Surge and Lightning Arresters: Functions, Types, and Testing Explained
Learn how surge and lightning arresters protect power systems from overvoltage. Discover working principles, MOV technology, arrester types, and key testing methods including insulation resistance, power factor, and V-I testing.
Surge and lightning arresters are essential protection devices in electrical power systems. Although they are often overlooked, they play a critical role in preventing equipment damage caused by overvoltage events such as lightning strikes and switching surges.
What Is the Purpose of Surge and Lightning Arresters?
Surge and lightning arresters are designed to protect electrical equipment from voltage spikes.
These spikes can be caused by:
- Lightning strikes
- Switching operations in the grid
- Fault conditions
- System resonance or ferroresonance
- Reconnection or disconnection events
Their main job is simple: divert dangerous overvoltage energy safely to ground before it damages transformers, cables, or substations.
How Do Surge Arresters Work in Power Systems?
Surge arresters are installed in parallel with protected equipment.
How do they behave during normal operation?
- They stay in a high-resistance (non-conducting) state
- Almost no current flows through them
What happens during a surge?
- Voltage rises above normal levels
- The arrester switches to a low-resistance state
- Excess energy is discharged to earth
- Voltage returns to normal, and the arrester resets
This automatic switching behavior protects the system from insulation breakdown.
What Are Surge Arresters Made Of?
Most modern arresters use metal-oxide varistors (MOVs).
Why are MOVs used?
- They behave like insulators at normal voltage
- They conduct heavily when voltage rises
- They respond very quickly to transients
A typical MOV shows a sharp “knee point” where resistance suddenly drops, allowing surge current to flow safely.
What Types of Surge Arresters Are Used in Power Systems?
Surge arresters are classified based on voltage level and application:
What are low-voltage surge protection devices?
- Used below 3 kV
- Protect household and low-voltage equipment
What are medium-voltage distribution arresters?
- Typically 3 kV to 30 kV
- Protect distribution transformers and cables
What are high-voltage station-class arresters?
- Used above 30 kV
- Protect substations and transmission transformers
What are magnetic blow or valve-type arresters?
- Used in high-energy systems up to 700 kV or more
- Suitable for large substations and communication networks
Why Is Proper Installation of Arresters Important?
Incorrect installation can reduce protection efficiency.
Key installation requirements include:
- Connection in parallel with equipment
- Strong and reliable grounding
- Correct voltage rating selection
- Proper mechanical mounting on bushings or structures
Without proper grounding, surge energy cannot be safely discharged.
What Happens Inside a Surge Arrester During a Surge?
Inside the arrester:
- Overvoltage appears
- MOV elements start conducting
- Surge current flows to ground
- Energy is dissipated as heat
- Voltage normalizes
- MOV returns to insulating state
This cycle happens within milliseconds.
How Do We Test Surge and Lightning Arresters?
Because arresters can fail without visible warning signs, regular testing is important.
Testing methods include:
- Visual inspection
- Thermal (infrared) inspection
- Electrical testing
What Is Checked During Visual Inspection?
Visual inspection helps identify:
- Cracks in housing
- Corrosion or moisture ingress
- Loose or damaged connections
- Surge counter records (if available)
A rising surge count may indicate the need for replacement.
How Is Thermal Inspection Used?
Thermal cameras or sensors detect:
- Hot spots caused by internal losses
- Poor connections
- Structural defects in cooling areas
Temperature trends over time can help identify early-stage failures.
What Electrical Tests Are Used for Arresters?
What is the watts loss (power factor) test?
- Measures leakage current and losses
- Helps identify moisture, contamination, or internal damage
- Common for station-class arresters
What is the V-I response test?
- Applies increasing voltage to observe conduction behavior
- Maps the arrester’s V-I curve
- Used to verify performance characteristics
⚠ High voltage risk: This test requires strict safety control.
What is the insulation resistance test?
- Uses a megohmmeter
- Quickly checks if arrester is short-circuited
- Best for low and medium voltage arresters
What is third harmonic analysis?
- Measures resistive current harmonics
- Very accurate but time-consuming
- Mostly used in factory testing
Which Test Method Is Best for Arresters?
The best method depends on voltage class:
- Low/medium voltage → Insulation resistance test
- High-voltage station arresters → Watts loss test
- Factory or detailed analysis → Third harmonic or V-I testing
Why Is the Guard Terminal Important in Testing?
Surface leakage currents can distort results.
Without a guard terminal:
- Moisture and dirt on surfaces create false readings
- Measured resistance may appear lower than actual condition
With a guard terminal:
- Surface leakage is removed from measurement
- Only internal arrester condition is tested
This ensures more accurate diagnostics.
What Are Common Causes of Arrester Failure?
Arrester performance can degrade due to:
- Moisture ingress
- Pollution or salt contamination
- Thermal aging of MOV blocks
- Repeated surge stress
- Mechanical or housing damage
FAQs (Long-Tail Questions)
What is the main function of a surge arrester in a power system?
It protects electrical equipment by diverting high-voltage surge energy safely to ground.
How does a metal oxide varistor in an arrester work?
It acts as an insulator under normal voltage and becomes conductive when voltage exceeds a threshold.
Can a surge arrester fail without visible signs?
Yes. Many arrester failures are internal and require electrical testing to detect.
How often should surge arresters be tested?
Typically during commissioning, periodically during maintenance, and after major fault events.
What is the difference between lightning arresters and surge arresters?
Lightning arresters are designed specifically for lightning energy, while surge arresters also handle switching and system transients.
Why do arresters need grounding?
Grounding provides a safe path for surge energy to be discharged without damaging equipment.
