The following clauses in Part 1 Section 1 of AS/NZS 3000:2007 provide the fundamental principles of electrical design.
1.6 DESIGN OF AN ELECTRICAL INSTALLATION
1.6.1 General
An electrical installation shall be designed to —
(a) Protect persons, livestock and property from harmful effects;
(b) Function correctly as intended;
(c) Connect, operate safely and be compatible with the electricity distribution system, or other source of supply, to which the electrical installation is to be connected;
(d) Minimise inconvenience in the event of a fault; and
(e) Facilitate safe operation, inspection, testing and maintenance.
1.6.2 Supply characteristics
The following characteristics of the electricity supply shall be determined:
(a) Nature of current, AC or DC;
(b) Nature and number of conductors, as follows:
(i) Active (phase), neutral and protective earthing conductors for AC.
(ii) Equivalent conductors for DC.
(c) Voltage and voltage tolerances.
Note: The nominal voltage and tolerances for low voltage supply systems and electrical installations is 230/400 V +10% to ?6% (in accordance with AS 60038);
(d) Frequency and frequency tolerances;
(e) Maximum current that can be supplied;
(f) Prospective short-circuit current;
Note: Information regarding prospective short-circuit and fault currents at the point of supply may be obtained from the local electricity distributor.
(g) Protective measures inherent in the supply, e.g. MEN earthing system;
(h) Limits on the use of equipment;
(i) Harmonic current or other limitations.
1.6.3 Maximum demand
The maximum demand of an electrical installation shall be determined, taking into account the capacity, physical distribution and intended use of electrical equipment in the electrical installation and the manner in which the present requirements might vary.
Consumers mains, submains and other electrical equipment of an electrical installation shall be designed and installed to meet the maximum demand.
Note: Clause 2.2.2 contains acceptable methods of determining maximum demand.
1.6.4 Utilisation voltage
The electrical installation shall be designed and installed to ensure that the voltage at the terminals of electrical appliances and equipment is suitable for the nominal operating voltage of electrical appliances and equipment to be supplied.
Note 1: Clause 3.6 contains requirements relating to voltage drop.
Note 2: A stand-alone system designed to provide a low voltage supply should be designed to maintain the voltage at the point of supply within the range noted in Clause 1.6.2(c) under normal full-load conditions.
1.6.5 Electrical installation circuit arrangement
Every electrical installation shall be divided into circuits as necessary to —
(a) Avoid danger and minimise inconvenience in the event of a fault; and
(b) Facilitate safe operation, inspection, testing and maintenance.
Note: Clause 2.3 contains requirements for the control and protection of electrical installations.
Part 2 of AS/NZS 3000 provides installation practices that achieve certainty of compliance with the essential safety requirements of Part 1.
The following clauses provide work methods and installation practices that are deemed to comply.
2.2 ARRANGEMENT OF ELECTRICAL INSTALLATION
2.2.1 Circuits
2.2.1.1 General
The electrical installation shall be arranged into an appropriate number of separate circuits taking the following into account:
(a) The relationship of the equipment, including any requirement for operation as a group and any special need identified by the user;
(b) The load and operating characteristics of the equipment in relation to the rating of the circuit components;
(c) The limitation of consequences of circuit failure including loss of supply to critical equipment, overload and the ability to locate a fault;
(d) The facility for maintenance work, and capacity for alterations and additions, to be performed without interrupting supply to other parts of the installation.
Circuits for safety services shall be separate from those used to supply the remainder of the electrical installation, as required by Clause 7.2.2.
Note: The most common distribution arrangement for a low voltage electrical installation is radial branched distribution, an example of which is shown in Figure B1 (Appendix B).
Division of circuits falls logically into several categories, each an individual circuit or group of circuits. Typically, the circuit groups selected are:
(a) Lighting.
(b) Socket-outlets.
(c) Heating and/or air conditioning appliances.
(d) Motor-driven plant.
(e) Auxiliary services, such as indication and control.
(f) Safety services.
Appendix C provides recommended circuit arrangements for basic applications.
2.3 CONTROL OF ELECTRICAL INSTALLATION
2.3.1 General
Electrical installations shall be provided with devices to prevent or remove hazards associated with the electrical installation and for maintenance of electrically activated equipment.
Note: The measures specified in this Clause 2.3 are in addition to, and not alternatives, to the protective measures specified in Clause 2.4.
Electrical installations shall include all switching devices or other means of disconnection necessary to enable operations, repairs and maintenance work to be carried out with safety.
Any device provided shall comply with the relevant requirements of this Clause 2.3, in accordance with the intended function or functions.
Such devices are classified according to one of the following functions:
(a) Isolation; in accordance with Clause 2.3.2.2.
(b) Emergency; in accordance with Clause 2.3.5.2.
(c) Mechanical maintenance; in accordance with Clause 2.3.6.2.
(d) Functional (control); in accordance with Clause 2.3.7.2.
Where two or more such functions are performed by a common device, that device shall comply with all the requirements for each of the functions concerned.
2.4 FAULT PROTECTION
2.4.1 General
The following methods of fault protection are recognised in this Standard:
(a) Automatic disconnection of supply, in accordance with Clause 1.5.5.3.
(b) The use of Class II equipment or equivalent insulation, in accordance with Clause 1.5.5.4.
(c) Electrical separation, in accordance with Clauses 1.5.5.5 and 7.4.
The requirements for protection by means of automatic disconnection of supply are set out in Clauses 2.4.2, 2.4.3, 2.5 and 2.6.
2.5 PROTECTION AGAINST OVERCURRENT
2.5.1 General
Active conductors shall be protected by one or more devices that automatically disconnect the supply in the event of overcurrent, before such overcurrent attains a magnitude or duration that could cause injury to persons or livestock or damage because of excessive temperatures or electromechanical stresses in the electrical installation.
No fuse shall be inserted in a neutral conductor. Protective devices that incorporate a switching function in the neutral conductor shall comply with the requirements of Clause 2.3.2.1.1(b).
Protection against overcurrent shall consist of protection against —
(a) Overload current, in accordance with Clause 2.5.2 and 2.5.3; and
(b) Short-circuit current, in accordance with Clause 2.5.2 and 2.5.4.
Protection against overload current and short-circuit current shall be coordinated, in accordance with Clause 2.5.6.
Note: Reduction in current-carrying capacity of conductors may occur by a change in cross-sectional area, method of installation, or type of cable or conductor.
2.6 ADDITIONAL PROTECTION BY RESIDUAL CURRENT DEVICES
2.6.1 General
The use of fixed setting RCDs with a rated operating residual current not exceeding 30mA, is recognised as providing additional protection in areas where excessive earth leakage current in the event of failure of other measures of protection or carelessness by users could present a significant risk of electric shock.
Note: The use of RCDs is intended only to augment other measures of basic protection.
RCDs do not provide protection against faults between live conductors, nor do they provide protection against voltages imported into the electrical installation earthing system through the supply system neutral conductor.
The use of such devices is not recognized as a sole means of protection and does not obviate the need to apply the protective measures specified in Clause 2.4.
Additional protection shall be provided, where required by Clause 2.6.3, to automatically disconnect the supply when an earth leakage current reaches a predetermined value.