Electrical Services Design Assignment
Introduction
In this Assignment you will undertake a design exercise for a commercial building. Data are provided from which you are to design parts of the electrical distribution. The building will have a public network supply with the following declared supply characteristics:
- 300 Ampere, 400/230 volt, TP&N, 50 Hz
- Prospective short circuit current: 30 kA
- TNCS, Earth fault loop impedance: 0.10 Ω
The incoming cable will terminate onto a main switchboard located on the ground floor. The Roof Distribution Board is fed from the Roof Plantroom Switchboard, but all other loads are fed radially from the main switchboard. The circuit details are shown in Table 1.
The following assumptions are to be made:
(i) All loads are three phase and operate at unity power factor.
(ii) The ambient temperature will not exceed 30 °C.
(iii) Cables are not installed in contact with any thermal insulation.
You will also have to make assumptions about the layout of the distribution, and choose the cabling system.
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Table 1 - Circuit Details
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Circuit Ref.
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Description
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Route Length (m)
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Design Current (A)
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Protective Device Rating (A)
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1
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IT server room D.B.
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80
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80
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160
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2
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Ground Floor D.B.
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70
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30
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63
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3
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First Floor D.B.(East)
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18
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20
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63
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4
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First Floor D.B. (West)
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73
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30
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63
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5
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Lift Motor Room D.B.
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85
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55
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63
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6
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Roof Plantroom Switchboard
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80
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110
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125
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7
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Roof D.B. (fed from the Roof Plantroom Switchboard, ref 6)
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30
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40
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63
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The Assignment
1. (a) How do we ensure that the live conductors of a cable are adequately protected against the effects of overload?
(b) What limitations do we need to impose on the volt drop in electrical distribution design?
(c) Why are we required to calculate the value of prospective short circuit current at points in the distribution system?
(d) Why do we impose an upper limit on the earth fault loop impedance, ZS, for each circuit?
(e) How do we ensure that the circuit protective conductor is adequately protected against the effects of fault current?
2. (a) Draw a single-line schematic diagram of the electrical distribution.
(b) For both of the circuits ref. 6 and 7 state your choice of the type (not the size) of the following:-
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for example...
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Circuit protective device
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type of fuse or circuit breaker
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Cabling system
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insulation, multi-core or single cores, containment
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Circuit protective conductor
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SWA, or steel trunking/conduit, or incorporated core or separate core, and any insulation used.
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(c) Calculate the cable sizes for circuits 6 and 7, demonstrating how you have satisfied each of the five requirements from question 1.
3. To exploit locally available wind power, an on-site micro-generator rated at less than 16 A is to be installed.
(a) Illustrate on your single-line schematic how this might be connected to the distribution system.
(b) Explain how the generator connection can be designed to be compliant with the requirements from question 1.