c Standard filters often have a much lower voltage withstand than that of the Influence of filtering capacitors on the electrotechnical equipment...

Pokaż mi serce nie opętane zwodniczymi marzeniami, a pokażę ci człowieka szczęśliwego.

various earthing systems v These filters are therefore more vulnerable to c In TN, they do not hamper the circuit-breakers common mode overvoltages: they may need and fuses and therefore do not affect the closer protection by a voltage suppressor protection of people provided by these devices.
(varistor). This poses the more global problem of c In IT, they do not increase the disturbance of coordination of insulating devices in LV.
short-circuit protection devices. But if there are v These filters are at the origin of 50 Hz leakage large amounts of electronic equipment
currents which, although limited by product incorporating these filters, they may hamper the Cahier Technique Schneider Electric no. 177 / p.20
operation of LF current injection PIMs.
Note that protection equipment has been the DC injection devices are therefore preferable.
subject of numerous improvements.
c In TT, leakage currents due to the filters, when For example, nowadays RCDs:
there are large amounts of electronic equipment, v are unaffected by steep edge disturbances and can cause accidental operation of high-transient currents,
sensitivity (30 mA), or even medium-sensitivity v are immune to pulsed unidirectional currents, (0.3 A) RCDs. It is for this reason that in normal v allow overvoltages due to lightning to flow to practice no more than three power sockets are earth via the lightning arrester without tripping protected by one 30 mA RCD.
(RCD with slight delay on tripping).
Cahier Technique Schneider Electric no. 177 / p.21
Conclusion
The various earthing systems are equivalent in For the TN-S system, in view of the high terms of protection of people.
disturbing fault currents, it is advisable to create But, with the development of communicating a functional EBS, separated from the earth digital systems and the proliferation of disturbing circuit (PE) and therefore truly equipotential (see devices, the design of electrical installations fig. 10) which, with the conductive floors and requires that the cohabitation of “high currents”
structures, will constitute a mitigating ground and “low currents” is controlled and hence that plane effect and Faraday cage.
installation methods are reconsidered, as well as The IT system offers the best continuity of the choice of earthing system.
service with a very low disturbance level, but if the occurrence of the double fault is taken into At installation level
account, the specifications are the same as in At this level, it is necessary to reduce both the TN-S.
sources of disturbances (power and radiation), The TT system generates the least amount of and the sensitivity of devices and especially of disturbances in the event of an insulation fault.
“low current” links.
It makes it possible to continue to mix both For this it is important to:
functional bonding conductors and electrical c Avoid connecting lightning rod downcomers exposed conductive parts closely and to make and MV exposed conductive parts to the neutral the most of the meshing and equipotentiality.
earth connection (elimination of common mode Without doubt, faced with the new problem of overvoltages resulting from common-impedance systems communicating via digital links, the coupling).
whole problem concerns the equipotentiality, c Ensure that the PE runs along the live in LF and HF, of all the exposed conductive conductors (reduction of inductive couplings) parts in the entire installation.
and is only connected, in the distribution, to the The answer in terms of implementation of exposed conductive parts of electrical loads, earthing systems is:
especially in TN.
c For all earthing systems: create mitigating c Use equipotential metallic cable trays in ground planes (floors, metallic cable trays), relation to the main equipotential bonding interconnect them and avoid “high current” -
(reduction in radiation from the electrical power
“low current” loops.
cables and effect of the parallel earthing conductor and ground plane for sensitive c For TN-S and IT earthing systems (2nd fault), separate the earth network (PE) from the circuits).
equipotential bonding system or very strongly c Clearly separate “low current” links from power mesh all the exposed conductive parts in order cables if they are on the same support, or to divide the 50 Hz fault currents and the HF
preferably place them on different neighboring disturbing currents.
cable trays.
Closely connecting anything metal is a solution In reality, the “high current” and “low current” links usually put forward by the English. In practice it often have different paths. Therefore, a parallel is only applicable in “mainly metal buildings”, earthing conductor (or equivalent) must be used developed vertically and where construction is for “low current” circuits and thus a functional closely monitored.
equipotential bonding system is created.
c The TT system is the one which best solves At earthing system level
the problem posed by the proliferation of digital links in buildings, as long as the earth The TN-C system, already prohibited in areas at connections of the loads are interconnected by risk of fire and explosion, must not be used since the PE.
the neutral currents circulating in the PEN
disturb equipotentiality.
Moreover, if some of the neutral and fault currents circulate in the metal structures of the building, these stray currents, as well as the phases + PEN cable become generators of disturbing magnetic fields.
Cahier Technique Schneider Electric no. 177 / p.22
Appendix 1: earthing systems according to IEC 60364
The three earthing systems standardized at a presentation of the risks and associated international level are nowadays included in protection devices.
many national standards.
A brief summary of their protection principles is These three neutral systems are studied in detail given below.
in “Cahier Technique” no. 172 with, for each one, The TN system (see fig. 18 ) c The transformer neutral is earthed.
If the outward circuit impedance equals that of the return circuit, the fault voltage (exposed c The exposed conductive parts of the electrical conductive part/deep earth) known as “indirect loads are connected to the neutral.