After consulting applicable regulations, Figures E16 and E17 can be used as an aid in deciding on divisions and possible galvanic isolation of appropriate sections of a proposed installation.
Division of source
This technique concerns the use of several transformers instead of employing one high-rated unit. In this way, a load that is a source of network disturbances (large motors, furnaces, etc.) can be supplied by its own transformer.
The quality and continuity of supply to the whole installation are thereby improved.
The cost of switchgear is reduced (short-circuit current level is lower).
The cost-effectiveness of separate transformers must be determined on a case by case basis.
Network islands
The creation of galvanically-separated “islands” by means of LV/LV transformers makes it possible to optimise the choice of earthing methods to meet specific requirements (see Fig. E18and Fig. E19 ).
Fig. E18: TN-S island within an IT system
Fig. E19: IT islands within a TN-S system
Conclusion
The optimisation of the performance of the whole installation governs the choice of earthing system.
Including:
- Initial investments, and
- Future operational expenditures, hard to assess, that can arise from insufficient reliability, quality of equipment, safety, continuity of service, etc.
An ideal structure would comprise normal power supply sources, local reserve power supply sources (see section 1.4 of Chapter E) and the appropriate earthing arrangements