Electrical Installation Design

[Darth Gupta]

Rangeof 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 itslife 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

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.)

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.

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.

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 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.

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.

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.

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.

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 theelectrical installation request some particular attention.

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.

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.

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 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).

The Electrical Installation Guide is now available here as a wiki (Electrical Installation Wiki). This wiki is a collaborative platform, brought to you by Schneider Electric: our experts are continuously improving its content, as they were doing for the guide. Collaboration to this wiki is also open to all.

There are workarounds for many of these tasks. But I fear there is no desire from the official library team to add symbols for a task that is better handled in other software. EPLAN is the king here (similar to Altium in the field of PCB design) but there are cheaper options available. One such option is DesignSpark Electrical by RS. But there are also open source alternatives like (Disclaimer: I have not personally used any of these tools.)

I have no experience with any such programs. A long time ago when I was using a DOS version of some PCB design program, one of my brothers told me electrical wiring diagrams for cabinets were made with autocad, with some special libraries and extensions.

If you want to make a lot of such drawings, specialized software is probably a better choice than drawing them in KiCad, but for occasional use or if you do not want to install and learn yet another program them Eeschema may be adequate.

I would suggest in answer to your question sufficient information to enable the installer to complete the installation which is compliant with BS 7671. Enough information to satisfy the requirements of the CDM Regulations 2015. Enough information for the inspector to inspect and test the installation.

I fully agree with JP (wouldn't dare do otherwise ? ) but any electrician who puts in any new work has designed it unless somebody else takes responsibility. That applies even to extending a circuit - is there spare capacity, is the Zs at the far end within limits and so on? Don't forget the wording of an MEIWC, part 5:

I have priced and completed jobs for builders where the architects have written an electrical work specifications including the sizes of circuit conductors and installation methods, I did seriously consider using a three part EIC and leaving the designer box empty rather than using a single signatory EIC.

However, the designer certifies that he (she) has designed the installation i.a.w. BS 7671. The installer certifies that he (she) has installed i.a.w. BS 7671. So any installer must be capable of verifying that the design is sound.

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