TT plus PE

There is no need to do it, so why would one want to (rhetorical question - NNTR)?

http://en.wikipedia.org/wiki/Earthing_system

in reality you are right, but to make the system more reliable ? Say the main PE breaks then is it reasonable?

Nonsense. Stay TT.

Maybe it looks like this:

"...TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point..."

nonsense? if main PE breaks then your rcd will not work . then again where is your protection against indirect contact?

Nonsense to you perhaps, in reality if the PE (Protective Earthing) conductor has no connection to earth/ground/neutral then there is no path for current to flow. The RCD only senses the current leaving it and the current returning to it and responds to the net difference between them.

If there is no path for current to flow through the (open circuited) PE then whatever current leaves a hot leg returns via neutral and/or the other hot legs, and all of those conductors are monitored by the RCD which nets them to zero.

Now if you think the RCD is going to protect you from contacting a hot leg and another hot leg(s) and/or neutral, then you are wrong because it has no way to distinguish between you and any other legitimate load.

Please read this thread for more info on what happens when this misunderstood and/or ignored.

On the other hand if the PE is disconnected from the bonding point at the distribution panel and the building steel is still earthed, then any contact from the phase conductor(s) to the building steel or anything else touching/connected to it, will cause the RCD to operate as you expect. Whether it saves anyone's life though depends on a whole host of variables especially the path through their body and their physical condition.

An RCD (30mA) on dometic type socket outlet feeds, is there to protect you from bad earthloop resistances. It measures an inbalance between the live and neutral and, if above 30mA trips. By indirect contact to an Exposed-conductive-part, you may create a path to earth (the ground) and, in doing so cause the 30mA inbalance between live and neutral. The 30mA tripping point is at a level considered not enough current to defibrillate the heart. So you don't die.

Are you saying that the building in question has a TN-S incoming supply configuration. But the earth of this supply has not been utilised for the earthing of your distributed electical supplies within the building. Instead the earth that is utilised for your distributed electrical supplies within the building is connected 'to the reinforcement metals' (I'm assuming this is the floor and any other reinforced concrete structure). Remember all this reinforcement metal has to be connected to earth anyway, as it is considered an Extraneous-conductive-part. Are you confusing this connection as being your Main Earthing Conductor?

If this is the case you need to get in contact with the electrical supply company that installed the supply to your building. I would think that the earth from the TN-S supply would be a much better/reliable earth than the earth offered by the reinforcement metals. As the TN-S supplies earth conductor size would've been calculated by the supply company and, gives you a gauranteed external earth loop impedance Ze.

You need to contact the electrical installation company that fitted out the building with it's distrbuted electrical supplies and ask the why they didn't utilise the earth offered by the TN-S supply. It may have been an oversite on their part and, something they will put right for you at no extra cost.

no, what I ask is I add an additional conductor which comes from wye point of the secondary of the transformer and goes to the tt earth schemed buildings grounding main terminal

Are you certain that the 'reinforcement metal' and it's associated earth conductor are your only earthing system for your facility? This structural metal work should be connected to your main earthing terminal and in turn connected to your supply earth.

:) it is my fault that I can not make you understand what I ask about.

Lets take it simple. I have a TN-S system including a secondary grounded transformer and a building power central which is supplied by this transformer. But I also install ground electrodes which are used in TT systems only and I add this ground electrodes to my TN-S system . What is wrong with such a scheme if there is ?

#1↑.

It can cause your TT power system to experience "noise" issues that are being passed through from the TN-S neutral.

This will cause severe performance issues with/in any electronic equipment especially digital devices and/or possibly total operational failure.

TT power systems are specifically isolated from the distribution neutral for this reason.

It is exactly how it is practised in many parts of India (i.e.) There would be a PE conductor running throughout the plant (as the 'S' conductor in a TN-S system) and this PE conductor would be bonded to local earth electrodes of the equipments (as in a TT system). Someone half-wittedly called this a "T-TN-S" System. And, it works in India.

I would differ here. Very rarely you will find dedicated earth electrodes provided for body earthing of electrical equipments separate from general earth grid. Only for neutral earthing after double wound isolation transformer and insulated (isolated) body earthing for telecommunication equipments, have I used additional earth electrodes beyond the general earth grid provided. Neutral earthing beyond isolation transformer conforms to TN-S system and insulated body earthing for Data and telecommunication equipments meets TT requirements. TT system coexists with TN-S system.

You may kindly refer to Figure 3 (Indian TN-S System) in IS 3043 (The Indian Standard Code of Practice for Earthing), wherein it is clearly shown that PE condutor is connected (bonded) to the LV Source Neutral which has its own Earth Electrode and also the same PE conductor is connected (bonded) to the equipment frame earthing conductor, which has its own local earth electrode.

May be your practice is different but I have designed & commissioned a number of electrification projects in various states of India and in almost all the projects it was this system of Indian TN-S that I followed.