Capítulo 1. Antecedentes: Motivos del levantamiento del EZLN, Chiapas 1994
2.1. Antecedentes históricos: Conflictos previos entre el EZLN y el Ejército
For several years, increasing malfunction of and damage to electrical and electronic equipment has been noticed, for example
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UnaccounTable faults in data transmission networks.
Desktop and server crashes.
Printer failures.
Slowdown of data transmission in local networks, even to complete standstill.
Triggering of alarm systems and fire detectors.
Corrosion in piping and ground conductorsThe reasons for these effects often lie in an old-style power supply system where the N conductor and the PE conduc-tor are combined to form a single PEN conducconduc-tor. This wasn’t a problem in the old days, as the number of elec-tronic equipment connected into supply was low. The phases were loaded nearly symmetrically, and conse-quently the PEN conductor was hardly loaded.
Owing to an increasing number of high-power single-phase loads, and loads with a high proportion of harmonic contents in the third order (switched power supply units), the phases are loaded extremely asymmetrically, and the N conductor is sometimes loaded with a higher current than the phase conductors. As the PE conductor is meant to carry current only in case of a fault, the PE conductor and the N conductor must be laid separately in new power supply systems (VDE 0100 Part 540 Appendix C.2). If this requirement is not observed in an electrical installation, part of the return current might be distributed through all grounding systems and equipotential conductors. Current flows back to the voltage source through the smallest resistors, so that unwanted currents might even flow through metal pipes and screens of data cables.
These “stray” currents may give rise to strong electromag-netic fields which cause strange failures and malfunction of electronic equipment. They may also cause corrosion in water pipes. Since higher currents may be present in the N conductor, as explained above, care must be taken not to reduce the cross section of the N conductor as compared to that of the phase conductors, but even to increase it.
Effects of conductor design on EMC
Fig. 91/7 demonstrates which problems must be expected if the PE and N conductors are combined to form a PEN conductor. The illustration shows a device through which the current IL flows during operation. Normally, this cur-rent should be taken back to the source through the PEN
screening and a parasitic current Ibuilding in the building.
The parasitic currents flowing through the cable screens interfere with or destroy equipment which is susceptible to overvoltages. Moreover, parasitic currents in the building may result in corrosion and give rise to magnetic fields which may cause further damage. Separate design of the N conductor and PE conductor will prevent such stray cur-rents. Thus, the PE conductor only carries current in case of a fault (Fig. 91/8). ISt = Parasitic currents in screens
∆U = Voltage drop in PE N conductor (external voltage)
Fig. 91/7: Current flow with combined PEN conductor
Distributor ISt = Parasitic currents in screens
ISt = 0
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Power supply systems
In order to avoid parasitic currents, the type of power supply system must be carefully selected. The following section explains two typical examples for coupling the normal power supply (NPS network) and the safety power supply (SPS network). In the first case, the SPS is installed in the immediate vicinity of the NPS (central feed-in) and in the second case, the SPS is installed remote from the NPS (distributed feed-in).
Power supply system for central feed-in
The power supply system shown in Fig. 91/9 is recom-mended for central feed-in, with EMC being ensured even when the supplying sources of sections A and B are oper-ated in parallel. We recommend that the PEN conductor be marked in light blue and, additionally, in green-yellow throughout its course.
The following should be observed for this kind of power supply system:
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The PEN conductor must be wired separately along its whole course, both in the SPS and in the NPS, as well as in the LVMD..
There must be no connection between the neutral points of transformer and generator, and ground or the PE conductor, respectively..
The feeder switches for supply from SPS and NPS must be in 3-pole design..
The supplying sources for sections A and B may be operated in parallel..
A connection between ground and the PE conductor may only be made at one point (central grounding point), as otherwise the PE conductor and the N conductor would be connected in parallel, resulting in unfavorable EMC conditions as shown in Fig. 91/9..
All load feeders are designed as a TN-S system, i.e. with distributed N-conductor function and separate PE and N conductors. 3-pole and 4-pole switching devices may be used.Power supply system for distributed feed-in
Fig. 91/10 depicts the recommended system for distributed feed-in. Distributed feed-in is encountered if the following applies to the distance between sections A and B:
a1 >> a2
As short-circuit currents decrease with distance from the main equipotential bonding conductor, and protective devices require a certain minimum value for safe tripping in the event of a fault, and as selective grading must also be taken into account, a second main equipotential bond-ing conductor is installed for distributed feed-in of the SPS.
The following should be observed for this kind of power supply system:
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The PEN conductor must be wired separately along its whole course in the NPS..
There must be no connection between the neutral point of the transformer and ground or the PE conductor, respectively. Between the neutral point of the generator and ground or the PE conductor, respectively, aconnection for an additional main equipotential bonding conductor is installed.
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A parallel operation between sections A and B is not permitted. The transformers may supply sections A and B at the same time. The generator, however, can only supply section B.Note: During changeover from transformer to generator operation, parallel operation is possible under unfavorable EMC conditions for a short time, for example during back synchronization.
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The switches of the changeover connection in the SPS and the generator supply must be in 4-pole design. The feeder switches for supply of section A must be in 3-pole design..
All load feeders are designed as a TN-S system, i.e. with distributed N-conductor function and separate PE and N conductors. 3-pole and 4-pole switching devices may be used.By implementing a central grounding point in the power supply systems described above, suiTable measuring devices can be used to make sure that no further – imper-missible – splitter bridge between the N conductor and the PE conductor was installed.
Overview of power supply systems according to their connection to earth and their relation to EMC
An overview and evaluation of the different power supply systems with regard to EMC can be found in the standard DIN VDE 0800-2-548. Besides the TN-S system, IT and TT systems are also EMC-friendly systems. Further details can be seen in Table N.1 in the standard.
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SPS NPS
Low-voltage main distributionSource
Section A Section B
Interlock
Central ground point for section B
Central ground
point for Main grounding Protective equipotential bonding – transformer
Main grounding terminal – Generator Protective
equipotential bonding – transformer
L1 L2 L3 PENPE
L1 L2 L3 PEN a1
a2 Fig. 91/9: Power supply system for central feed-in
SPS NPS
Low-voltage main distributionSource
Section A Section B
Central ground point for sections A and B
Protective equipotential bonding – transformer
Equipotential bonding – Generator Protective
equipotential bonding – transformer
Main grounding busbar
The PEN conductor must be wired separately along its whole course!
L1 L2 L3 PENPE
L1 L2 L3
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Interference limits
Electromagnetic alternating fields caused by current trans-mission can negatively influence the undisturbed function of sensitive equipment like computers or measuring tools.
To ensure undisturbed and reliable operation, the interfer-ence limits of the respective equipment should always be observed. DIN VDE 0100-710 defines limit values of mag-netic fields with supply frequency (mains frequency) in hospitals. At a patient's place, the magnetic induction at 50 Hz must not exceed the following values (T = Tesla, magnetic induction):
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0.2 μT for EEG (electroencephalogram).
0.4 μT for ECG (electrocardiogram)The limit value for inductive interference between multi-core cables and lines of the power installation with a conductor cross section > 185 mm2 and the patient places to be protected will certainly be undershot, if the minimum distance of 9 m is kept as recommended by
DIN VDE 0100-710.
When a busbar system is used, this distance may usually be smaller, as the design properties of busbar systems effec-tively reduce magnetic interference fields for the surround-ings.
In order to observe these limits, the magnetic flux density can be reduced by both increasing conductor clearance and a suiTable conductor arrangement. A busbar system
can possibly be used. As an example, the course of the magnetic flux density and the interference limits for ECG and EEG are depicted in Fig. 91/11. This illustration shows the minimum distances, when cables or busbar systems are used, for which the interference limits are observed in hospitals. The magnetic fields of busbar systems depend on the construction (suiTable and symmetrical conductor arrangement and conductor clearances) of the busbar system and the amperage. The illustration compares a SIVACON LXC01 busbar system with a rated current of 1000 A to a conductor arrangement of cables. As it can be seen, the field of the busbar system is initially greater in the close area, but it decreases much more with an in-creasing distance and already causes a weaker magnetic field at a distance of < 1 m than a cable arrangement. For possible applications, characteristic curves of more busbar systems can be found in the engineering manual “Planning with SIVACON 8PS.” Additionally, the illustration shows that even a small asymmetrical load greatly increases the magnetic field. Generally, the following aspects have a favorable impact on the reduction of the course of flux lines:
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Symmetrical conductor arrangement.
Small clearances between conductors.
Symmetrical conductor loads.
Large clearances between conductors and the potentially susceptible equipmentConductor arrangements
Magnetic flux density B in µT
Distance to source of interference in m Interference limit ECG