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Lalith A. Samaliarachchi
Session 4
Assessment of general characteristics
Contents
4.1 Assesment of general characteristics 4.1.1 Purpose, Supplies and Stucture 4.1.2 Clasifications of external influances 4.1.3 Compatibility 4.1.4 Maintainability 4.1.5 Recognized safety services 4.1.6 Continuity of service 4.2 Type of earthing systems 4.3 General Aim The aim of this lesson is to teach you how to assess the general characteristics of an electrical installation. Specific objectives At the end of this lesson you will be able to:
- give an abbreviated check list for the designer to assess the relevant facts
- State the main types of earthing systems and subdivisions where applicable, and discuss the significance of the designation used.
- Diagrammatically describe the types of systems taking an example of a single phase installation and/or a 3-phase installation.
- Give the main parameters to be assessed Under Nature of Supply.
Lalith A. Samaliarachchi
4.1 Assessment of general characteristics
The first step in the design of an electrical installation is the assessment of the general characteristics of the installation. Before planning or carrying out an electrical installation, thought must be given to a number of factors that will effect its operation. If these factors are not properly assessed, the completed installation may not be safe or may be unsuitable for its purpose. 4.1.1 Purpose, Supplies and Structure In here, the purpose for which the installation is intended to be used, its general structure, and its supplies are assessed.
- Maximum Demand and Diversity -For economic and reliable design, the maximum demand of an installation shall be assessed. In determining the maximum demand of an installation or part thereof, diversity may be taken into account.
- Arrangement of Live Conductors and Type of Earthing- In general the characteristics mentioned in Regulation 312.2.1 and 312.3.1 shall be ascertained and appropriate methods of protection for safety should be selected in compliance with Part 4. The number and type of live conductors (e.g. 1-phase 2-wire a.c., 3-phase 4-wire a.c.) shall be determined, both for the source of energy and for each circuit to be used within the installation. Where the source of energy is provided by a distributor, the distributor shall be consulted if necessary.
- Type of earthing arrangement- The type of earthing system to be used for the installation shall be determined, due account being taken of the characteristics of the source of energy, and in particular of any facilities for earthing. Types of systems are: TN-C, TN-S, TN-C-S, TT and IT (discussed later in this lesson) When we talk about the Supplies in general, the following characteristics of the supply or supplies, from whatever source, and the normal range of those characteristics were appropriate, shall be determined by calculation, measurement, enquiry or inspection:
- The nominal voltage(s) and its characteristics including harmonic distortion
- The nature of the current and frequency
- The prospective short-circuit current at the origin of the installation
- The earth fault loop impedance of that part of the system external to the installation, Ze
- The suitability for the requirement of the installation, including the maximum demand
- The type and rating of the overcurrent protective device(s) acting at the origin of the installation. The structure of an electrical installation should be divided in to circuits, as necessary to:
Lalith A. Samaliarachchi number. For example AA4 means Environment Ambient temperature in the range of -5 0 C to +40 0 C. The first capital letter relates to the general category of external influence: A - Environment B -Utilization C - Construction of buildings The second capital letter relates to the nature of the external influence: A - Temperature B -Humidity C -Altitude D -Water E -foreign bodies F -Corrosion G -Impact H -Vibration etc. The third number relates to the class within each external influence: 1, 2, 3, etc. Note that the above codification is not intended to be used for marking equipments. You may refer to Chapter 51 and Appendix 5 of IEE wiring regulation 17th^ edition for further details. 4.1.3 Compatibility It is very important that one part of an electrical installation does not produce effects which are harmful to another part. For example, the heavy transient starting current from electric motors could result in large voltage reductions affecting the operation of filament and discharge lamps. Again, the generation of harmonics in discharge lamps or motor controllers may upset the operation of electronic and communications equipment in another part of the same installation. An assessment of such harmful influences at an early stage is likely to prevent difficulties. Some of the influences concerned are listed as follows:
- Transient overvoltages
- Undervoltages
- Unbalanced loads
- Rapidly fluctuating loads
- Starting currents
- Harmonic currents
- Leakage current
- Excessive protective conductor current
- d.c. feed back
- High-frequency oscillations
- Necessity for additional connections to earth
- Power factor For an external source of energy the distributor shall be consulted regarding any equipment of the installation having a characteristic l i k e l y to have significant influence on the supply. All fixed i n s t a l l a t i o n s shall be in accordance with the relevant EMC regulations. Consideration s h a l l be given by the planner and designer of the electrical installation to measures reducing the effect of induced voltage disturbances and electromagnetic interferences (EMI). Measures are given in Chapter 44 of the IEE Regulations.
Lalith A. Samaliarachchi 4.1.4 Maintainability Maintainability is also a very important factor to consider when deciding on the design. An assessment shall be made of the frequency and quality of maintenance that the installation can reasonably be expected to receive during its intended life. This assessment shall, wherever practicable, include consultation with the person or body who will be responsible for the operation and maintenance of the installation. Only then can the regulations be applied so that:
- Any periodic inspection, testing, maintenance and repairs likely to be necessary during the intended life can be readily and safely carried out, and
- The protective measures for safety remain effective during the intended life, and
- The reliability of equipment is appropriate to the intended life. 4.1.5 Recognized safety services The need for safety services and their nature are frequently regulated by statutory authorities whose requirements have to be observed. Examples of safety services are: emergency escape lighting, fire alarm systems, installations for fire pumps, fire rescue service lifts, smoke and heat extraction equipment. The following sources for safety services are recognised:
- Storage batteries
- Primary cells
- Generator sets independent of the normal supply
- A separate feeder of the supply network effectively independent of the normal feeder 4.1.6 Continuity of service An assessment shall be made for each circuit of any need for continuity of service considered necessary during the intended life of the installation e.g. life-support systems. The following characteristics shall be considered:
- Selection of the earthing system
- Selection of the protective device in order to achieve discrimination
- Number of circuits
- Multiple power supplies
- Use of monitoring devices.
Lalith A. Samaliarachchi The second letter in the designation denotes the relationship of the exposed conductive parts of the installation to Earth, and T = direct electrical connection of the exposed conductive parts to earth, independently of the earthing of any point of the source of energy. ('Terra' gives the letter 'T') N = direct electrical connection of the exposed conductive parts to the earthed point of the source of energy, which, for a. c., is usually the neutral point ('Neutral' gives the letter 'N') The system designated TN is further subdivided depending on the arrangement of the neutral and protective conductors. This subdivision is denoted by a further letter or letters separated by a hyphen. S = neutral and protective functions provided by separate conductors ('Separate' gives 'S') C = neutral and protective functions combined in a single conductor ('Combined' gives 'C') If the neutral and protective functions are combined in part of the system and separate elsewhere, then the letters C-S is used. Question Why do you think that it is important to know the type of system in an installation design? Discussion The choice of measures for protection against electric shock depends on, amongst other factors, the earthing arrangement at the source of energy and the type of path intended for earth fault current. Thus it is important that the type of system is known prior to designing the installation. To understand the differences between the types of systems further, let us examine the diagrams shown in figure 4.1 to 4.
Lalith A. Samaliarachchi Figure 4.1 -TN-C System
In here the system Neutral and protective functions combined in a single
conductor throughout system. All exposed-conductive-parts of an installation are connected to the PEN conductor. An example of the TN-C arrangement is earthed concentric wring but where it is intended to use this, special authorization must be obtain from the appropriate authority. Consumer shall not combine the neutral and protective functions in a single conductor in his consumer’s installation. Figure 4.2 -TN-S System Here you find the separate neutral and protective conductors throughout system The protective conductor (PE) is the metallic covering of the cable supplying the installations or a separates conductor.
Lalith A. Samaliarachchi Figure 4.3 -TT System All exposed-conductive-parts of an installation are connected to an earth electrode which is electrically independent of the source earth. Figure 4.4 – IT System All exposed-conductive parts of an installation are connected to the earth electrode. The source is either connected to Earth through a deliberately introduced earthing impedance or is isolated from earth. UNDER THE ELECTRICITY SUPPLY REGULATIONS 1937 THIS SYSTEM MUST NOT BE USED FOR PUBLIC SUPPLIES. You would probably have noticed that each of the diagrams show a typical 3 phase installation and a typical single phase installation. Perhaps you would also have noticed that each of these installations show not only the conductors but also exposed conductive parts (such as the metallic frame of an equipment). They also show a 3 phase source of energy from which 3 or more wires originate. Let us now examine each one in turn. Question If you examine figure 4.1, what special arrangements do you notice? Discussion You would have noticed that 3 phase wires and an earthed neutral wire originate from the source. You would also have perhaps noticed that this neutral wire is additionally earthed elsewhere on the neutral conductor (multiple earthing). You may even have noticed that the exposed conductive parts (such as equipment frames) of both types of installations are connected directly to the earthed neutral. Thus the neutral conductor in this case combines the functions of the neutral (which is to
Lalith A. Samaliarachchi return the out of balance current of the 3 phases) and the protective conductor (which is to keep the equipment frames near earth potential). Such a conductor is referred to as a PEN conductor (Protective-earthed-neutral). It is to be noted that whether a neutral is a PEN conductor or not depends on the supply authority and not on the consumer. In the TN-C system, the individual consumer does not generally have his own earth electrode, and the neutral would thus carry earth fault currents as well. Since earth fault currents are combined with neutral currents, it becomes difficult to detect fault currents and operate protective devices at the consumer's installation. Question If you now examine figure 4.2, what differences do you see? Is the difficulty mentioned above rectified? Discussion You would have noticed that there are 2 wires in addition to the phase wires and that both these wires are earthed at the source. You would also have noticed that one of these is the neutral wire while the other is a protective conductor supplied by the supply authority. This has overcome the difficulty of the combined protective conductor and neutral and hence protection of the consumer's installation becomes easy. However this
would mean additional expense for the supply authority as he would have to draw a
fifth wire throughout. In the TN-S system all exposed conductive parts are connected to the supply authorities protective conductor so that a good earth is ensured. The consumer does not need his own earth electrode. Question Examine figure 4.3 and compare it with the 2 earlier diagrams. Are the problems encountered overcome? Discussion You would have noticed that in the TN-C-S system there is just 1 wire in addition to the 3 phase wires up to the consumer's installation. However within the installation the neutral wire and the protective conductor are separated allowing the consumers protection to operate properly. Thus this system has overcome the difficulty of the TN-C system while at the same time not increasing the cost to the supply authority by have only 4 wires. The PEN conductor is again earthed at a number of additional points and, this type of distribution is also known as Protective Multiple Earthing (PME). At the consumer’s premises, no earthing electrode is necessary, but may be present as the PEN conductor is earthed at several points. In the TN-C-S system, all exposed conductive parts of an installation are connected to the PEN conductor through the main earthing terminal and the neutral terminal, with these terminals, being linked together.
Lalith A. Samaliarachchi Reference: Requirements for Electrical Installations, IEE Wiring Regulations 17th^ Edition, 2008.
