Diagram of connection to the ground

In electricity, a Diagram of Connection to the Earth , or SLT (In the past Mode of neutral ) defines the mode of connection in the ground of a Transformateur of distribution and masses side user.

The diagrams of connection to the ground have as an aim of protection the people and the material by controlling the defects of Isolement. Indeed, for safety reasons, very conducting part of an installation is isolated compared to the Masse S. This insulation can be done by distance, or by the use of materials Isolant S. But with time, the insulation can worsen (because of the Vibration S, the shock, dust, etc), and thus put a mass (the metal carcass of a machine for example) under a potential dangerous. This defect found of the risks for the people, the goods but also the continuity of service.

According to the standard CEI 60364 (replaced by the guide of load CEI 60076-7 ED. 1), a diagram of connection to the ground is characterized by two letters. The first indicates the connection of the neutral of the transformer, it can be:

T for connected to the ground;
I for insulated (or impédant ) compared to the ground.

The second letter indicates the way of connecting the masses users, it can be:

T for connected to the ground;
NR for connected to the neutral, which is connected to the ground.

Diagram TN

General information

In the SLT TN , the neutral of the transformer secondary is connected to the ground and the masses users are connected to the driver of protection principal itself connected to the earth electrode. The unit is thus inter-connected with a collecting copper bar whose electrode earth bottom trench is connected there. Standards CEI 60364 and NF C 15-100 define 3 subschema for SLT TN: TN-C (ground and confused neutral), TN-C/S (TNC for the circuit principal and TNS for the final circuits and section of the driver <10mm ² copper) and TN-S (ground and separate neutral).

the cost of a diagram of connection to ground TN-C is less because the driver of protection " PE" and conducting neutral " N" is confused what gives us a driver " PEN" (saving of a cable and a pole of protection overcurrent). This driver " PEN" in priority a driver of protection is before being a neutral conductive, it should not be crossed in order to ensure the protection of the people.
the current fault being limited only by the impedance cables, the intensity of short-circuit is more important. The diagram of connection to the ground (in the past " mode of neutre") TN-C is prohibited by standard NF C 15-100 in the buildings where there is an explosion or fire hazard).

However, it is necessary to refer on standard NF-C 15-100 for more precision.

TN-C

In TN-C , the drivers of neutral (NR) and protection (PE) are confused to form the PEN.

This SLT makes it possible to save a cable (as well as a pole on each apparatus of protection), but it is necessary to use a special cable mechanically reinforced in order to limit the risk of cut of the driver of protection (PE). If the EP is cut, the masses users are put under the potential of the phase.
to ensure the safety of the people in the event of too long line, it is necessary to use a Residual Differential Dispositif (GDR).

TN-S

In TN-S , the driver of protection and the neutral are connected only to the station of distribution but to any other point.

TN-S is obligatory for the networks having drivers with a section ≤ 10 mm ² out of Copper.

TN-C-S

The driver of protection (PE) and the neutral (NR) are confused transformer until the point of distribution, and then separate on the final circuits and section of driver < 10 mm ² copper. One can also find a résitance which connects the neutral to the ground. That makes it possible to limit the current of short-circuit of a hundred amps. Thus Id (Fault Current) will be function of resistance (If R high… Weak Id).

Diagram TT

The neutral of the transformer is connected to the ground, and the masses of the equipment of the users have their own connection with the ground.

This diagram of connection to the ground is most frequent at the private individuals in France.
the use of a GDR is obligatory at the head of installation to ensure the protection of the people (like that of maximum value 30mA on the circuits taken).

Diagram IT

Characteristics

The principal characteristic of this diagram is that the transformer upstream of the installation is completely isolated from the ground (it is known as “floating”, gràce with the galvanic insulation suitable for the transformer ). The three phases and especially the neutral are not connected to the ground, contrary to the other diagrams. Actually, the neutral can be connected to the ground via the stray capacities of the cables, or voluntarily via an impedance of strong value (1500Ω). The masses user are inter-connected normally and connected to the ground.

One speaks about first defect when an apparatus or a user connects one of the three phases to the ground (through the frame of the apparatus for example).

One speaks about second defect when a second short-circuit with the ground appears after a first defect, either on the same phase, or on one of both others.

strong points

In the case of a first defect, there exists in theory no danger to the people and the equipment: because of insulation of the transformer upstream, the fact of putting a phase at the ground does not induce any electric current. Contrary to the other diagrams, this case does not oblige the cut of the supply of electricity: this very important point explains its use in the fields where the supply of electricity is vital: operating room suites of the hospitals, buildings at the risks of explosion, installations of emergency lighting, as well as the industrial fields which have a requirement of continuity of service: electrical communications MT.

limitations

If the first defect is not quickly treated, a second defect can appear and prove to be dangerous, even mortal. It is thus necessary to use a permanent Contrôleur of insulation ( CPI , not represented on the diagram) to announce a first defect. This controller must announce the defect to a team of maintenance which must leave to its research. The security standards thus impose the permanent availability of a personnel of maintenance qualified on the site.

There exists a case for which a mortal risk can appear as of the first defect: if two buildings having their own ground are supplied by same network IT, and that a defect appears on two different phases in each building, then a cable connecting the two buildings (such as a cable of telecommunication) could be carried with the potential of the sector (generally 400V) in one of the two buildings. This is why it is strongly advised to inter-connect together all the grounds of the same network IT.

The use of electrical material with important leakage currents (stray capacities between phase and frame), or in great number will amplify the current in CPI, at the point to present fire hazards.

The material and protections must be adapted in order to accept important tensions between neutral/phase and the ground. Because of the character floating of the neutral, of disturbances BF of common mode can be at the origin of these overpressures. An impedance about the kohm can be connected between the neutral of the transformer and the ground, this in order to reduce the variations of potential between the network and the ground: it is thus particularly important in the networks feeding of the sensitive apparatuses.

The localization of a defect is difficult, even practically impossible in the case of a second defect on the same phase. A technique of localization consists in injecting a current of 10Hz on the level of CPI, and to detect the leakage using an amperometric grip and of a selective filter.

To protect the installation against overpressures (the lightning for example) from with dimensions the high-tension, standard NF C 15-100 obliges to place a low-voltage surge limiter between the neutral point of the transformer and the ground (not represented on the diagram).

All these constraints explain that this diagram is disadvised, even interdict in the domestic facilities for example.

Protection of the neutral according to the SLT

the neutral must be cut in mode TT and TN if the section of the neutral is lower than that of the phase.
the neutral must be protected and cut in mode IT (because one of the defect can be on the neutral) and in mode TT and TN-S if the section of the neutral is lower than that of the phase.
In TN-C the neutral should not not be crossed, because it is also the driver of protection.
the neutral must be protected, some is the diagram of connection to the ground, if its section is lower than that of the phases

Uses of the SLT in the world

In the United States, TN-C is mainly used. Earthing of the neutral is made at subscriber BT.
In France and Belgium, TT is obligatory in public distribution with protection of the socket-outlets by a GDR of sensitivity 30mA.
In Great Britain, the new installations are in TN-C . The earth electrode of the neutral is provided by the supplier of energy.
In Germany, TT and TN-C cohabit, but the earth electrode is at the subscriber.
In Norway, the buildings being out of insulating materials and the earth electrodes from bad quality, the selected SLT is IT with use of GDR of sensitivity 30mA in indication and cut to the second defect by the circuit breaker.

Electromagnetic compatibility

TN-C is bad from the point of view of magnetic compatibility because of strong currents circulate in the PEN and modify the equipotentiality.
In TN-S , it is advised to separate the driver from protection (PE) of the functional masses.
In IT , due to the very low value of the current of first defect, the electromagnetic disturbance is weak. With the second defect, the problem is the same one as in TN-S
In TT , very few disturbances are generated in the event of defect, the driver of protection and the functional masses can be separate.

Random links: