaper then a TN-CS OR TN-S. Cost. In a TN-CS they have to spike the earth cable a number of times in case there is a break in the earth -neutral.
 
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TonyM58

  • Thread Starter Thread Starter
  • #11
Minky, i have spent years (actually decades) working with mobile and static generators. we essentially used a TNS system in that we run a pysicall three core (or 5 core for three phase) cable to our distribution boxes.

the gennys were star connected with the neutral strapped to the generator frame. We then used to conect the generator frame to earth with an earth spike.

The point is, the Zs path for any earth fault would have been from phase and back down the physical earth cable to the star point.

The earth spike was used (for want of a better term) as equipotential bonding between the generator frame and the earth, preventing a shock for anone touching the generator frame under fault conditions.

The spike served a double puropse in that it guaranteed an earth path (like a TT) in the event of the physical earth cable connection being lost (I have worked in some pretty harsh environments)

The point i am getting to is: on the system you have just converted from TNS to TT, theoretically if you had left the TNS system connected, and put a TT spike as a 'back up' (like my genny) would that have worked.

Wonder if it says you CANT do it in the regs!

like i said, this interesting.......

regards

Tony

I have found a bit that says you CAN do it!

Under defenitions (in the 16th) for TNCS it says

"the supply system PEN conductor is earthed at several points and an earth electrode may be necessry at or near a consumers installation"

Got me thinking again.........
 
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