Electrical Installation Design: Rules & Standards

General Rules of Electrical Installation Design

• Rules and statutory regulations must be followed
• Power loads need to be properly installed, considering their characteristics
• Connections to both Medium Voltage (MV) and Low Voltage (LV) public distribution networks are essential
• LV distribution should be carefully planned to provide adequate power
• Protection against electric shocks and electrical fires are critical safety measures
• Sizing and protection of conductors is important for ensuring safe and efficient operation
• LV switchgear should have appropriate functions that meet the installation's needs
• Energy efficiency is crucial for reducing costs and environmental impact
• It is possible to use reactive energy by doing power factor correction
• Accurate energy measurement is necessary for monitoring consumption and optimizing performance

Regulations and Standards

• Quality and safety must be ensured for electrical installations including the wiring rules
• It must also ensure that electrical equipment complies with relevant product standards
• Initial checks are required to conform the electrical installation with standards and regulations
• Before connecting an installation to the supply network, strict pre-commissioning electrical and visual inspections are needed
• Electrical continuity and conductivity tests are crucial for protective, equipotential, and earth-bonding conductors
• Insulation resistance tests between live conductors and protective conductors must be performed where connected to the earthing arrangement
• Compliance tests of SELV (Safety Extra Low Voltage) and PELV (Protection by Extra Low Voltage) circuits must be performed or for electrical separation.
• Measure the Insulation resistance or impedance of floors and walls.
• Protection with automatic disconnection of the supply is also needed.
• Periodic check-testing is necessary verification of RCD (Residual Current Device) effectiveness and adjustments
• Appropriate measurements must be taken in providing safety against effects of electric shock and damage to property from fire and heat
• Confirmation that the installation is not damaged through this testing process.
• Identification of installation defects must occur

Installed Power Loads – Characteristics for Induction Motors

• The rated current I₂ supplied to the motor is given by the formulae: 3-phase motor: I₂ = Pₙ × 1000 / (√3 x U x η x cos φ), I-phase motor: I₂ = Pₙ x 1000 / (U x η x cos φ)
  • Iₙ is rated current (in amps)
  • Pₙ is nominal power (in kW)
  • U is voltage between phases for 3-phase motors and voltage between the terminals for single-phase motors (in volts)
  • η is per-unit efficiency (output kW / input kW)
  • cos φ is power factor (kW input / kVA input)
• Induction motors have a motor starting current that can be reduced with a star-delta starter
• Compensation of reactive-power (kVar) in induction motors to reduce the current supplied to induction motors and can be achieved by using capacitors without affecting the power output of the motors
• The current supplied to the motor after power-factor correction is given by I = Iₐ * (cos φ / cos φ'), where cos φ is power factor before compensation and cos φ' is power factor after compensation. Iₐ is the original current.

Installed Power Loads – Characteristics for other Items

• For resistive-type heating appliances and incandescent lamps (conventional or halogen) The current demand of a heating appliance or an incandescent lamp can be obtained from the nominal power Pₙ quoted by the manufacturer when cos φ = 1
• Installation must consider the power Pn (watts) indicated on the tube of a fluorescent lamp which doesn't include the power dissipated in the ballast
  • The current is given by I₂= (Pballast+Pn)/(U cos φ)
  • U= voltage applied to the lamp
  • Where no power-loss is indicated for the ballast, a figure of 25% of Pn may be used.
  • cos ф = 0.6 with no-power factor correction capacitor
  • cos ф = 0.86 with power factor correction capacitor (single or twin tubes)
  • cos ф = 0.96 for electronic ballast.
• Discharge lamps which have longer start-up times must be installed
• To calculate the current in (amps) through a circuit, the equation la= (kVA10^3)/(Usqrt(3)) can be used. kVA being the maximum 3-phase apparent power circuit value, and U is the line voltage
• LED lamps or luminaires typically use a driver and has very short start-up times.

Power loading of an Installation

• Most electrical appliances and equipment are marked to indicate their nominal power ratiing (Pₙ).
• Total power is sum of power-consuming devices
• Power demands vary through utilization and diversity. (ku and ks)
  • ku: factor of maximum utilization.
  • ks: Diversity factor.
• The total kVA of apparent power that is supplied is not equal to the arithmetic sum of the calculated kVA ratings of individual loads, it should be optimized
• In order to select cable sizes for the distribution circuits of an installation, the current I (in amps) through a circuit is determined from the equation.
• Standard transformer rating is always needed when doing calculations.