Electrical Power Systems

Power Distribution

The design of the incoming power supply is critical for future expansion capacity and reliability of supply. The main criteria to establish early are the following:

• Capacity of supply;
• Location of the main power distribution centre; and
• The required level of reliability and future flexibility of the incoming supply.

The capacity for the site is usually determined at concept stage on a watts/m2 basis, which is calculated depending on usage, i.e. commercial, retail, educational, etc. and diversity between usage types.

The major cost of the electrical installation can be attributed to the cost of distributing the energy. It is therefore preferable to locate the main distribution centre centrally to the main electrical loads, which in addition to the built area would also include mechanical plant rooms, lifts and water heating installations. The distribution generally originates at the low voltage Main Switchboard. Submains cables then feed Distribution Switchboards which are located locally to the electrical loads. These would typically be one per floor or building wing. From the Distribution Switchboards final subcircuits feed individual outlets or equipment. Typically the subcircuits will feed outlets or loads within approximately a 100m radius. If longer lengths are required then additional Distribution Switchboards should be considered.

If the electrical installation requires an incoming supply in excess of 250kW a high voltage supply should be considered. The economics of whether a low voltage or high voltage supply is more efficient will depend on the actual location and access to a suitable Supply Authority network. If a high voltage supply is required then a transformer substation will be required. Typically transformer substations come in two general types as follows:

• Indoor substation, which is where the high voltage switchgear and transformers are located within the building. Supply Authority regulations stipulate minimum size and access requirements.
• External kiosk type, which is where the high voltage switchgear and transformer are housed in a weatherproof enclosure. Supply Authority regulations stipulate minimum size, clearance from the building and access requirements.

Power distribution over a long distance is more effective at high voltage rather than low voltage. Therefore, if a high voltage supply is required due to the capacity then the substation should be located as close as possible to the low voltage Main Switchboard.

Since the usage of the building sometimes changes over the building life and the relatively high cost for the incoming supply it is reasonable to allow spare capacity for future expansion. It would be reasonable to provide an additional 20% for future expansion for normal applications. Special applications need to be determined based on the current use and the envisaged future use.

As a rule of thumb, this additional component should only be utilised with a change of building use.

Special applications may also require higher levels of supply reliability then the standard single incoming supply. This would usually apply to critical business, process or health facilities. In these cases special supply arrangements need to be designed as part of the concept design.

Power Distribution Space Requirements

The following spaces should be provided for the standard electrical installations:

• Incoming Supply Authority fuses and metering. This will vary from a simple space on an external wall not subject to mechanical damage to a separate room/s for larger buildings depending on application. The fuses and metering need to be accessible by the Supply Authority during normal business hours.
• Room for the Main Switchboard which can be combined with the supply Authority Fuses and Metering if required. These would usually be ventilated rooms with easy access for maintenance. The Australian Standard AS 3000 has specific requirements for access, space and signage.
• For larger installations electrical risers are required to accommodate the submains cabling running up through the building.
• For larger installations wall enclosures or small rooms will be required for the Distribution Switchboards.

Internal Lighting

The lighting design should be developed in a manner which addresses the following attributes:

Lighting level defined as illuminance measured in Lux. Minimum standards defined by AS 1680 and the Building Code of Australia:

• Consideration of the impact of glare, recommended standards defined by Australian Standards;
• Consideration of any colour requirements for the lighting;
• Aesthetics of the space and the influence of the lighting as part of this;
• Flexibility to accommodate typical use of the space, i.e. open plan or modular office;
• Means of lighting control;
• Lamp types and the expected lamp life; and
• Energy efficiency, which is defined as efficacy.

The best lighting design solution seeks to resolve all of the above points. However, in practice often different aspects need to be traded off and the lighting design will recognise which aspects are most important and which aspects are more nominal.

For general commercial applications it is crucial to meet the minimum defined illumination levels and prescribed glare criteria. The design calculations for these should include a realist maintenance factor which allows for degraded light output due to dust build up, lamp survival factor, etc. and based on average light output taking into consideration the reduced light output after a defined period of operation.

Emergency Evacuation Lighting and Exit Signage

Emergency lighting and illuminated exit signage is generally required to provide occupants with a safely illuminated escape route from a building in the case of emergency and particularly when mains power may not be available. The need for emergency and exit lighting is defined in the Building Code of Australia (BCA). The mandatory requirements for this system, if required, are defined in the AS 2293.

In general, there are several kinds of emergency lighting system, which meets the BCA and Australian Standard requirements:

• Maintained, i.e. the emergency lamp/s of the fitting is continuously illuminated; and
• Non-maintained, i.e. the emergency lamp/s of the fitting is not illuminated when normal supply is available.

The emergency and exit luminaries can be supplied via either one of two systems:

• Central battery system, where the emergency batteries, battery charger, inverter and controls are located centrally and a cable distribution network connects each of the emergency and/or exit signs together; and
• Self-contained system, where each emergency exit light or exit sign has it’s own battery, battery charger, inverter and controls integral with the light fitting or mounted very close to it.

If the number of self-contained luminaires is larger and regular maintenance difficult then all the luminaries can be monitored by a centralised computer system. This simplifies the regular statutory testing required and general maintenance of the system. However, it is usually only required on very large installations.

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