EARTH RESISTANCE INSULATION RESISTANCE
Two of the most confused electrical tests on an AGL system — earth resistance checks the effectiveness of the grounding/earthing path, while insulation resistance checks the quality of electrical insulation and its ability to prevent unwanted current leakage. One wants a low reading, the other wants a high one, and passing either test says nothing about the other.
At a Glance
| Feature | Earth Resistance Test | Insulation Resistance Test |
|---|---|---|
| Main purpose | Verify effectiveness of earthing system | Verify insulation integrity |
| Main concern | Fault-current path to earth | Leakage-current path through insulation |
| Desired resistance | Low | High |
| Typical unit | Ω | MΩ / GΩ |
| Test instrument | Earth resistance tester | Insulation resistance tester |
| Test location | Earth electrode/grid/system | Cable, winding, equipment insulation |
| Typical AGL use | CCR/equipment earthing | AGL primary cable insulation |
| Main fault detected | High-resistance earth connection | Insulation deterioration/breakdown |
Earth Resistance
Earth resistance determines how effectively an earthing electrode or grounding system is connected to the general mass of earth.
EQUIPMENT
│
│
EARTH CONDUCTOR
│
▼
EARTH ELECTRODE
│
▼
EARTH
The objective is to provide a sufficiently low-impedance path for fault current and help maintain accessible metalwork near earth potential.
Insulation Resistance
Insulation resistance measures the resistance between electrically separated parts.
PRIMARY CONDUCTOR
════════════════════
│
│
INSULATION
│
│
EARTH
The objective is to verify that the insulation is preventing excessive leakage current.
Two Paths, Two Opposite Goals
Think of an electrical system as offering a current two possible paths — the one it's meant to take, and the one it must never take.
LIVE CONDUCTOR
│
│
┌──────┴──────┐
│ │
▼ ▼
INTENDED PATH UNWANTED PATH
│ │
▼ ▼
LOAD INSULATION
│ │
▼ ▼
RETURN EARTH
Earth Resistance Checks
The intended safety path to earth.
Insulation Resistance Checks
The unwanted path from conductor through insulation to earth or another conductor.
Worked Example: The CCR
Consider an AGL Constant Current Regulator (CCR) installed in a substation. Both tests apply to it, but they check completely different things.
CCR ENCLOSURE
│
▼
EARTH CONDUCTOR
│
▼
EARTH GRID / ELECTRODE
Verifies the grounding arrangement of the enclosure according to the project's requirements.
LIVE CIRCUIT
│
▼
INSULATION
│
▼
EARTH / OTHER CONDUCTOR
A low IR value can point to moisture, contamination, cable damage, or insulation deterioration.
Worked Example: The Primary Cable Circuit
For a typical AGL series circuit, both tests exist along the same run of equipment but answer different questions.
CCR │ ▼ PRIMARY CABLE │ ▼ ISOLATION TRANSFORMER │ ▼ LIGHT
Insulation Resistance Test
Checks the primary cable insulation.
PRIMARY CONDUCTOR
══════════════════
│
▼
INSULATION
│
▼
EARTH
The expected result is high insulation resistance according to the specified test criteria.
Earth Resistance Test
Checks the resistance of an earthing system/electrode.
EARTH BAR │ ▼ EARTH CONDUCTOR │ ▼ EARTH ELECTRODE │ ▼ SOIL
It does not tell you whether the AGL cable insulation is healthy.
Fault Coverage Comparison
| Condition | Earth Resistance | Insulation Resistance |
|---|---|---|
| Poor earth electrode connection | ✔ | — |
| High earth resistance | ✔ | — |
| Damaged cable insulation | — | ✔ |
| Moisture in cable insulation | — | ✔ |
| Leakage through insulation | — | ✔ |
| Broken earth conductor | Can help identify, depending on method | — |
| Poor grounding arrangement | ✔ | — |
What Each Test Is Performed With
Earth Resistance
Typically uses an earth resistance tester / earth tester. Common methods include:
- 3-point fall-of-potential
- 4-point methods for specific applications
- Clamp-on/stakeless methods where suitable
Insulation Resistance
Uses an insulation resistance tester, often referred to as a megohmmeter / megger. It applies a specified DC test voltage and measures insulation resistance.
The appropriate earthing test method depends on the earthing system and test requirements — always follow the applicable standard.
Desired Values Are Opposite
This is the easiest way to remember the difference between the two tests.
EARTH RESISTANCE
↓
LOW Ω
↓
GOOD EARTHING
INSULATION RESISTANCE
↓
HIGH MΩ/GΩ
↓
GOOD INSULATION
Do not apply a universal numerical limit. The acceptable value must always come from the applicable standard, equipment manufacturer, design, and project specification.
Two Almost Opposite Safety Roles
Earthing — During a Fault
FAULT ↓ EQUIPMENT BODY ↓ EARTH CONDUCTOR ↓ EARTH SYSTEM ↓ FAULT-CURRENT PATH
A properly designed earthing system helps provide a safe fault-current path and supports protective-device operation.
Insulation — During Normal Operation
LIVE CONDUCTOR
↓
GOOD INSULATION
↓
NO UNINTENDED PATH
↓
MINIMAL LEAKAGE
Good insulation prevents the fault from occurring in the first place.
Don't Confuse the Two Systems
For AGL installations, don't confuse the earth resistance of the airport earthing system with the insulation resistance of the AGL series circuit. They answer different questions.
AGL SYSTEM
│
┌─────┴─────┐
▼ ▼
EARTHING INSULATION
│ │
▼ ▼
"Is the earth "Is the circuit
path effective?" properly insulated?"
│ │
▼ ▼
LOW Ω HIGH MΩ/GΩ
Two Different Ways to Fail
The AGL equipment has a poor earth connection, so earth resistance reads high and fails — but the cable insulation could still be perfectly good.
The AGL cable insulation has deteriorated, so insulation resistance reads low and fails — but the earthing system could still have excellent earth resistance.
Passing one test does not prove that the other condition is satisfactory. Both tests must be performed and both must pass.
Full Engineering Parameters
| Parameter | Earth Resistance | Insulation Resistance |
|---|---|---|
| Question | Is the earth path effective? | Is the insulation effective? |
| Desired value | Low | High |
| Measurement | Ω | MΩ/GΩ |
| Test object | Earth electrode/grid/earthing system | Cable/equipment insulation |
| Test instrument | Earth tester | IR tester |
| Fault type | Poor grounding | Insulation leakage/breakdown |
| AGL cable relevance | Earthing system | Primary circuit insulation |
| Electrical safety role | Fault-current path | Prevents unintended current path |
How to Remember the Difference
EARTH RESISTANCE
↓
LOW Ω
↓
"FAULT CURRENT CAN
FIND A GOOD EARTH PATH"
INSULATION RESISTANCE
↓
HIGH MΩ/GΩ
↓
"CURRENT IS PREVENTED
FROM LEAKING"
One-line distinction: earth resistance checks the path you WANT fault current to take, while insulation resistance checks the paths you DON'T want current to take.
Earth Resistance vs Insulation Resistance
Earth resistance verifies the resistance/effectiveness of the earthing system, ensuring a suitable fault-current path to earth.
Insulation resistance verifies the electrical integrity of insulation by measuring resistance between electrically separated parts. For AGL systems, both tests are required and serve almost opposite purposes: earthing provides the safe path for a fault to happen on, while insulation prevents the fault from happening at all.
Earth electrode/grid → earth resistance test. CCR enclosure earthing → earthing continuity/resistance as applicable. AGL primary cable and isolation transformer insulation → insulation resistance test, as applicable.
Engineering & Expert Notes
Do Not Compare the Two Numbers
InterpretationOne wants a low value and the other a high value. Interpret each against its own purpose.
Record Soil and Weather Conditions for Earth Tests
ConditionsMoisture changes earth readings. Note conditions to compare results over time.
Isolate Before Any Insulation Test
SafetyFollow the approved procedure for isolation, discharge and test voltage.
Trend Both Over the Years
TrendingGradual change in either is an early warning of deterioration.
Frequently Asked Questions
Earth resistance checks the effectiveness of the grounding/earthing path so fault current can safely flow to earth. Insulation resistance checks the quality of insulation and its ability to prevent unwanted current leakage.
A low earth resistance means fault current has an easy, low-impedance route to earth, which is what you want. A low insulation resistance means current is leaking somewhere it shouldn't, which indicates a fault.
No. The two tests are independent. Passing one test does not prove that the other condition is satisfactory — both should be performed.
Earth resistance is measured with an earth resistance tester, commonly using fall-of-potential, 4-point, or clamp-on/stakeless methods. Insulation resistance is measured with an insulation resistance tester, often called a megohmmeter or megger.
Earth resistance typically applies to the earth electrode, earth grid, and CCR enclosure earthing. Insulation resistance typically applies to the AGL primary cable and isolation transformer insulation.
No. The acceptable value must always come from the applicable standard, equipment manufacturer, design, and project specification rather than a generic number.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
IEEE 81 / IEC 62305 Series
Guidance on earth/ground resistance measurement methods including fall-of-potential and related test techniques for earthing/grounding systems.
IEC 60364 / Local Wiring Regulations
Requirements for insulation resistance testing of low-voltage installations and cabling, including AGL series and control circuits.
DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Do not select an acceptance value from a generic web diagram. The final test criteria should always be developed from the current governing standard, approved design, and manufacturer-approved installation arrangement.
Earth resistance and insulation resistance are not two versions of the same test — one confirms the path you want current to take, the other confirms the paths it must never take.
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