Primary & Secondary Equipment (Circuits) - Detailed Guide

Primary Equipment, Secondary Equipment, and Civil Auxiliary Facilities in Power Systems

---

1. Primary Equipment (Main Power Circuit)

Primary equipment refers to devices directly involved in the generation, transmission, transformation, distribution, and utilization of electrical energy.
They usually handle high voltage and large current.
The circuits they form are called the primary circuit or primary wiring system.

Examples of Primary Equipment

• Generator
• Transformer
• Circuit breaker
• Disconnect switch (isolator)
• Automatic switch
• Contactor
• Knife switch
• Busbar
• Transmission line
• Power cable
• Reactor
• Motor
  
  

---

2. Secondary Equipment (Protection & Control Circuit)

Secondary equipment mainly performs monitoring, control, regulation, protection, and communication functions for the primary system.

Main Functions

• Monitoring – real-time measurement of voltage, current, power, etc.
• Control – remote or automatic start/stop and switching of primary equipment
• Regulation – automatic adjustment of voltage, current, and frequency
• Protection – isolates faulty equipment (relay protection)
• Information transmission – sends operating data to monitoring systems
• Alarm indication – alerts operators when abnormal conditions occur
  
  

Common Secondary Equipment

• Fuse — protects equipment when current exceeds safe value
• Control switches — manual or remote operation of equipment
• Energy meter — records power consumption
• Relays — detect faults such as overcurrent or overvoltage
• Control cables — transmit control signals and data
• Indicator lamps — display operating status
• Automation monitoring system — remote monitoring and data acquisition
  
  

---

3. Equipment Used in Both Primary and Secondary Systems

Some devices serve both power flow and monitoring/protection roles.

Instrument Transformers (CT / PT)

• Primary side: sample high voltage/current
• Secondary side: convert to low voltage/current for protection and metering
  
  

Surge Arresters

• Primary: protect transformers and breakers from lightning/overvoltage
• Secondary: protect protection and control equipment from induced surges
  
  

Cables

• Primary: transmit power
• Secondary: transmit control and measurement signals
  
  

Circuit Breakers (Smart Breakers)

• Primary: connect or disconnect load current
• Secondary: provide monitoring data to automation systems
  
  

Grounding System

• Primary: personnel safety and system protection
• Secondary: noise shielding and stable operation of electronics
  
  

---

4. Civil Auxiliary Structures

Civil auxiliary structures provide the physical support and environment for electrical equipment.

Simply put: they are the “building and foundation” of the power system.

Common Structures

• Utility poles — support overhead lines
• Substation buildings — protect equipment from weather
• Cable trenches and conduits — protect buried cables
• Transmission towers — support high-voltage lines
• Foundations, supports, fences, drainage systems
• Grounding grid — protects personnel and equipment
  
  

---

Civil auxiliary facilities ensure electrical equipment operates safely and reliably within a stable environment.