Electrical Schematics

Understanding electrical drawings:

Types of Electrical Drawings:

Schematic Diagrams:
Show electrical logic and function:
- How circuit works
- All components and connections
- Not physical location
- Primary troubleshooting tool

Wiring Diagrams:
Show physical connections:
- Terminal numbers
- Wire numbers
- Cable designations
- Panel-to-field wiring

Panel Layout:
Physical component location:
- Component placement
- Dimensions
- Heat dissipation
- Maintenance access

Single Line (One-Line):
Simplified power overview:
- Power distribution
- Protection devices
- Transformer connections
- System architecture

Symbol Standards:

Common Standards:
- NEMA: North American
- IEC: International
- JIC: Joint Industry Council
- Mix often seen in practice

Basic Symbols:
- Contacts (NO, NC)
- Coils (relays, solenoids)
- Motors
- Switches
- Lights
- Fuses/breakers

Reading Direction:
- Power flows top to bottom
- Control sequence left to right
- Rungs numbered for reference
- Cross-references between pages

Component Designations:

Standard Prefixes:
- M: Motor
- CR: Control relay
- PB: Pushbutton
- LS: Limit switch
- PS: Pressure switch
- SOL: Solenoid
- PL: Pilot light

Numbering:
- Sequential within type
- Related components same base
- Consistent throughout project

Understanding control logic:

Basic Control Circuits:

Two-Wire Control:
- Simple on/off
- Maintained contact (switch)
- Direct motor control
- No memory

Three-Wire Control:
- Start/Stop stations
- Momentary contacts
- Seal-in circuit (memory)
- Industry standard

Seal-In Circuit:
How motors stay running:
1. Press START (momentary NO)
2. Contactor energizes
3. Auxiliary contact seals around START
4. Release START, motor runs
5. Press STOP (momentary NC) to stop

Multiple Stations:
- Stop buttons in series (any stops)
- Start buttons in parallel (any starts)
- Master stop overrides all

Interlocking:

Prevent conflicting operations:
- Mechanical interlock (contactors)
- Electrical interlock (aux contacts)
- Push-pull interlock (forward/reverse)

Example - Forward/Reverse:
- Forward contact opens reverse circuit
- Reverse contact opens forward circuit
- Prevents simultaneous operation
- Protects motor and driven equipment

Sequence Control:

Operations in order:
- Pilot devices (LS, PS, FS)
- Timer sequences
- Interlock conditions
- Step-by-step operation

Example - Conveyor System:
- Downstream starts first
- Upstream starts after delay
- Jam detection stops upstream
- Emergency stop stops all

Troubleshooting with Schematics:

Logical Approach:
1. Understand normal operation
2. Identify what is not working
3. Trace circuit from power to load
4. Check each component in series
5. Find the open or short

Voltage Tracing:
- Power present = closed path to source
- No power = open circuit upstream
- Check at each junction
- Identify where power is lost

Developing clear, complete drawings:

CAD Software:

Popular Platforms:
- AutoCAD Electrical
- EPLAN
- SEE Electrical
- ePLAN Electric P8
- SolidWorks Electrical

CAD Advantages:
- Symbol libraries
- Automatic cross-referencing
- Bill of materials
- Error checking
- Revision control

Drawing Organization:

Page Numbering:
- Logical grouping
- Power circuits separate from control
- Consistent structure
- Easy navigation

Cross-References:
Show related components:
- Relay coil references contact locations
- Contact references coil location
- Page and line numbers
- Essential for troubleshooting

Title Block:
Standard information:
- Drawing number
- Revision level
- Project information
- Approval signatures
- Date

Best Practices:

Clarity:
- Consistent symbol use
- Clear line intersections
- Adequate spacing
- Logical flow

Completeness:
- All wires numbered
- Terminal numbers shown
- Component designations
- Notes where needed

Standards Compliance:
- Follow company standards
- Industry conventions
- Customer requirements
- Code compliance

Documentation Package:

Complete Set Includes:
- Schematics
- Wiring diagrams
- Panel layouts
- Bill of materials
- Operating descriptions
- Interconnection diagrams

Schematic skills are universally valuable:

Maintenance Technician:
Use schematics for troubleshooting:
- Interpret existing drawings
- Trace circuits
- Document changes
- $50,000-$75,000

Panel Builder:
Build from schematics:
- Read and follow drawings
- Component installation
- Wiring accuracy
- $45,000-$65,000

Controls Technician:
Create and modify schematics:
- As-built documentation
- Modifications
- Troubleshooting complex systems
- $55,000-$80,000

Controls Engineer:
Design electrical systems:
- Complete schematic development
- System design
- Standards compliance
- $75,000-$110,000

Skills Progression:

Reading (Foundation):
- Symbol recognition
- Circuit tracing
- Logic understanding
- Cross-reference use

Interpretation (Intermediate):
- Complex circuit analysis
- System understanding
- Troubleshooting application
- Modification planning

Creation (Advanced):
- CAD proficiency
- Design capability
- Standards compliance
- Documentation development

Learning Resources:

Training:
- Technical school courses
- Vendor training
- Online courses
- On-the-job experience

Practice:
- Trace existing drawings
- Build simple circuits
- Redline actual panels
- Create as-built drawings

Standards:
- NFPA 79 (Industrial Machinery)
- IEC 60204 (Machine Safety)
- Company standards
- Customer requirements

Certifications:
- CAD software certifications
- Electrical certifications
- Vendor credentials

Schematic skills are essential for all electrical and controls professionals.