TSI

1. INTRODUCTION TO TRUNKING SYSTEM INSTALLATION

An electrical trunking system is an enclosure, usually rectangular with a removable lid, that protects and organizes electrical cables and wires. It is a larger and more accessible alternative to a conduit system, and is widely used in commercial and industrial settings where multiple cables need to be run and easily accessed. 

Types of trunking

Trunking systems are categorized primarily by material and function. 

·         PVC trunking: The most popular and affordable type, made from polyvinyl chloride. It is easy to install and lightweight, making it a common choice for domestic and office installations.

o    Mini-trunking: A very small PVC trunking ideal for surface wiring in residential and light commercial applications.

o    Skirting/dado trunking: Designed to blend into the room's decor by incorporating the trunking into the skirting board or dado rail. It can have multiple compartments for different services.

·         Metallic trunking: Formed from mild steel and coated with either enamel paint for dry indoor use or a hot-dipped galvanized coating for protection against damp conditions. It is extensively used in industrial settings due to its strength and durability. Galvanized Iron (GI) trunking is a specific, corrosion-resistant type of metallic trunking.

·         Multi-compartment trunking: Features internal dividers to segregate different types of circuits, such as power, data, and communication cables, preventing electrical interference.

·         Bus-bar trunking: Used for distributing high currents in factories and industrial facilities. It consists of insulated copper or aluminum busbars enclosed in a protective trunking shell, with tap-off points for easy connection of machinery.

·         Lighting trunking: Specifically designed for continuous runs of light fittings, especially in industrial settings. The trunking carries the supply cables and can also house control gear.

·         Floor trunking: Installed underfloor in commercial buildings and offices, with accessible junction boxes for power, data, and telephone outlets. 

Advantages of trunking systems

·         Flexibility and adaptability: It is easy to add, remove, or change circuits, which is ideal for environments that require frequent changes, such as offices and factories.

·         Cable protection: The enclosure shields cables from physical damage, dust, and moisture, extending their lifespan and ensuring safety.

·         Aesthetics: Trunking provides a neat and organized way to conceal unsightly cables, contributing to a professional appearance.

·         Safety: By containing wires, trunking reduces the risk of electrical shocks, fires, and tripping hazards.

·         Ease of maintenance: The removable lids allow for quick and easy access to cables for inspection and maintenance.

·         Cable management: Keeps cables organized and makes it easier to trace specific wires. 

Best practices for installation

·         Follow space factor rules: Adhere to regulations (such as the IEE's recommended 45% maximum fill) to ensure cables do not overheat.

·         Ensure proper segregation: Use multi-compartment trunking to separate different cable types, such as high-voltage and low-voltage circuits, to avoid interference and meet safety regulations.

·         Provide adequate support: Secure trunking directly to surfaces or suspend it using brackets, ensuring supports are placed at regular intervals, especially at joints and bends.

·         Plan the installation carefully: Lay out the trunking route beforehand, taking into account obstacles and expansion needs. Ensure a secure and level alignment.

·         Finish joints and entries properly: File all sharp edges and burrs after cutting or drilling. Use appropriate fittings and bushings for connections to prevent damage to the cables.

·         Do not overcrowd: Leave spare space for future expansion and proper heat dissipation. Overcrowding can damage cables. 

2. IEE REGULATIONS ON TRNKING SYSTEM OF INSTALLATION

The Institution of Electrical Engineers (IEE) Wiring Regulations, now known as BS 7671, provide the definitive standards for the design and installation of electrical systems in the UK. While specific editions and regulations change over time, the core principles regarding trunking remain consistent and are focused on safety, maintenance, and prevention of damage. 

Cable capacity and space factor

A critical aspect of trunking installation is managing the number of cables to prevent overheating. 

a)       45% space factor: The current regulations require that the total cross-sectional area of all cables in a trunking system must not exceed 45% of the trunking's internal cross-sectional area. This ensures sufficient free air for heat dissipation.

b)       Reduced capacity: The space factor must be reduced in sections where cables are pulled in rather than laid in, or where numerous cables cross over each other.

c)       Cable factors: Sizing is done by adding up "cable factors" for each individual cable and comparing the total to tables of "trunking factors". This accounts for different cable sizes and insulation types. 

Cable segregation

To prevent mutual interference and ensure safety, different types of circuits must be segregated within trunking. 

·         Voltage bands:

BS 7671 classifies circuits into voltage bands. Band 1 circuits (e.g., data, telecommunications) must be separated from Band 2 circuits (e.g., mains power, emergency lighting).

·         Separation methods:

Segregation can be achieved by:

Ø  Using multi-compartment trunking with a solid, earthed divider.

Ø  Maintaining a minimum separation distance.

Ø  Ensuring that all cables are insulated to the highest voltage present in the trunking.

·         Electromagnetic compatibility (EMC):

Segregation is crucial to prevent electromagnetic interference (EMI) from power cables affecting sensitive data cables. A metallic, earthed divider offers the best protection. 

Earthing and bonding

Proper earthing and bonding are vital for safety, particularly with metallic trunking. 

·         Metallic trunking: The entire metallic trunking system must be effectively earthed to provide a continuous path for fault currents. This can be achieved through secure mechanical joints or by fitting a separate earth continuity conductor.

·         Non-metallic trunking: With PVC trunking, an insulated circuit protective conductor (CPC) must be run within the trunking.

·         Accessories: All metallic accessories, such as bends and junction boxes, must have earth continuity across their joints. 

Fire stopping

Regulations require the use of fire barriers to prevent the spread of fire through trunking systems. 

·         Location: Fire barriers must be fitted where trunking passes through fire-rated walls, partitions, or floors.

·         Frequency: For long, vertical trunking runs, fire barriers are required at every floor level or at intervals not exceeding 5 meters. 

·          

General installation requirements

·         Protection: Trunking must protect cables from mechanical damage and corrosion.

·         Support: The system must be securely supported at regular intervals to prevent sagging or displacement.

·         Cable entry: All openings and entries must be smooth and free of burrs to prevent damage to cable insulation.

·         Cable support: In long vertical runs, cables must be adequately supported within the trunking to prevent them from slipping.

·         Environmental protection: Installation must be suitable for the environment, with specific requirements for areas with high temperatures, moisture, or corrosive substances.

·         Accessibility: All connections and joints must be accessible for inspection, testing, and maintenance, though exceptions exist for certain joints. 

3. ELECTRICAL DIAGRAMS AND SYMBOLS

ELECTRICAL DIAGRAMS

Electrical diagrams are visual maps that show how an electrical system works and is wired. They are crucial tools for electricians and engineers during design, installation, maintenance, and troubleshooting. These diagrams use standardized symbols and lines to represent components and connections, enabling clear communication. 

Types of electrical diagrams

Different types of diagrams serve specific purposes, providing varied levels of detail. 

• Schematic diagram:

This diagram focuses on the functional logic of a circuit using abstract symbols, not the physical layout. It shows the connections between components to help understand how the circuit works, making it ideal for troubleshooting.

• Single-line diagram (SLD):

Also called a one-line diagram, this offers a simplified overview of an electrical power system. It uses a single line to represent multiple conductors (like a three-phase system) and shows how major components like transformers and circuit breakers are interconnected.

• Wiring diagram:

This shows the physical layout and connections of an electrical system. It provides details on wire colors, sizes, and the relative positions of components, which is critical for installation and repair.

• Ladder diagram:

A specialized schematic often used for industrial control logic, it resembles a ladder with two vertical rails (power supply) and horizontal rungs (control circuits). It is used to document how electromechanical switches and relays are interconnected.

• Electrical floor plan:

This is an architectural drawing that shows the location of electrical fixtures, outlets, and switches within a building's physical layout. It is essential for new construction or remodeling.

• Riser diagram:

Illustrates the physical layout of electrical distribution in a multi-level building using a single line. It shows the path of electrical distribution from the point of entry to each floor and sub-circuit. 

How to read electrical diagrams

To correctly interpret an electrical diagram, follow these general steps: 

1. Start with the legend:

Diagrams use standardized symbols and abbreviations, which are explained in a key or legend. Study this first to understand what each symbol represents.

2. Follow the flow:

For schematics, read from left to right and top to bottom, following the path of the current from the power source to the components.

3. Trace the path:

Identify the power source and trace the circuit through switches, fuses, and loads. Wires that are connected are often indicated with a small dot at their intersection, while those that cross without a connection have no dot.

4. Pay attention to details:

Look for wire tags, component labels (e.g., R1 for resistor 1), and notes that provide specific details on components and their values. 

Electrical diagrams and trunking systems

Electrical diagrams are used to detail the specific requirements for a trunking installation. 

• Single-line diagrams

Might show the main trunking runs and how power is distributed to different areas of a building.

• Wiring diagrams 

Would detail the actual cables being laid in the trunking, including segregation requirements for different types of circuits (e.g., power vs. data).

• Floor plans

Would show the physical route of the trunking system along walls, ceilings, or floors. 

3.1. ELECTRICAL SYMBOLS

ELECTRICAL SYMBOLS