Source of information: http://www.schneider-electric.com/documents/technical-publications/en/shared/electrical-engineering/dependability-availability-safety/low-voltage-minus-1kv/ect172.pdf
Today the 3 system earthings such as defined in IEC 60364 and French standard NF C 15-100, are:
- Exposed-conductive parts connected to neutral -TN;
- Earthed neutral - TT;
- Unearthed (or impedance-earthed) neutral - IT.
The purpose of these three earthings is identical as regards protection of persons and property: Mastery of insulation fault effects. They are considered to be equivalent with respect to safety of persons against indirect contacts. However, the same is not necessarily true for dependability of the LV electrical installation with respect to:
- Electrical power availability;
- Installation maintenance.
These quantities, which can be calculated, are subjected to increasingly exacting requirements in factories and tertiary buildings. Moreover, the control and monitoring systems of buildings -TBM- and electrical power distribution management systems -TEM- play an increasingly important role in management and dependability.
This evolution in dependability requirements therefore affects the choice of system earthing.
It should be borne in mind that the concern with continuity of service (keeping a sound network in public distribution by disconnecting consumers with insulation faults) played a role when system earthings first emerged.
In order to ensure protection of persons and continuity of service, conductors and live parts of electrical installations are "insulated" from the frames connected to the earth.
Insulation is achieved by:
- Use of insulating materials;
- Distancing, which calls for clearances in gases (e.g. in air) and creepage distances (concerning switchgear, e.g. an insulator flash over path)
Insulation is characterised by specified voltages which, in accordance with standards, are applied to new products and equipment:
- Insulating voltage (highest network voltage);
- Lightning impulse withstand voltage (1.2; 50 ms wave);
- Power frequency withstand voltage (2 U + 1,000 V/1mn).
Example for a LV PRISMA type switchboard:
- Insulating voltage: 1,000 V;
- Impulse voltage: 12 kV.
When a new installation is commissioned, produced as per proper practices with products manufactured as in standards, the risk of insulation faults is extremely small; as the installation ages, however, this risk increases.
In point of fact, the installation is subject to various aggressions
which give rise to insulation faults, for example:
- During installation;
- Mechanical damage to a cable insulator;
- During operation;
- Conductive dust;
- Thermal ageing of insulators due to excessive temperature caused by:
- Climate;
- Too many cables in a duct;
- A poorly ventilated cubicle;
- Harmonics;
- Overcurrents, etc.
- The electrodynamic forces developed during a short-circuit which may damage a cable or reduce a clearance;
- The operating and lightning overvoltages;
- The 50 Hz return overvoltages, resulting from an insulation fault in MV.
It is normally a combination of these primary causes which results in the insulation fault.