electrical syllabus summary part 2 Flashcards
(47 cards)
Two main types of excitation
Static and Brushless
Types of excitation
- Static Excitation
DC current is supplied from an external DC source (like a battery or rectifier).
The DC power is fed into the rotor field windings through slip rings and brushes.
Used in older or smaller alternators.
- Brushless Excitation
Common in modern alternators, especially in power stations.
Uses an exciter alternator mounted on the same shaft β this avoids brushes and slip rings.
π What is a Pilot Exciter?
It is a small AC generator mounted on the same shaft as the main alternator, used to supply power to the main exciterβs field in a brushless excitation system.
Operation and structure of a pilot exciter
𧱠Structure:
Mounted on the same shaft as the main alternator.
Has a permanent magnet rotor and a stationary armature.
Does not need external power β starts working as soon as the shaft rotates.
βοΈ Working:
Pilot exciter generates AC as the shaft turns.
AC is rectified to DC and used to energize the main exciterβs field.
The main exciter produces AC for the main rotor field, which is rectified and fed to the rotor.
This produces the rotating magnetic field needed in the main alternator.
β Advantages of Using a Pilot Exciter
Fully self-excited system β no need for external power.
No brushes or slip rings, so lower maintenance.
Reliable and safe for high-voltage alternators.
Excellent for large power stations and critical backup generators.
Summary
Excitation Supplies DC to rotor to create a magnetic field.
Static Excitation Uses external DC and brushes/slip rings.
Brushless Excitation Uses internal exciters; no brushes/slip rings.
Pilot Exciter Small permanent-magnet alternator that powers the main exciter.
π What is a Change-Over Switch?
A change-over switch is a switching device that allows you to manually or automatically switch the power supply from the main grid to a standby power source (like a generator) and back again when needed.
βοΈ Structure of a Change-Over Switch
Switching Mechanism
This is the main component that opens one power line and closes the other.
Contact System
Heavy-duty electrical contacts to handle high current from both the utility and generator.
Designed to prevent backfeeding (feeding power into the wrong source).
Enclosure
Usually housed in a metal or plastic box.
Weatherproof and insulated for safety.
Control Circuitry (in Automatic Change-Over Switches β ATS)
Contains sensors and relays to detect power loss.
Automatically starts the generator and switches l
β‘ Function of a Change-Over Switch
The main function is to ensure a safe and reliable transition between two power sources:
When main power is available
The switch connects the load to the utility supply.
When main power fails
The switch disconnects the utility and connects the generator.
In automatic systems, this happens automatically within seconds.
When utility power returns
The switch transfers the load back to the main supply.
The generator is then stopped.
β Why Itβs Important? ^^^^
β
Why Itβs Important
Prevents backfeeding, which can damage equipment or injure utility workers.
Ensures continuous power supply during outages.
Essential for critical services like hospitals, data centers, and industrial plants.
π§ Types of Change-Over Switches
π§ Types of Change-Over Switches
Manual Change-Over Switch
Requires a person to operate.
Simple and cost-effective.
Automatic Transfer Switch (ATS)
Fully automatic.
Detects power failure, starts generator, and switches load instantly.
03.1 Purpose of Wiring Plans and Other Drawings
Wiring plans and diagrams are essential tools for:
Showing the layout and route of electrical circuits.
Identifying locations of devices, outlets, lights, switches, and panels.
Helping electricians follow the correct installation and ensure safety and compliance.
Reducing errors and saving time during installation.
Serving as a reference for troubleshooting and maintenance later.
Types of drawings:
Schematic diagrams β show circuit functions.
Wiring layout plans β show physical placement.
Single-line diagrams β used for high-level power flow.
Installation drawings β detailed views for cable routing and component placement.
03.2 Interpret Architectural Layouts
Architectural drawings provide:
Floor plans, wall structures, ceiling heights.
Details on rooms, doors, and fittings.
Information needed to:
Position outlets and switches.
Plan routing for conduits and wires.
Ensure installations do not conflict with plumbing, HVAC, etc.
Electricians use these drawings to plan safe and accessible wiring paths while working with other trades (plumbers, builders).
03.3 Interpret Electrical Specifications and Data
Electrical specifications include:
Cable sizes, circuit breakers, and load ratings.
Voltage, current, and power requirements.
Installation instructions, safety margins, and performance data.
They ensure:
Proper material selection.
Systems meet load demand.
Compliance with codes and standards.
Understanding specs avoids overloading, under-sizing, and ensures efficient energy use.
03.4 Special-Purpose Installations (Weather-proof & Flame-proof)
Special conditions require special equipment:
Weather-proof installations (e.g., outdoor, marine, wet areas):
Use IP-rated enclosures, UV-resistant cables, sealed fittings.
Flame-proof/explosion-proof installations (e.g., fuel stations, chemical plants):
Use ATEX-rated or intrinsically safe gear.
Seal all entries, prevent sparks, maintain temperature limits.
Properly interpreting these specs ensures safety in hazardous environments.
03.5 Colour Coding & Regulations (Local and Foreign Standards)
Colour codes vary by country:
03.5 Colour Coding & Regulations (Local and Foreign Standards)
Colour codes vary by country:
Local standards (e.g., Jamaica/British standard):
Live: Brown
Neutral: Blue
Earth: Green/Yellow
US standards (NEC):
Live: Black/Red
Neutral: White
Earth: Green
Understanding both local and foreign standards is important for:
International work or imported equipment.
Preventing wrong connections.
Staying code-compliant and ensuring safety.
Also includes knowing labeling systems, wiring zones, and cable marking rules.
05.1 Determine the Specifications of Materials and Systems
Before estimating, you must understand the type of installation and its requirements. This helps in choosing the right:
Cables (size, insulation type, armoured vs flexible)
Conduits and trunking (PVC, metal, weatherproof)
Switchgear and protective devices (breakers, isolators)
Lighting systems (LED, emergency, energy-saving)
Control systems (sensors, timers, automation)
Different installations (residential, commercial, industrial) have different specifications due to load size, safety, and environment.
05.2 Research Costing of Materials and Equipment
You should:
Check prices from local suppliers, manufacturer catalogs, and online platforms.
Compare brands, quality, and warranty.
Consider bulk prices, discounts, or delivery charges.
Keep updated on market fluctuations (e.g., copper wire cost).
Tools like Excel, quotation forms, or construction estimating software help keep estimates accurate and organized.
05.3 Discuss How Materials and Systems Affect Cost
Some examples:
PVC conduit is cheaper than metal trunking, but metal is better for industrial durability.
LED lighting has a higher initial cost but lower long-term energy costs.
Using standard cable lengths avoids waste and reduces cost.
Choosing higher quality breakers prevents future faults and reduces maintenance.
You must balance cost, safety, quality, and customer needs.
05.4 Identify Approved Sources of Materials and Equipment
Use only materials from reputable suppliers who provide certified, code-compliant products:
Examples of Approved Sources:
Commercial Installations:
Reputable electrical wholesalers, large suppliers like Schneider, ABB, Legrand, etc.
Industrial Installations:
Suppliers of heavy-duty equipment and components rated for high loads or automation.
Damp Conditions:
Use IP-rated (Ingress Protection) waterproof gear from trusted brands.
Hazardous Environments:
ATEX-certified (explosion-proof) materials.
Flame-proof enclosures and intrinsically safe devices.
Approved suppliers provide certification, warranty, and technical support.
05.5 Prepare Material Listings and Estimates for Customers
This includes:
Listing all materials with descriptions, quantities, and unit prices.
Adding labour costs, contingency, and markup/profit.
Presenting in a clear, professional format (quotation sheet or spreadsheet).
Tailoring estimates to customer budgets and explaining choices.
Example
Example Electrical Estimate
Item: 2.5mmΒ² twin & earth cable
Quantity: 100 meters
Unit Price: $1.00 per meter
Total: $100
Item: 16-way breaker panel
Quantity: 1
Unit Price: $120.00
Total: $120
Item: Labour (installation)
Total: $300
Grand Total: $520
β 06.1 Various Types of Conductor Materials and Their Applications
β
06.1 Various Types of Conductor Materials and Their Applications
1. Copper
Most commonly used conductor.
Excellent electrical conductivity and flexibility.
Used in residential, commercial, and industrial wiring.
Found in wiring, motor windings, transformer coils.
- Aluminum
Lighter and cheaper than copper, but less conductive.
Common in overhead transmission lines and large cable installations where weight and cost are factors.
Must be carefully installed to avoid loose connections.
- Silver
Best conductor of electricity.
Very expensive, used in precision instruments, RF circuits, and military applications.
- Gold
Excellent corrosion resistance.
Used in high-end electronics, connectors, and aerospace systems.
β 06.2 Importance of Cable Insulations (Emphasis on Mechanical Protection)
Functions of Insulation:
Prevents short circuits and electric shock.
Maintains the integrity of the cable over time.
Protects the conductor from moisture, chemicals, and physical damage.
Common Insulation Materials:
PVC (Polyvinyl Chloride): Cheap and flexible. Common in household wiring.
XLPE (Cross-linked Polyethylene): High temperature and mechanical strength. Used in industrial settings.
Rubber/Neoprene: Flexible and durable. Suitable for flexible cords and portable equipment.
Mechanical Protection Aspects:
Insulation must resist abrasion, impact, and bending.
For underground or industrial cables, armor or outer sheathing adds extra protection.
In hazardous areas, flame-retardant or fire-resistant insulation is critical.
