Schneider
technical updates
VOLUME 4 – DECEMBER 2004
Role and Protection of Neutral in a LV Installation In a well-designed three-phase installation, with the exception of singlephase terminal distribution circuits, the Neutral conductor does not convey current (or only very little: less than 15% of phase current). Its potential with respect to the ground is consequently often zero. In practice, this conductor is very rarely "neutral" and can be a source of disturbance for the operator if no precautions are taken. The function of the Neutral in the Earthing systems is to ensure: • Proper operation operation of the installation (safety (safety with power power ON)
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• Proper protection protection of persons in contact contact with de-energized de-energized parts parts of the installation (safety with power OFF)
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Protecting the Neutral against overloads and short circuit is essential.
Overloads and Short-Circuits In the event of overloads on the Neutral or of a Phase to Neutral shortcircuit, the same fault current, Id, flows through the conductors. Consequently, two possibilities must be examined: Neutral Conductor and Phases with the Same Cross-Section
If Sn = Sph, the Neutral conductor is protected in the event of a phase to Neutral fault by the phase overcurrent protection. The IEC 60364 standard then stipulates in Section 431.2.1: a) When the cross-section, Sn, of the Neutral conductor is at least equivalent to the cross-section, Sph, of the phase conductors, there is no need to provide an overcurrent overcurrent detector or a breaking device on the Neutral conductor. puush too p triip
Sn = Sph 0 A In=25
N
1
2
3
4D 4D
Id N
Figure 2: Phase protection on the Phase to Neutral fault
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Figure 1: Sn = Sph
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technical updates VOLUME 4 Sn = Sph N
1
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DECEMBER 2004
Neutral Conductor with a Smaller Cross-Section than the Phases 2
3
Imax
The Neutral conductor only conveys currents when there is a high unbalance and even then these currents rarely exceed 10% of phase current in well-designed installations. Consequently, it is tempting and economically advantageous to reduce its cross-section. The crosssection Sn = Sph / 2 is the one normally chosen. Thus, if Sn = Sph / 2, the Neutral conductor must be protected against Phase to Neutral faults by a specific overcurrent protection. If this protection acts directly on the phases, protection of the Neutral is guaranteed without need to break.
Figure 3: Sn < Sph
The IEC 60364 standard then stipulates in Section 431.2.1: b) When the cross-section, Sn, of the Neutral conductor, is smaller than the cross-section, Sph, of the phase conductors, it is necessary to provide an overcurrent detector on the Neutral conductor, adapted to the cross-section of this conductor. This detector must cause the Phases conductors, but not necessarily the neutral conductor, to break.
In << Iph
Figure 4: Breaking of the Neutral is not compulsory
4P3D-N/2
Figure 5: However, protection y the 4P 3D-N/2 circuitbreaker is an optimum solution
In point of fact, this type of protection is not always reliable or economic. The use of four-pole 4P 3D-N/2 circuit-breakers (“Half Neutral” ) is an optimum solution which also guarantees breaking (often recommended) of the Neutral conductor.
Effect on the Neutral Conductor in TT System: p
Energised Installation*
P
Protection: No effect as the installation fault current (small) does not flow through this conductor.
P
Neutral breaking: No effect for the same reason
P
De-energised installation
P
Disconnection compulsory as in the IEC 60364 Section 536.2
Indeed in the event of overvoltage on the MV (transformer breakdown or fault), the Neutral potential rises causing a very dangerous potential to appear (a few hundred volts) between the Neutral and the application ground (Figure 6). Consequently, a person performing maintenance on the machine may in this case be in direct contact with the Neutral conductor at this high voltage and the risk is at its greatest. Installation standards, in particular the IEC 60364 Section 536.2, take account of this risk by stipulating disconnection of Neutral conductors. If disconnection is performed by a multipole breaking function ensuring both simultaneous breaking then disconnection of the phases and the neutral, the result is increased safety of maintenance with power off. Disconnection is thus a necessity. A four-pole circuit-breaker enabling multipole breaking and disconnection naturally meets all the requirements of the IEC 60364 standard.
Effect on the Neutral Conductor in TN-S System: Figure 6: Effect of a fault on MV – Dangerous contact risk
* Please refer to Electricity Regulations 1994, Part II, Regulation 18.
p
Energised Installation*
P
Protection: No effect as the fault current does not flow through this conductor.
P
Breaking: No effect for the same reason.
P
De-energised installation
P
Disconnection: The IEC 60364 standard recommends disconnection in Section 536.2.
Some countries make disconnection a requirement on the basis that, just as in TT, the potential of this conductor cannot be guaranteed. The two examples below highlight these problems.
technical updates VOLUME 4
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DECEMBER 2004
Example 1: If a lightning wave (frequency around MHz) reaches the MV, it will not be stopped by grounding the transformer (as the inductive component – Lω – of the grounding Neutral connection is dominant at these frequencies since ω = 2π f) and the potential (dangerous) will be automatically transmitted to the Neutral conductor.
Figure 7: Consequence of a MV lightning stroke
In this case, the maintenance operator will be in direct contact with the overvoltage. Disconnection is recommended and thus breaking of the Neutral in the TN-S system. Example 2: High-rise buildings
It is harder to guarantee the quality of grounding connections in the various storeys of high-rise buildings. The potential of the frames moves away from the potential of the ground at the bottom of the foundations due to the exceptionally long cables used. To avoid generating dangerous situations, breaking the Neutral is recommended.
Effect on the Neutral Conductor in IT System: p
Energised Installation*
It is recommended not to distribute the Neutral. If a double fault occurs, one of which concerns the Neutral conductor, the conductor may be subjected to overload independently from the current flowing in the Phases (see Figure 8). Figure 8: IT system in a double fault situation
The Neutral of B is designed for 100 A. The phase of A, designed for 1000 A, will not protect it, hence: P
Protection: Compulsory
P
Breaking: Compulsory (multipole)
P
De-energised installation
P
Disconnection
As the neutral is not grounded, the effects of MV overvoltages are greater than with a TN/TT grounding system, and disconnection is compulsory (IEC 60364 Section 431.2.2.).
General Problem Relating to Source Coupling in the Event of an Insulation Fault and Neutral Not Broken S1
S2
GFP
GFP
When two sources, S1 and S2, are coupled using the TN-S system without breaking of the Neutral (Figure 9), if an insulation fault occurs, part of the fault current (ld1) returns normally to the source via the PE, but another part (ld2) may return to the source via the metal structures.
Id1 Id2
PE
Service ground
This ld2 current is detected by the Ground Fault Protection (GFP) protection of source S2.
Neutral
Id2
Ground bus
Loads
Service ground
Figure 9: Insulation fault with Neutral not broken
According to current distribution, the protection of source S1 where the fault occurred may not trip as the sensitivity threshold is no longer reached. However the ld2 current may, on the contrary, disturb the GFP and cause untimely tripping if its threshold is lower than that of the coupling switch.
Problem Relating to Source Coupling when Harmonics are Present Let us now reconsider the two sources, S1 and S2, still coupled in TN-S without breaking of the Neutral but this time without an insulation fault and with the presence of 3rd order harmonics (Figure 10).
* Please refer to Electricity Regulations 1994, Part II, Regulation 18.
These harmonics cumulate in the Neutral which conveys a non-negligible current. As a result, of the Neutral connections with the metal frames, this current may return to the source S1 via the Neutral conductor and the PE of the installation, and a non-negligible current may thus flow in the structures even if the two networks are independent.
technical updates VOLUME 4
S1
GFP
PE
GFP Neutral
Ground bus
Service ground
DECEMBER 2004
This natural “pollution” of S2 can be considerable and serious if the equipment supplied is sensitive.
S2
Id2
–
Loads
Service ground
Figure 10: Harmonic current without insulation fault with Neutral not broken
For example, if in Figure 10 a distribution of 3000 A is considered, 5% of 3rd order harmonics cumulating in the Neutral generate a flow of 3 x 3000 x 5% = 450 A. With 10% of current returned via the coupling, this gives a current of 45 A in the PE and structures, a zero sequence current, which thus generates electromagnetic radiation. Multipole breaking on the coupling switch will eliminate this harmonic pollution. Moreover if breaking is performed on both the source circuitbreakers, proper operation of the installation is guaranteed in all circumstances. A transfer switch of four-pole is recommended by IEC 60364 Section 444.4.9.
Conclusion Circuit-Breaker Protection
Article 530.3.1 of the IEC 60364 standard stipulates that: “ All the moving contacts of all the poles of the multipole devices must be mechanically coupled so that their opening and closing is virtually simultaneous.” In this case, a four-pole circuit-breaker must be used which ensures simultaneous opening and closing of phases and Neutral. As a result, protection of circuits using a four pole circuit breaker/disconnector guarantees the quality of the electrical power supply during operation and safety with power off.
Masterpact NW/NT
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