None of the earthing systems is basically bad. They all ensure safety of persons. Each system has its own advantages and disadvantages and the user must therefore be guided according to his needs, with the exception, however, of prescription or of standard or legislative bans.
When designing an industrial network, a suitable neutral earth arrangement must be selected:
The neutral can either be insulated, or it can be connected to earth. The use of an insulated neutral in an HV network has the advantage of ensuring operational continuity since it does not trip on the first fault, however the network capacitance must be such that an earth fault current is not likely to endanger personnel or damage equipment.
On the other hand, an insulated neutral implies the following:
- the risk of high overvoltages likely to favourize multiple faults,
- the use of super insulated equipment,
- compulsory monitoring of the insulation,
- protection against overvoltages, which will become compulsory in the near future,
- the need for complex, selective protection against earth faults which cannot usually be ensured by simple current-measuring relays. An earthedneutral generally implies mandatory tripping on the first fault, however:
-it reduces overvoltages,
- it provides a simple, reliable, selective means of protection,
- it allows the use of equipment, and in particular cables, with lower insulation levels than for an insulated neutral.
Between 1880 and 1920, transmission and distribution of electrical power took place in «unearthed neutral».
Lines are uninsulated, placed out of reach, supported by insulators; no points of the network are deliberately earthed. In homes, voltage is 100/110 V AC. Throughout this period, fuses blow and persons «receive Electric Shocks» However, in view of distribution voltage level, few persons are electrocuted.
- in UK, in the last quarter of 19 th century, electric arc lighting was developing rapidly. When it was introduced into houses, insurance companies became concerned about danger of fire due to undersized cables, poor jointing and insulation breakdown. Many insurance companies produced sets of rules to minimise their risks.
In May 1882, the Council of the Society of Telegraph Engineers and of
Electricians (later to become the Institution of Electrical Engineers), appointed a committee to consider rules for the prevention of fire risks due to electic light. These rules were not popular with the insurance companies who continued to publish their own. The IEE had yet to become a recognized authority on the subject. By the third edition of the IEE rules in 1897, there was still strong opposition from insurance companies and it was not until 1916 that the final opposition crumbled and the IEE rules became universally accepted in the UK.
In the first edition of the rules, in 1882, two items were concerned with danger to people: no one should be exposed to more than 60 V and the potential between two points in the same room should not exceed 200 V. The earthing of metalwork of appliances working at domestic voltages was first required in the eighth edition in 1924, althought it was soon recognised that an adequate earth was not always easy to obtain.
In 1930, the requirement for an earth leakage trip operating at 30 mA or less was introduced (since deleted).
In France in 1923 a «standard» for electrical installations makes earthing of frames a «requirement»:
- casings of fixed and moving motors, which may be touched in a noninsulated area, in installations with a voltage greater than 150 V,
- fixed and portable electrical household appliances with a power greater than 4 kW,
- electrical bath heater enclosures installed in bathrooms,
- metal parts placed in premise steeped in conductive liquids and which, due to insulation faults, might become live.
The standard provides absolutely no information on earthing conditions or on the value of earth connection resistance, and stipulates no protection device. Although it contains a few rules for fuses these are only for installation conditions.
In order to prevent fuses blowing on a double insulation fault, it quickly become obvious that indication of the presence of the first fault was a good idea.
For this reason, the first failsafe insulation monitor was installed in industrial installations If a lamp goes out, there is a fault between the corresponding phase and the earth.