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#144818 - 01/19/06 07:47 PM Are European neutrals multigrounded?  
tdhorne  Offline
Joined: Mar 2001
Posts: 345
Maryland, USA
Are the utility distribution neutrals grounded at multiple points in the system? Is the grounded conductor of the transformer secondary that serves individual homes grounded at the transformer, the customer service unit, or both? This Yank electrician would like to know.
Tom Horne

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#144819 - 01/20/06 03:48 AM Re: Are European neutrals multigrounded?  
Wolfgang  Offline
Joined: Sep 2005
Posts: 153
the very West of Germany
There is almost everything possible in Europe. It depends on the country, the PoCo etc.

It is one of the first things that you have to know when you start the design of an installation if you working at different places or in different countries.

The immense work of harmonization was to cover all these historical systems with a general logic. The result was the idea of TN, TT and IT systems.

#144820 - 01/20/06 04:43 AM Re: Are European neutrals multigrounded?  
kiwi  Offline
Joined: Dec 2004
Posts: 354
christchurch new zealand
Wolfgang I think the best thing here would be to explain to tdhorne the specifics of TN, TT and IT systems, and which system the different European countries use.

Although I've read a lot about these transformer neutral systems, I've never known what TN, TT and IT stands for ?

Can someone tell me ?

#144821 - 01/20/06 04:46 AM Re: Are European neutrals multigrounded?  
djk  Offline
Joined: Dec 2002
Posts: 1,237
In Ireland, in most installations, the supply neutral is earthed at multiple points along its route. It's also bonded to the local earthing system at a point just before the meter.

However, there are TT installations too. In these cases, the entire supply passes through an RCD just after the meter.

#144822 - 01/20/06 11:06 AM Re: Are European neutrals multigrounded?  
C-H  Offline
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
It appears my website is down at the moment (can't complain. It cost next to nothing) so here is a short explanation:

{Edit to insert my website }

The first letter is the transformer side:

I (isolated) Neutral not connected to earth
T (terra) Neutral connected to earth

The second is how the earth fault current is sent back to the transformer:

N (neutral) Through the neutral through some connection point between the transformer and the equipment served.
T (terra) Trough the earth

The third letter is only used for TN-systems and explains where the connection point between the transformer neutral and protective bonding of the equipment is.

-C (Common) The neutral is brought all the way to the equipment and serves as protective bonding/earthing

-S (Separate) The neutral and bonding/earthing conductors are separated all the way

-C-S (Common-Separate) Starts out as a common conductor which is then separated somewhere, commonly at the point of entry to a building.

Therefore there now are:

IT Transformer or generator not connected to earth, equipment locally bonded toghether and to earth rod.

TT Transformer and equipment connected to local earth rods. Fault trips earth fault protection device (e.g. RCD/GFCI)

TN-C / TN-S / TN-C-S
If fault occurs, the overcurrent protection sees a short circuit and trips.

(A system where you didn't have any earth connection at all would be II but I don't think this term is ever used)

[This message has been edited by C-H (edited 01-23-2006).]

#144823 - 01/20/06 11:19 AM Re: Are European neutrals multigrounded?  
C-H  Offline
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
To answer the original question:

In MV systems there often is no neutral.

Swedish wiring regs require multigrounding of overhead conductors for LV systems. I know this is the case in several other countries, but I don't think it is in all.

Some countries (e.g. Denmark) requires permission from the utility if you want to use the neutral as grounding conductor.

One country (Norway) doesn't even have a neutral in the first place. The system is IT as described above. (But new systems are with multigrounded neutral as I understand it.)

#144824 - 01/20/06 11:21 AM Re: Are European neutrals multigrounded?  
Texas_Ranger  Offline
Joined: Dec 2001
Posts: 2,400
Vienna, Austria
TN-C-S is fairly similar to standard US practice, TN-C more or less equals the old US 3w dryer hookups.
IT systems are only to be used for separately derived systems not exceeding a certain size, running off their own transformer or generator. As far as I know mostly used for hospital equipment.

TN-C used to be very common in Germany. Imagine an old US house with only 2-prong outlets. Then somebody comes in and puts in 3-prong outlets, running the white wire to the green screw and a white jumper to the silver screw. IMHO scary.

#144825 - 01/21/06 05:27 AM Re: Are European neutrals multigrounded?  
kiwi  Offline
Joined: Dec 2004
Posts: 354
christchurch new zealand
Thanks C-H, great explanation. I look forward to reading your website once its back up again.

I've also noticed that the transformer description codes are followed by the connection names, like "WYE" for star connection etc.

How are these decided ?

#144826 - 01/21/06 08:18 AM Re: Are European neutrals multigrounded?  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
In the U.K., we have TN-S, TN-C-S, and TT, although those designations were not adopted here until the 1980s.

In times past, almost all urban areas used TN-S. The neutral was grounded only at the star point of the transformer secondary. There were no multiple grounds, and no neutral-ground bonds at each service entrance. The most usual distribution arrangement in such systems employed armored underground cables, with the cable armor providing a separate fault path back to the sub-station.

TN-S diagram

In rural areas, the system which would now be called TT was used. Originally, these networks also had only a single ground on the neutral at the transformer. Again, there were no neutral-ground bonds at each service, and the sole ground fault path was actually by way of a local rod and the earth itself (an arrangement which in the U.S. of course, is prohibited by the NEC).

Because the fault path on a TT system can be of relatively high impedance, some form of earth-leakage (ground fault) circuit breaker is needed to provide proper protection.

TT diagram

Although the current-balance ELCB (Earth Leakage Circuit Breaker) was used, a voltage-operated ELCB was the norm for many years on domestic systems. It detects faults a different way, but the supply distribution arrangement is still the same. The voltage ELCB was abandoned in the 1970s, but many are still in service.

TT diagram with voltage ELCB

The TN-C-S system is the one which most closely resembles American practice. The neutral is grounded at the xfmr, then again at regular intervals along its route. At the service entrance, the EGCs and bonds to water pipes etc. are then all bonded to the incoming neutral. We still have a separate neutral and ground bars in our panels -- The bond is made to the neutral at the service head before the meter.

This arrangement has been in use since at least the 1930s, and was named PME -- Protective Multiple Earthing. In the original scheme of things, PME distribution was used only in rural areas in which soil conditions made it very difficult to achieve fault protection with a TT arrangement. In fact at one time, the utilities needed special permission from the government for each PME distribution network installed.

PME/ TN-C-S diagram

Over time, PME/TN-C-S has become more widespread and utilities have added multiple grounds to their distribution lines so as to make PME available. The PME-ing of rural lines gradually accelerated over the last 20 to 25 years, and I believe that all lines might now be PME. They certainly are in my area,

Although the distribution lines might be PME, that doesn't necessarily mean that the house has to be wired for PME as well. It just means that PME connections may be used at the service if required. Even though the lines are now PME, there are still many services in rural homes which are wired for TT. Our regs. specify stricter bonding requirements for PME, so to change a house from TT to TN-C-S can often mean considerable work. In fact there are still new services being installed as TT even though TN-C-S could be employed.

Even the old TN-S urban systems have now been multiple earthed, so PME bonding is available in those areas as well.

As far as MV/HV distribution is concerned, British practice is to distribute with no neutral. Thus all 3-ph xfmr primaries are delta connected, and 1-ph spur lines are always run as two phases.

[This message has been edited by pauluk (edited 01-21-2006).]

#144827 - 01/21/06 08:25 AM Re: Are European neutrals multigrounded?  
djk  Offline
Joined: Dec 2002
Posts: 1,237
The following are the main types of installation used in Ireland:
i) TN-C-S system.
The standard installation system in Ireland. (See National Rules for Electrical Installations, Third Edition, ET101, Figure 2A). RCDs are mainly used on this system for protection against Direct Contact, and they also provide protection against Indirect Contact. The rated residual operating current of RCDs intended for protection against Direct Contact must not exceed 30mA.

ii) TT system.
The use of TT systems in Ireland is relatively rare. The TT system differs from the TN-C-S system in that there is no connection between the protective conductor (PE) and the neutral, and instead protection relies on local earth electrodes. Resultant impedances in the earth return path between the load and the origin of the supply may prevent the operation of overcurrent protection devices in the event of an earth fault. This can result in bonded metalwork reaching dangerous touch voltages and also give rise to fire hazards due to the sustained flow of earth fault currents if the overcurrent protection device fails to operate. The use of RCDs on TT systems for earth fault protection is therefore essential in all cases. RCDs are used to provide protection against Direct Contact and Indirect Contact in TT systems.

iii) IT system.
Generally used where continuity of supply is required. The use of IT systems in Ireland is confined to special applications such as hospital operating theatres, mines, etc., where continuity of supply under a first earth fault condition is of paramount importance. IT systems are characterised by the absence of a direct connection of the supply to earth or connection to earth via a relatively high impedance which negates the use of RCDs on such systems.

[This message has been edited by djk (edited 01-21-2006).]

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