OL04 - Low voltage distribution

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Course content

The module is designed for non-electrical Building Services engineers who now find that they have an increasing need to be able to design electrical distribution schemes, or for those with a more traditional electrical background who wish to apply their knowledge practically to the design of electrical distribution within buildings.

The module covers both the principles and practices of electrical power distribution from the optimum location for the main electrical intake position through to the choice of power distribution systems and their containment. It also introduces the use of essential and non-essential supplies and the possible choices for standby electrical supplies.

Learning Outcomes:

Upon completion of the module, students should be able to:

• Make suitable load assessments for buildings to enable supply application including the maximum demand requirement
• Consider the effect of: usage and occupancy, daylight availability and other building services such as air-conditioning on a building's electrical power requirements
• Utilise rules of thumb and diversity tables in the estimation of load factor
• Make initial decisions with regards the main electrical intake locations, such as:
  • Intake location
  • Use of high-voltage vs. low voltage supplies
  • Access and metering requirements
• Make initial decisions with regards to the distribution of electrical supplies within a building including the choice of system, its protection and its containment
• Appreciate the factors which affect the choice of cables and their installed current ratings
• Consider which supplies are essential and which non-essential and possible forms of standby/emergency supply.

Module Contents

Choosing electricity supplies

• Usage and occupancy
• Seasonal occupancy
• Effect of other building services
• Air conditioning - energy usage
• Daylight linking
• Daylight penetration
• The effect of Part L

Load assessment - preliminary stage

• Rules of Thumb
• Example of 'Rules of Thumb' calculation
• Definitions
• Maximum demand
• MD averaging time
• Diversity
• Diversity table
• Diversity - sources of guidance
• Load calculations - summary
• Availability charge
• Maximum demand - summary
• Other factors that affect the supply system

Power factor

• Power factor correction

Load factor

• Load factor improvement

Harmonics

• Non-linear loads
• Problems caused by harmonics
• Which loads produce harmonics?
• Electronic ballasts
• UPS systems
• Learning about harmonics
• Harmonics across three-phases
• Transformers and harmonics
• Supply limits
• G5/4

Design decisions for the main electrical intake

• Site logistics
• Turning-moments
• HV to LV Substation layout
• The need for access
• Ingress
• Future expansion
• A learning process
• Electrical intake - typical plans and sections
• Main intakes - HV metering
• Main intakes - LV metering
• Distribution within the building
• Typical distribution diagram
• Rising busbars
• Working access

Energy efficiency in offices

• Designing to reduce energy
• Economic operation
• Assessing loads
• Office loads - a reality check

Distribution systems and equipment

• HV vs. LV distribution
• LV site distribution

Distribution inside the building

• Distribution inside the building - conventional
• Distribution inside the building - rising busbars
• Critical decision - riser location
• Switch-panels
• Fault-levels
• Circuit-breakers
• Keeping records
• Specifying switchgear - key points
• High-voltage switchgear
• Open ring-main distribution
• Low-voltage switchgear
• Types of fuse/switchgear
• BS EN 60269 (BS 88) fuses
• Types of circuit-breaker
• MCB operation
• Fault-levels at transformer terminals
• Fault-levels throughout the distribution system
• Transformers
• Distribution transformers
• Distribution substations - key points

Cabling - design factors

• Supply voltage and full-load current
• Mechanical protection
• Ambient temperature
• Bending radius
• Distribution circuits - methods of containment
• Rating factor for grouping
• Space for the cores to spread
• Forward planning
• SWA (Steel-Wire Armoured) cables
• SWA - construction
• Single-core cables
• MI cable
• MI cable - spatial requirements
• Cable types - common abbreviations
• High-voltage cable types
• Prospective fault current
• Adiabatic equation
• Earth-faults
• Earth Fault Loop Impedance
• RCDs
• RCD operation
• Discrimination
• System earthing and bonding
• MET - main earthing terminal
• System bonding arrangement
• Supply system classification
• Classification notation - IEC terminology
• TN variants
• TNCS (PME)
• TNCS - broken supply neutral
• TNCS - size of main bonding conductors
• TNCS - common practice
• TNS
• TT
• TT - earth electrodes
• "Earthing" within the installation
• CPCs (Circuit Protective Conductors)
• Bonding conductors

Distribution within buildings

• Wiring systems and cable management
• Technical suitability
• Economic suitability
• Conditions of service
• Cable support systems
• Builder's work ducts
• Enclosed wiring systems
• Cable trunking
• Under-floor distribution systems
• Skirting and dado trunking
• Mini-trunking
• Service poles
• Conduit systems

Essential and non-essential loads

• Essential services
• Lighting

Standby supplies

• Dual source feeders
• Standby generators
• Manual vs. automatic changeover
• Diesel engines
• Gas turbines
• Power output
• Sizing the set
• Generators and motor loads
• Generators and non-linear loads
• Parallel operation
• Generator specification

Uninterruptible power supplies (UPS)

• Rotary or static?
• UPS operation
• Rotary UPS
• Static UPS systems
• Static UPS operation
• Static system configurations
• Diverse routing of supplies

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