Electrical installation design methodology

For the best results in electrical installation design it is recommended to read all the chapters of this guide in the order in which they are presented.

Listing of power demands

The study of a proposed electrical installation requires an adequate understanding of all governing rules and regulations. The total power demand can be calculated from the data relative to the location and power of each load, together with the knowledge of the operating modes (steady state demand, starting conditions, non simultaneous operation, etc.)
From these data, the power required from the supply source and (where appropriate) the number of sources necessary for an adequate supply to the installation are readily obtained.
Local information regarding tariff structures is also required to allow the best choice of connection arrangement to the power-supply network, e.g. at medium voltage or low voltage level.

Service connection

This connection can be made at:

• Medium Voltage level
  A consumer-type substation will then have to be studied, built and equipped. This substation may be an outdoor or indoor installation conforming to relevant standards and regulations (the low-voltage section may be studied separately if necessary). Metering at medium-voltage or low-voltage is possible in this case.
• Low Voltage level
  The installation will be connected to the local power network and will (necessarily) be metered according to LV tariffs.

Electrical Distribution architecture

The whole installation distribution network is studied as a complete system. A selection guide is proposed for determination of the most suitable architecture. MV/LV main distribution and LV power distribution levels are covered.

Neutral earthing arrangements are chosen according to local regulations, constraints related to the power-supply, and to the type of loads.

The distribution equipment (panelboards, switchgears, circuit connections, ...) are determined from building plans and from the location and grouping of loads. The type of premises and allocation can influence their immunity to external disturbances.

Protection against electric shocks

The earthing system (TT, IT or TN) having been previously determined, then the appropriate protective devices must be implemented in order to achieve protection against hazards of direct or indirect contact.

Circuits and switchgear

Each circuit is then studied in detail. From the rated currents of the loads, the level of short-circuit current, and the type of protective device, the cross-sectional area of circuit conductors can be determined, taking into account the nature of the cableways and their influence on the current rating of conductors.

Before adopting the conductor size indicated above, the following requirements must be satisfied:

• The voltage drop complies with the relevant standard
• Motor starting is satisfactory
• Protection against electric shock is assured

The short-circuit current Isc is then determined, and the thermal and electrodynamic withstand capability of the circuit is checked.

These calculations may indicate that it is necessary to use a conductor size larger than the size originally chosen.

The performance required by the switchgear will determine its type and characteristics.

The use of cascading techniques and the discriminative operation of fuses and tripping of circuit breakers are examined.

Protection against overvoltages

Direct or indirect lightning strokes can damage electrical equipment at a distance of several kilometers. Operating voltage surges, transient and industrial frequency over-voltage can also produce the same consequences.The effects are examined and solutions are proposed.

Energy efficiency in electrical distribution

Implementation of measuring devices with an adequate communication system within the electrical installation can produce high benefits for the user or owner: reduced power consumption, reduced cost of energy, better use of electrical equipment.

Reactive energy

The power factor correction within electrical installations is carried out locally, globally or as a combination of both methods.

Harmonics

Harmonics in the network affect the quality of energy and are at the origin of many disturbances as overloads, vibrations, ageing of equipment, trouble of sensitive equipment, of local area networks, telephone networks. This chapter deals with the origins and the effects of harmonics and explain how to measure them and present the solutions.

Particular supply sources and loads

Particular items or equipment are studied:

• Specific sources such as alternators or inverters
• Specific loads with special characteristics, such as induction motors, lighting circuits or LV/LV transformers
• Specific systems, such as direct-current networks

A green and economical energy

The solar energy development has to respect specific installation rules.

Generic applications

Certain premises and locations are subject to particularly strict regulations: the most common example being residential dwellings.

EMC Guidelines

Some basic rules must be followed in order to ensure Electromagnetic Compatibility. Non observance of these rules may have serious consequences in the operation of the electrical installation: disturbance of communication systems, nuisance tripping of protection devices, and even destruction of sensitive devices.

Ecodial software

Ecodial software⁽¹⁾ provides a complete design package for LV installations, in accordance with IEC standards and recommendations.
The following features are included:

• Construction of one-line diagrams
• Calculation of short-circuit currents
• Calculation of voltage drops
• Optimization of cable sizes
• Required ratings of switchgear and fusegear
• Discrimination of protective devices
• Recommendations for cascading schemes
• Verification of the protection of people
• Comprehensive print-out of the foregoing calculated design data