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Assessment of General Characteristics (Note: All the mentioned tables in this course refer to, unless otherwise specified, Low Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004) Chapter 2 Electrical Installation II 1 Introduction Before detailed design and installation work of an electrical installation commences, considerations should be given to: – purpose for which the installation is intended to be used, its general structure, and its supplies; – external influences – compatibility of its equipment – maintainability of the installation Electrical Installation II 2 General Assessment Purpose of installation Supplies and structure of an electrical installation – – – – – maximum demand diversity of installation supply and earthing arrangement nature of supply circuit arrangement Electrical Installation II 3 Maximum Demand & Diversity Definition – sum of all connected loads multiplied by a percentage of loads that is likely to be used at the same time. The percentage is known as diversity factor, i.e. Maximum Demand = Connected Load x Diversity Factor Electrical Installation II 4 Maximum Demand & Diversity There is no standard methods and values for diversity factor for every type of electrical installations. The allowance for diversity figures given in Table 2.2 may need increasing or decreasing as decided by the registered electrical worker responsible for the design of the installation. Electrical Installation II 5 Maximum Demand & Diversity For current demand under 400A, refer to Tables 2.1 and 2.2 For current demand over 400A, reference can be made to Tables 2.3, 2.4 and 2.5 Electrical Installation II 6 Supply Characteristics – Type of earthing arrangement • TT, TN-S, or combination of both – Nominal voltage, nature of current and frequency – Prospective short-circuit current at the origin of the installation – Earth fault loop impedance external to the installation – Supply suitability for installation, including maximum demand – Type and rating of the over-current protective device at the origin of installation – Supplies for safety services and standby purposes Electrical Installation II 7 Types of earthing arrangements EWR and CoP stipulate that the earthing arrangement in Hong Kong is a TT system However in order to minimize the rise of potential difference between the supply company’s exposed conductive parts and the consumer’s exposed and extraneous conductive parts, there is a bonding conductor connecting the electricity company’s transformer earth point to the consumer’s main earthing terminal. Electrical Installation II 8 Types of earthing arrangements Technically, this is a combination of TT and TN-S earthing system. However, the CoP further requires that the installation earthing be effective even if the bonding conductor is disconnected Electrical Installation II 9 Nominal voltage, current and frequency The declared limits of the supply voltage and frequency are: – Low voltage: 220V+6%, 220/380V+6% – High voltage: 11kV and 132kV – plus 10% or minus 2.5% – Frequency: 50Hz ±2%. Electrical Installation II 10 Prospective short-circuit current at the origin of the installation It is not possible for the electricity supply companies to give an accurate value of prospective short-circuit current as its magnitude varies with changes in infeed cable network. The max. prospective short-circuit current is declared to be 40kA at low voltage side for the following supply arrangements: i) Feeding from a transformer provided and installed by CLP Power or HEC ii) Feeding from LV underground cables when supplied by HEC Electrical Installation II 11 Earth fault loop impedance external to the installation To be covered in Chapter 7 Electrical Installation II 12 External Influences External influences – Environmental conditions – Utilisation of premises – Type of building construction Electrical Installation II 13 Compatibility The following need to be considered carefully: To minimised harmful effects to the supply Transient overvoltages Fluctuating loads Starting currents Harmonic currents Additional connections to earth - Clean Earth Electrical Installation II 14 Maintainability To ensure CONTINUAL SAFETY Electrical Installation II 15