TT vs TN-C-S

[Napa]

I am trying to understand why there is a difference between the two.
I know that TN-C-S has PME along its lenght to an installation, so Neutral and Earth are the same. The Neutral and Earth are separated at the installation, the Earth then provides a 'Faraday cage' for protection within the installation.

1) Why can't TT be treated the same way at the installation? The supply transformer as I understand it, has its Neutral Earthed at the transformer. The Neutral could be split at the entry to the installation to give the same effect as the TN-C-S.
2) If the Neutral is Earthed at the transformer why can't it be Earthed a the installation so any volt drop along the supply cable is nulled out?

 

[mapj1]

TT is normally used when the neutral is not of high enough integrity (may be broken too easily) to allow use as PME. The usual one is if L and N come in as separate bare overhead lines, where a falling tree might break one not the other (and it CAN happen, and even on some overheads that are PME, leading to very odd fault symptoms) or where the supplier knows that a single terminal 'line tap' (think giant choc bloc) has been used in the neutral. There are strict rules about minimising the risk of a broken neutral, and in cases where the risk is not low enough PME cannot be used.
Building sites and agricultural installations, where there is risk of damage to the cables and/or the earthing system, and there is significat chance of equipment in use outdoors (where PME with a broken neutral is most dangerous) are 2 cases that come to mind where PME is not considered good enough.
Hope that helps.
M.

 

[ban-all-sheds]

1) Why can't TT be treated the same way at the installation? The supply transformer as I understand it, has its Neutral Earthed at the transformer. The Neutral could be split at the entry to the installation to give the same effect as the TN-C-S.

Yes - the same effect as TN-C-S, but not the same as PME, and TN-C-S without PME is extremely risky.

2) If the Neutral is Earthed at the transformer why can't it be Earthed a the installation so any volt drop along the supply cable is nulled out?

I'm not sure I understand what you mean by that volt-drop bit. But the reason it can't be is that without PME, if you create your own private TN-C-S supply and the neutral breaks then all your earthed metal in the house goes to 230V, and this is not good.

 

[theshogun]

TN-C-S supply and the neutral breaks then all your earthed metal in the house goes to 230V, and this is not good.

I got called out to a school recently to upgrade it's bonding after this had happened


Supply was,
3phase for night storage heating afternoon boost, total heating load 48kw + normal load of 15kw there about

Lost neutral on the over head supply 16mm bonding cable became like a 1 bar fire when the heating cut in people had been getting small shocks off toilet sink taps they thought it was static, but were not sure.

School shut fire engines lucky someone did not get killed or the school burned down.

I don't trust T,N,C,S

 

[mikely]

The form of system you are suggesting is the norm in the USA. The TT earthing system is not allowed by their national electrical code (NEC). Their typical system is an overhead distribution with an earth electrode at the installation as well as at the distribution transformer. It seems that the Americans do not trust RCD protective devices to clear earth fault currents and so do not allow the TT system to be used.

It has been suggested that the addition of a local earth electrode would be a sensible precaution in PME systems in the UK. See http://www.iee.org/Publish/WireRegs/PME_ESQC.pdf

As has been said, it is the reliability of the neutral connection that is important. The American NEC requires the low impedance characteristic of a TN-C system for earth fault clearance through high current overload of protective devices. The TT system depends on the reliable operation of RCD devices to clear earth fault currents.

 

[plugwash]

i still think TN-S is the best way but its more expensive for the recs so they don't tend to install it anymore.

 

[HDRW]

i still think TN-S is the best way but its more expensive for the recs so they don't tend to install it anymore.

Why is that? What does it involve beyond fitting a clamp to the armour of the incoming cable (or do they not use armoured cable these days?)

Cheers,

Howard

 

[Big_Spark]

The form of system you are suggesting is the norm in the USA. The TT earthing system is not allowed by their national electrical code (NEC). Their typical system is an overhead distribution with an earth electrode at the installation as well as at the distribution transformer. It seems that the Americans do not trust RCD protective devices to clear earth fault currents and so do not allow the TT system to be used.

Mikely...what you have just described is a CLASSIC TT system.

The Yanks do not call RCD's by the same name, they call them Earth Fault Interruptros (EFI's), and I can certainly attest that they do rely on them.

Even in towns in the US the supplies are overhead, and 90% if US installations are fed by, what in the UK, is a TT system.

For reference:

TT = Supply Tx Earthed by stake or plate. Service Lines consisting of Neutral and Phase conductors on Overhead Poles, Earth Rod or plate at Consumer Installation end. RCD main switch for safety.

TN-S = Supply TX earthed by Plate or Stake, Service line is one of several types of SWA in which there are seprate Neutral and Phase conductors. Some cables rely ont he SWA as the Earth, some have a seperate Earth conductor (Very Rare). The Consumer Installation earth is taken from the SWA of the supply cable.

TN-C-S = Supply Tx earthed by Stake or Plate, Service line is one of several types of Wire Armoured Cable in which the Neutral and protective conductors are combined, and it also forms the protective armouring of the cable. This is also known sometimes as a Concentric cable as the phase cores are embedded inside the Combined N/E armouring. The Earth at the Consumer installation is supplied by the splitting of the combined conductor into seperate Earth and Neutral cores.

Protective Multiple Earthing (PME) is a variation of TN-C-S in which, at various points along the supply line, the supply has had the Combined Conductor earthed to ensure that Neutral is always at Zero potential difference in respect of local earth. This system has been replacing older overhead TT lines over the last decade. although installed in underground supplies for in excess of 20 years. Overhead the combined conductor is earthed every 1km or so by a stake located beside a Pole or by a stake connected directly to a line Tx in circuit.

NOT ALL TN-C-S SUPPLIES ARE PME AND NOT ALL PME SUPPLIES ARE TN-C-S.

 

[mikely]

Mikely...what you have just described is a CLASSIC TT system.

My description was deficient. As I understood it, they normally use a TN-C system but locally earth the neutral at the installation. This was purely based on what I had read about different earthing systems worldwide - I stand corrected.

 

[plugwash]

i still think TN-S is the best way but its more expensive for the recs so they don't tend to install it anymore.

Why is that? What does it involve beyond fitting a clamp to the armour of the incoming cable (or do they not use armoured cable these days?)

Cheers,

Howard

TN-S requires more expensive types of cable.

usually the recs dont use a cable with a seperate sheath/armour anymore they use a concentric cable with the neutral surrounding the live.

There are modern cables for TN-S (split concentric and 4 core concentric) but they cost more than the ones needed for TN-C-S

 

[mapj1]

Actually the modern underground supply cable is a strange fish, the three phase cores are almost triangular, like slices of a circular cake, with relitivly thin looking insulation for their size, and the relativly 'whispy' braid is deliberately made slack, so it can be splayed, to allow the phase cores to be pierced (think vampire!!) into by shearhead screws, so a tap for a single house would be a screw into the core for a single phase , plus a clamp onto the braid for Neut /Earth. There is no additional earthed outer. (Whole joint thing set in a sausage shaped mould full of either tar or epoxy, depending on the era.)
Cant find a picture right now, but will look.
The neut /earth can be reduced in X section because on average, the phase currents cancel (or if the current is sine wave they do, but not if they are rectified peaks from switch mode power supplies....)
regards M.

 

[Napa]

Thanks, that is starting to make sense. Basically it looks like there is a safety issue with respect to a possible broken Neutral.

But for a TT system why can't one earth the Neutral at the input of the installation like the diagram in http://www.iee.org/Publish/WireRegs/PME_ESQC.pdf
for at PME system?
Does this not help stop the Neutral go up to Line voltages in the case of a broken neutral in the supply?

I read somewhere on the web that transformers are not necessarily grounded in all cases. Maybe 'small' ones up poles. I don't know if anyone can confirm that is the case?

Somewhere else I was reading about earth loops from other nearby installations. I'll look into that some more.


With regard to volt drop, I was thinking about the V=IR loss in the Neutral in a TT system. If the Neutral is grounded all the way back at the transformer then the local Neutral in the installation might be at a voltage compared to Earth but I guess this is negligible under normal circumstances.

I going to read
http://www.schneider-electric.com/cahier_technique/en/abstracts/earthing_systems_LV.htm
and
http://www.schneider-electric.com/c...acts/earthing_systems_evolution_worldwide.htm
sometime. That might help me understand more about grounding/earthing issues.

Cheers. J.

 

[mapj1]

Adding a rod to PME at the house may help in some cases, but as a domestic (4 foot), earth rod will have a resistance of 10 to greater than 100 ohms perhaps, depending on soil dampness, this can easily be raised to 240V by even a modest fault current (say 10 of amps), and all that happens is you get a larger than planned leccy bill (oh and a piece of garden where standing in bare feet is painful.) It is very hard to get an earth good enough to carry the likely fault current, hammering in more rods soon comes up against diminishing returns for the effort..
The DNO/RECS bury long lengths of copper tape (10s of metres) or very big metal plates. Unless something similar is done at the house end, a low impedance earth other than via the supplier, is quite elusive.
regards M.

 

[felix]

A cracking bit of reference information there from FWL_Engineer. You rarely find this stuff in DIY books. This has prompted me to take a closer look at my own supply cable which dates back to 1935.

It comes in underground and is coated in what looks like bitumen. There is an earth clamp at the meter to which is connected the earth wire of a modern consumer unit. This is the only true earth in the house because the water pipe is plastic and the iron gas pipe is, like the power cable, encased in bitumen. Would anybody like to hazard a guess as to what kind of supply this is?

 

[mapj1]

TN-S if the clamp goes to the jacket of the incomer. If its 1935, it won't be TN-C or PME, as it would have been against the rules as they stood then... but there may be a fuse in the neutral.
Its probably the imperial equivalent of ~ 6mm copper, and you may find that the lights dim noticibly on heavy loads, compared with a mdern house.
As an electronics guy you will be able to estimate your earth impedance by measuring the on and off-load voltage of a known load current between L and E, this is what a proper Earth loop tester does, but it only puts the load on for a very short time (to minimise the shock risk if the earth impedance is high).
You may find free electricity of a few volts between N and E, when the neighbours are using electricity in such a way as to unbalance the neutral.!

regards M.
Ps post a picture, and we can all take bets..