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Electrical installation design methodology

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

Rules and statutory regulations

Range of low-voltage extends from 0 V to 1 000 V in a.c. and from 0 V to 1 500V in d.c. One of the first decision is the selection of type of current between the alternative current which corresponds to the most common type of current through out the world and the direct current. Then designers have to select the most appropriate rated voltage within these ranges of voltages. When connected to a LV public network, the type of current and the rated voltage are already selected and imposed by the Utility.

Compliance with national regulations is then the second priority of the designers of electrical installation. Regulations may be based on national or international standards such as the IEC 60364 series.

Selection of equipment complying with national or international product standards and appropriate verification of the completed installation is a powerful mean for providing a safe installation with the expected quality. Defining and complying with the verification and testing of the electrical installation at its completion as well as periodic time will guarantee the safety and the quality of this installation all along its life cycle. Conformity of equipment according to the appropriate product standards used within the installation is also of prime importance for the level of safety and quality.

Environmental conditions will become more and more stringent and will need to be considered at the design stage of the installation. This may include national or regional regulations considering the material used in the equipment as well as the dismantling of the installation at its end of life

Installed power loads - Characteristics

A review of all applications needing to be supplied with electricity is to be done. Any possible extensions or modifications during the whole life of the electrical installation are to be considered. Such a review aimed to estimate the current flowing in each circuit of the installation and the power supplies needed.

The total current or 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.)

Estimation of the maximum power demand may use various factors depending on the type of application; type of equipment and type of circuits used within the electrical installation.

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 is 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.

Connection to the MV public distribution network

Where this connection is made at the Medium Voltage level a consumer-type substation will 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 mediumvoltage or low-voltage is possible in this case.

Connection to the LV utility distribution network

Where the connection is made at the Low Voltage level the installation will be connected to the local power network and will (necessarily) be metered according to LV tariffs.

MV & LV architecture selection guide

The whole electrical system including the MV installation and the LV installation is to be studied as a complete system. The customer expectations and technical parameters will impact the architecture of the system as well as the electrical installation characteristics.

Determination of the most suitable architecture of the MV/LV main distribution and LV power distribution level is often the result of optimization and compromise.

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.

LV distribution

The system earthing is one protective measure commonly used for the protection against electric shocks. These systems earthings have a major impact on the LV electrical installation architecture and they need to be analysed as early as possible. Advantages and drawbacks are to be analysed for a correct selection.

Another aspect needing to be considered at the earlier stage is the external influences. In large electrical installation, different external influences may be encountered and need to be considered independently. As a result of these external influences proper selection of equipment according to their IP or IK codes has to be made.

Protection against electric shocks & electrical fires

Protection against electric shock consists in providing provision for basic protection (protection against direct contact) with provision for fault protection (protection against indirect contact). Coordinated provisions result in a protective measure.

One of the most common protective measures consists in “automatic disconnection of supply” where the provision for fault protection consists in the implementation of a system earthing. Deep understanding of each standardized system (TT, TN and IT system) is necessary for a correct implementation.

Electrical fires are caused by overloads, short circuits and earth leakage currents, but also by electric arcs in cables and connections. These dangerous electric arcs are not detected by residual current devices nor by circuit breakers or fuses. The arc fault detector technology makes it possible to detect dangerous arcs and thus provide additional protection of installations. See Protection against arc faults in cables and connections (AFDD) for more information.

Sizing and protection of conductors

Selection of cross-sectional-areas of cables or isolated conductors for line conductors is certainly one of the most important tasks of the design process of an electrical installation as this greatly influences the selection of overcurrent protective devices, the voltage drop along these conductors and the estimation of the prospective short-circuit currents: the maximum value relates to the overcurrent protection and the minimum value relates to the fault protection by automatic disconnection of supply. This has to be done for each circuit of the installation. Similar task is to be done for the neutral conductor and for the Protective Earth (PE) conductor.

LV switchgear: functions & selection

Once the short-circuit current are estimated, protective devices can be selected for the overcurrent protection. Circuit-breakers have also other possible functions such as switching and isolation. A complete understanding of the functionalities offered by all switchgear and controlgear within the installation is necessary. Correct selection of all devices can now be done.

A comprehensive understanding of all functionalities offered by the circuit-breakers is of prime importance as this is the device offering the largest variety of functions.

Overvoltage protection

Direct or indirect lightning strokes can damage electrical equipment at a distance of several kilometres. Operating voltage surges, transient and industrial frequency overvoltage can also produce the same consequences.All protective measures against overvoltage need to be assessed. One of the most used corresponds to the use of Surge Protective Devices (SPD). Their selection; installation and protection within the electrical installation request some particular attention.

Energy efficiency in electrical distribution

Implementation of active energy efficiency measures 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. These measures will most of the time request specific design for the installation as measuring electricity consumption either per application (lighting, heating, process…) or per area (floor, workshop) present particular interest for reducing the electricity consumption stillkeeping the same level of service provided to the user.

Reactive energy

The power factor correction within electrical installations is carried out locally, globally or as a combination of both methods. Improving the power factor has a direct impact on the billing of consumed electricity and may also have an impact on the energy efficiency.

Harmonics

Harmonic currents 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.

Residential and other special locations

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.

Measurement

Measurement is becoming more and more an essential part of the electrical installations. Chapter S is an introduction to the different applications of measurements, such as energy efficiency, energy usage analysis, billing, cost allocation, power quality ... It also provides a panorama of the relevant standards for these applications, with a special focus on the IEC 61557-12 related to Power Metering and monitoring devices (PMD).

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 according to several operating modes (normal, back-up, load shedding)
  • Calculation of voltage drops
  • Optimization of cable sizes
  • Required ratings and settings of switchgear and fusegear
  • Selectivity of protective devices
  • Optimization of switchgear using cascading
  • Verification of the protection of people and circuits
  • Comprehensive print-out of the foregoing calculated design data


Notes

  1. ^ Ecodial is a Schneider Electric software available in several languages and according to different electrical installation standards