what is the Insulation Resistance (IR) Test for cables ?

What is the purpose of cable insulation?

Cable insulation is an important protective material for cable conductors. It is non-conductive, used to resist electrical leakage, prevents cable conductors from contact with other conductors, and protects the conductor from environmental threats such as heat, water, and chemicals. Poor or damaged insulation may result in short circuit, electric shock, or fire.
Because the insulation of a cable is so important in determining the cable's safety and electrical conductivity, at Keystone Cable, we ensure that all our cables are subject to passing the insulation resistance (IR) test (as part of our many tests) before product delivery to customers.

What is Insulation Resistance (IR) Test?

An insulation resistance (IR) test measures the resistance to current flow across it on a completed cable; it applies a test voltage to determine how effective the insulation is in preventing the flow of electric current out of the insulation. This is analogous to how you would pump pressurized water in a water pipe to identify leaks.

Since insulation starts to age after it is made, over time, the performance of a high quality insulation material versus one of lower quality will become more apparent. Hence it is important that after the cable is manufactured there is a good pass rate for the IR test to help ensure the longevity of your cable.


Insulation Resistance Test Process

IR test is conducted using an IR tester. The IR tester is a portable ohmmeter (MΩ.km) with a built-in generator that produces a high DC voltage. The DC voltage usually measures 500V and causes a current to flow around the surface of the insulation. This resistance reading measures leakage current; a high IR reading means very little current is escaping through the insulation and a low IR reading indicates stronger current leakage and may indicate a break in the insulation.

At Keystone's quality control laboratory, we adhere to International Standards IEC 60502-1 for our IR tests. To pass, the cables would need to obtain a minimum insulation resistance constant Ki (refer to the table below) while tested at its maximum operating temperature (e.g. 70 °C for PVC insulated cables and 90 °C for XLPE and rubber insulated cables).




For single core cables, the cables are tested in water while for multi core cables they are tested in air. Test results will also vary for cables across different types of insulation, length of run and ambient temperature. To be certain for your cable type, feel free to check in with our team on the IR tests we perform for your cables.

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INSULATION RESISTANCE OF A CABLE

Cable conductor is provided with an insulation of suitable thickness to avoid the leakage of current. The thickness of any cable depends on the purpose of its design. The path of current leakage in such cable is radial. The resistance or opposition offered by the insulation to the flow of current is also radial throughout its length.

For a single core cable conductor of radius r1, internal sheath radius r2, length l and insulation material resistivity ρ, the perimeter of the conductor is 2πrl. The thickness of the insulation will be given as dr.

Rins = ρdr/2πrl

When integrated, we will have:

Rins = ρ/2πl[loge r2 /r2 ]

Rins is inversely proportional to 1/l contrary to R = ρl. Where ρ (rho) is a constant known as resistivity.
There are some cables that have more than one insulating layers and more than one core. The main wire being at the center, serve as the main conductor. The other core serves the purpose of grounding and preventing the electromagnetic waves and radiations from escaping from the cabled. It serve as a shield. Cables under this category is the Coaxial cables.

Coaxial cable conducts electrical signal using an inner conductor (the inner or main conductor could be any good conductor but copper is mostly preferred because of it's low resistivity, the copper could also be plated) is contained in mostly PVC case. Before the outer PVC case, there are two or more other insulators with either aluminum foil or copper strand between them. The cables are protected from external environment by the outermost PVC case. While voltage is passed through the inner conductor, the shield or case has little or no voltage passing through it.

The advantage of coaxial design is that electric and magnetic fields are confined to the dielectric with little leakage outside the shield. Due to the level of insulation in the cables which prevents outside electromagnetic fields and radiations from penetrating into it, interference is avoided. Since conductors with large diameter have less resistance, less electromagnetic field will be leaked. The same goes for cables with more insulation. Knowing that weaker signals are easily interrupted by little interference, cables with more layers of insulation are always good choice for conveying such signals.

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R = KM log10(D / d )

R is insulation resistance (MΩ⋅km)
K is insulation resistance constant
M is the temperature correction factor to 15.6 °C
D is the diameter over the insulation
d is the diameter under the insulation

The insulation resistance of the sample under test is determined by the following formula:

R = 0.001* Rmeas

where
R is the insulation resistance (MΩ⋅km)
Rmeas is the measured insulation resistance (MΩ)
L is the length of the test sample (m)

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What is the Insulation Resistance (IR) Test for Cables?

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Introduction
The Insulation Resistance (IR) test is a critical diagnostic tool used to assess the quality of the insulation in electrical cables. It helps ensure the safety, reliability, and longevity of cable systems by measuring the resistance offered by the insulation material to the flow of current. This test is commonly performed during cable manufacturing, installation, and maintenance to detect any potential issues that could lead to failures or hazards.

This article explains the purpose, procedure, standards, and significance of the IR test for cables.

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1. Purpose of the Insulation Resistance Test

The primary purposes of the insulation resistance test are:

• Safety: To ensure that the insulation is effective in preventing electric shock hazards.
• Reliability: To detect any deterioration or damage to the insulation that could compromise the performance of the cable.
• Preventive Maintenance: To identify potential issues early, allowing for corrective action before a failure occurs.
• Quality Assurance: To verify that the cable meets the required insulation resistance standards before commissioning.

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2. How the Insulation Resistance Test Works

The insulation resistance test involves applying a direct current (DC) voltage to the cable and measuring the resulting leakage current through the insulation. The resistance is calculated using the following formula:

IR = K × log10(D/d)
Where:

• IR = Insulation resistance in MΩ·1000 feet or MΩ·km at 15.6°C (60°F)
• K = Specific insulation resistance constant in MΩ·1000 feet or MΩ·km at 15.6°C (60°F)
• D = Diameter over insulation (mils)
• d = Diameter under insulation (mils)

Table 1: Minimum Values of K at 60°F (15.6°C)

Insulation Type	K (MΩ·1000 feet)
Thermoset (Crosslinked) Polyethylene - Low Voltage (rated 0 - 2kV)	10,000
Thermoset (Crosslinked) Polyethylene - High Voltage (rated > 2kV)	20,000
Thermoplastic Polyethylene	50,000
Polyethylene – PVC Composite	30,000
PVC	500
PVC (@ 75°C)	50
Ethylene Propylene Rubber - Low Voltage (rated 0 - 2kV)	10,000
Ethylene Propylene Rubber - High Voltage (rated > 2kV)	20,000

Equipment Used:

• Insulation resistance tester (megohmmeter)
• Test leads and probes

The megohmmeter typically applies a high DC voltage (250V, 500V, 1000V, or higher) and measures the resistance in megohms (MΩ).

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3. Procedure for Performing the IR Test

The following steps outline the general procedure for conducting an insulation resistance test on cables:

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• Preparation: Ensure the cable is de-energized and disconnected from any equipment. Verify that the test environment is safe.
• Connection: Connect the megohmmeter to the conductor and the insulation (or sheath) of the cable using appropriate test leads.
• Voltage Selection: Select the appropriate test voltage based on the cable's rated voltage. Refer to applicable standards for guidance.
• Test Execution: Apply the test voltage for a specified duration (typically 1 minute) and record the insulation resistance value.
• Result Interpretation: Compare the measured resistance value with the minimum acceptable resistance specified by standards or manufacturer guidelines.
• Disconnection: Safely disconnect the test equipment and restore the cable to its normal configuration.

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4. Standards and Guidelines

Several international standards provide guidelines for insulation resistance testing, including:

• IEC 60364: Electrical installations of buildings
• IEEE 400: Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems
• BS 7671: Requirements for Electrical Installations (IET Wiring Regulations)

These standards specify the minimum acceptable insulation resistance values and the appropriate test voltages for various types of cables.

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5. Factors Affecting Insulation Resistance

Several factors can influence the results of an insulation resistance test:

• Temperature: Higher temperatures reduce the insulation resistance, so temperature correction factors may be applied.
• Moisture: The presence of moisture significantly lowers the insulation resistance, indicating potential issues with the cable.
• Cable Length: Longer cables exhibit lower insulation resistance due to the increased surface area of the insulation.
• Cable Age: Aging and degradation of the insulation material over time can reduce resistance.

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6. Interpretation of Results

The insulation resistance value should be compared with the recommended minimum values provided by standards or the cable manufacturer. Typical minimum acceptable values are:

• Low-voltage cables (up to 1 kV): Minimum 1 MΩ
• Medium-voltage cables (1 kV to 35 kV): Minimum 100 MΩ
• High-voltage cables (above 35 kV): Minimum 1000 MΩ

A sudden drop in insulation resistance over time indicates potential insulation deterioration or damage, requiring further investigation.

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7. Applications of the Insulation Resistance Test

The IR test is widely used in various applications, including:

• Cable Manufacturing: To ensure the quality of insulation during production.
• Installation Testing: To verify the integrity of cable insulation before commissioning.
• Maintenance Testing: To monitor the condition of cable insulation during periodic maintenance.
• Troubleshooting: To identify faulty cables in an electrical system.

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Conclusion
The insulation resistance test is a vital tool for ensuring the safety and reliability of electrical cables. By measuring the resistance offered by the insulation, the test helps detect potential issues such as moisture ingress, insulation degradation, and mechanical damage. Regular IR testing, combined with proper interpretation of results, can prevent electrical failures, enhance system performance, and extend the lifespan of cables.

Adhering to international standards and best practices during insulation resistance testing ensures accurate and reliable results, contributing to the overall safety of electrical installations.

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References

• IEC 60364 - Electrical Installations of Buildings
• IEEE 400 - Guide for Field Testing of Shielded Power Cable Systems
• BS 7671 - Requirements for Electrical Installations
• Manufacturer Technical Manuals