IT System : the neutral is insulated or connected to earth through an impedance. The earths are connected to a local
earthing system, which may be separate or connected to that of the neutral.
It should be emphasised that if the generating set provides energy for safety services, the interruption of which could
jeopardise the safety of persons, an IT system should preferably be adopted, since it avoids cut-off for first earth fault.
Circuits fed by mobile generating sets usually have limited dimensions and in theory the protective measure for electrical
separation could be applied, Fig. 4. This protective measure is not advisable, however, for a complete temporary
installation, e.g. on a building site, because a wide-scale use of flexible cables is incompatible with the high level of
earthing insulation required.
A double earth fault would jeopardise safety against indirect contacts, Fig. 5. Efficient protection may be provided by
means of differential switches placed on the individual shunts, as shown in Fig. 6. A single differential device inserted to
protect the entire plant would have little effect, as can be seen clearly from Fig. 7. The system shown in Fig. 8, where the
earths are connected to the neutral insulated from earth, is dangerous (false TN), because at the first earth fault of a phase,
e.g. on a cable, the person in contact with an earth is in danger, even if the appliance is intact.
A snap-action differential on/off switch may act, Fig. 9, but it is the same current that goes through the person that activates
the protective device, a factor that makes this system of protection unsuitable, but tolerable in the specific case of non-
general application. In particular situations, the generating set feeds a small user plant totally situated on a continuous metal
structure, e.g. a generating set placed on board a mobile means, where earth faults are not anticipated, only to the actual
metal structure (there are no sockets from which appliances outside the metal structure can be powered).
Metallwarenfabrik Gemmingen Telefon: 07267 / 806 193
Stromerzeuger
&
Telefax: 07267 / 806 198
Industriestr. 1 www.metalwarenfabrik.com
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