Electrical design of Building project of a three bedroom house (Part 2)
All new domestic housing, including conversions and Electrical Design of Building Project must have:
- A fire detection and alarm system in accordance with the recommendations of British standard BS 5839-6; or
- Mains powered smoke detectors, with at least one on each floor and interconnected.
Load assessment of Electrical Design:
In Electrical Design of Building the Regulations require that the characteristics of the supply, including an estimate of the maximum demand, be determined by calculation, measurement, investigation or inspection.
It is assumed that this house will be connected to a basement 230VAC power supply and the maximum load demand is less than 100A.
The other characteristics can be determined by request from the electricity company (distributor). Some distributors publish guidelines on power supply characteristics and system requirements.
As a rule of thumb, do the following to calculate the total requested load:
- Step I: indicate the buildings (rooms) in the project
- Step II: find the total connected load of a building in kVA for example
- Step III: Apply the demand factor, load factor and diversity factor appropriately.
- Stage IV: Consider the future expansion of the service (25%).
A typical domestic offer
Except in exceptional circumstances, the characteristics of the offer are as follows:
- Prospective short-circuit current at the origin:
Never more than 16 kA and more likely to be less than 2 kA ;
- External earth fault loop impedance Z e :
Not exceeding 0.35 Ω (for TN-CS) and more likely to be less than 0.2 Ω;
- Main fuse:
This will be the distributor’s usual standard, BS 88-3 (formerly BS 1361, part 2) 100 A. Subject to the application of these values, there should be no problem in the application of standardized electrical design.
Specifications of Electrical Design:
Electrical Design of Building it is necessary to produce a project specification, as in Table 2 below. This will be used initially for pricing and technical purposes. It will eventually be updated to form the basis of a user manual.
Cabling systems and cable sizes
Circuits and circuit design are two important discussions that every electrical engineer should know. Traditionally, domestic installations were wired using the lighting system with three rosettes and ring circuits for the sockets.
As a rule of thumb, follow these steps to design your cable circuits:
Determine the power specifications, including the grounding diagram. I prefer TN-CS . Value adopted for Z e is 0.35 Ω . Then find the design current I re from the total load requested.
Select the size of the cable and the protection device. Always make sure I pd ≥ I d . Then select a wire size from Appendix 4 of BS 7671 that can support the load. I t . Always make sure that:
• C g = Classification factor for the grouping. Table 4 C1 of BS 7671
• C a = Evaluation factor for the ambient temperature. Table 4 B1 of BS 7671
• C i = Estimation factor for conductors surrounded by thermal insulation. Table 52.2 of BS 7671
• C F = Rating factor for the semi-closed fuse. BS 3036
Check the voltage drop. Use this equation to make sure the minimum section is correct.
• V d = Voltage drop in volts.
• L = Cable length in meters
• mV / A / m = The value from the appropriate table. Annex 4 of BS 7671
This voltage drop must be equal to or less than the appropriate voltage drop percentages given in table 4Ab of appendix 4 of BS 7671. (V d / 230) · 100% for a nominal voltage of 230V.
Check for electric shocks. the earth fault loop impedance Z s at the furthest point in the circuit should be determined using the following formula:
• R 1 + R 2 is the resistance per meter of the line and of the protective conductor.
• C r is the evaluation factor of the operating temperature
• Z e is the value of the external earth fault loop impedance.
The value of Z s must be less than the value given in tables 41.2, 41.3 or 41.4 of BS 7671.
Check the possible fault current. Live conductors are protected against short-circuit currents if their value of k 2 S 2 must be greater than the value of the energy allowed to pass i 2 t for the protection device.
• S is the section of the conductors
• k is the value taken from table 43.1 of BS 7671.
First check for the short-circuit current by:
• Z LN is the line-to-neutral impedance at the origin of the installation
• R 1 + R n is the resistance per meter of the line conductor and the neutral conductor
• t is the operating time of the protection device determined from the time / current characteristic given in
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