Cable Testing

Cable Insulation Resistance Test is an important procedure to assess the quality and integrity of electrical conductors. Conductors that are subjected to excessive stress or adverse environmental conditions can deteriorate over time. Regular testing can detect early signs of wear, corrosion, or other issues that may lead to failures and costly breakdowns.

Electrical cable insulation test is typically performed for the following reasons:

Low Voltage Cables

Installation (Acceptance Testing) :
Insulation tests are often performed after the installation of new electrical cables to verify that the insulation has been correctly installed and is free from any initial faults or damage. This helps identify any issues that may have occurred during the installation process before the system is energized.

Periodic Maintenance:
Regular preventive maintenance programs often include insulation testing as part of their routine. Periodic insulation tests help assess the condition of the insulation over time, detect any deterioration or degradation, and identify potential insulation faults before they cause problems or lead to system failures.

Troubleshooting:
Insulation tests are valuable in troubleshooting electrical issues. When there is an unexpected electrical fault, insulation tests can help determine if insulation damage or degradation is the cause. By pinpointing the location and severity of insulation problems, it becomes easier to plan repairs or replacements accordingly.

Following Significant Events:
Insulation tests are recommended after significant events that may impact the electrical system, such as power surges, lightning strikes, equipment failures, or other electrical disturbances. These events can cause insulation stress or damage, and conducting insulation tests helps assess the system's integrity and identify any resulting issues.

Cable Insulation Resistance Test procedure, Low Voltage (Less than 600V)

Purpose

Cable testing is conducted to chart the gradual deterioration over the years, to do acceptance testing after installation, for verification of splices and joints, and for special repair testing.

Acceptance Testing (New Cables)

Acceptance testing is performed after the installation of all cable and accessories, but before energizing the cable with system voltage. Its purpose is to detect installation damage in both the cable and cable accessories.

Maintenance Testing

Over time, cable insulation will degrade. A true evaluation is based on a trend in readings over a time period.

Maintenance testing is performed during the operating life of the cable system. Its purpose is to assess the condition and check the serviceability of the cable system so that suitable maintenance procedures can be initiated.

Test resistance values can vary, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture) Sometimes the drop in insulation resistance is sudden, as when equipment is flooded. Usually, however, it drops gradually A trend showing declining resistance values will be gradual, giving plenty of warning, of a possible service failure

Typically, cables have a design life of at least 20-30 years, but various stresses can increase the natural aging process and decreasing the normal working life of the cable.

Common environmental and mechanical stresses found in a plant’s electrical system are:

  • Vibration
  • Excessive Dirt
  • Oil
  • Corrosive Vapors
  • Ambient Temperature, both heat and cold
  • Humidity
  • Moisture from processes
Test Equipment

The insulation resistance is measured using a Megohmmeter (or it can be measured using a portable instrument consisting of a direct voltage source, such as a generator, battery, or rectifier, and a high-range ohmmeter that gives insulation resistance readings in megohms or ohms). This is a nondestructive method of determining the condition of the cable insulation to check contamination due to moisture, dirt, or carbonization.

Low Voltage Cables
Fluke 1587 Insulation Resistance Test Set

Test Procedure

Visual and Mechanical Inspection:
  1. Compare cable data with drawings and specifications.
  2. Inspect exposed sections of cable for physical damage and correct connection in accordance with the single-line diagram.
  3. Inspect for correct identification and arrangements.
  4. Inspect cable jacket insulation and condition.
Insulation Test:
  1. Perform insulation-resistance test by applying test voltage on each conductor with respect to ground and adjacent conductors. Applied potential shall be 500 volts dc for 300-volt rated cable and 1000 volts dc for 600-volt rated cable.
  1. Phase conductor to adjacent Phase conductor
    (A-ph & B-ph, B-ph & C-ph, C-ph & A-ph)
  2. Phase to Ground Bus
    (A-ph & ground, B-ph & ground, C-ph & ground)
  3. Phase to Neutral Bus
    (A-ph & neutral, B-ph & neutral, C-ph & neutral)
  1. Test voltage shall be applied to each conductor for one minute.

Neta Table 100.1
NETA Table 100.1 shows recommended minimum test result values
Test Results
  1. Insulation-resistance values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.Values of insulation resistance less than this table or manufacturer’s recommendations shall be investigated.
  2. Cable shall exhibit continuity.
  3. Deviations in resistance between parallel conductors shall be investigated.
NETA Test Procedure, Low Voltage

NETA ATS

7.3.2 Cable Low-Voltage, 600-Volt Maximum

A. Visual and Mechanical Inspection:
  1. Compare cable data with drawings and specifications.
  2. Inspect exposed sections of cable for physical damage and correct connection in accordance with the single-line diagram.
  3. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.3.2.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Results of the Perform thermographic survey shall be in accordance with Section 9. (optional)
  4. Inspect compression-applied connectors for correct cable match and indentation.
  5. Inspect for correct identification and arrangements.
  6. Inspect cable jacket insulation and condition.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with low-resistance ohmmeter, .
  2. Perform insulation-resistance test on each conductor with respect to ground and adjacent conductors. Applied potential shall be 500 volts dc for 300-volt rated cable and 1000 volts dc for 600-volt rated cable. Test duration shall be one minute.
  3. Perform continuity tests to insure correct cable connection. (optional)
  4. Verify uniform resistance of parallel conductors.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9.(optional)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.
    Values of insulation resistance less than this table or manufacturer’s recommendations shall be investigated.
  3. Cable shall exhibit continuity.
  4. Deviations in resistance between parallel conductors shall be investigated.

NETA MTS

7.3.2 Cable Low-Voltage, 600-Volt Maximum

A. Visual and Mechanical Inspection:
  1. Inspect exposed sections of cables for physical damage and evidence of overheating.
  2. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.3.2.B.1 (optional).
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform a thermographic survey shall be in accordance with Section 9. (optional)
  3. Inspect compression-applied connectors for correct cable match and indentation.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter in accordance with Section 7.3.2.A.2.1.
  2. Perform an insulation-resistance test on each conductor with respect to ground and adjacent conductors. The applied potential shall be 500 volts dc for 300-volt rated cable and 1000 volts dc for 600-volt rated cable. The test duration shall be one minute.
  3. Verify uniform resistance of parallel conductors.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt-torque levels should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9.(optional)
D. Test Values – Electrical
  1. compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values should be comparable to previously obtained results and similar circuits but not less than two megohms.
  3. Deviations in resistance between parallel conductors shall be investigated.
NETA ATS / MTS
NETA ATS / MTS
Neta Table 100.5
Neta Table 100.5
Medium Voltage Cable Testing, VLF Method

LSIG Curve
Model layout performed on SKM software Interface
Introduction

VLF testing is considered a safe and efficient method for medium voltage cable testing. The method is used for assessing the integrity and reliability of high voltage cables. This testing method helps identify cable defects, insulation degradation, and potential weaknesses in the cable insulation.

VLF testing is considered a non-destructive testing technique that can be used for both new cable installations and routine maintenance of existing cable systems. In addition, VLF testing can be performed on longer cable lengths compared to DC testing methods.

Principals of VLF Testing

VLF testing utilizes an alternating current (AC) source with a frequency typically in the range of 0.01 Hz to 0.1 Hz, which is much lower than the power frequency (50 Hz or 60 Hz). This low-frequency AC is applied to the cable under test, creating an electric field that stresses the insulation and reveals potential weaknesses.

Test Equipment

VLF test equipment consists of a VLF high voltage source, which applies a low frequency sinusoidal waveform with the desired frequency to the cable. The test set also monitors and measures test parameters, including voltage, current, and waveforms.

Test Procedure
Visual and Mechanical Inspection:
  1. Inspect exposed sections of cables for physical damage.
  2. Inspect shield grounding, cable supports, and terminations.
  3. If cables are terminated through window-type current transformers, inspect to verify that neutral and ground conductors are correctly placed and that shields are correctly terminated for operation of protective devices.
  4. Inspect for correct identification and arrangements.
  5. Inspect cable jacket and insulation condition.
Shield-continuity and Insulation Test:
  1. Perform a shield-continuity test on each power cable.Shielding shall exhibit continuity. Investigate resistance values in excess of ten ohms per 1000 feet of cable.
  2. Perform an insulation-resistance test individually on each conductor and each shield with all other conductors and shields grounded.
    3. Neta Table 100.1
    NETA Table 100.1 shows recommended minimum test result values
VLF Test Result:
  1. Test voltage is maintained for a duration 60 minutes, to stress the cable insulation. Test voltage values can be found in IEEE std 400.2, Table 3.
  2. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the test, the test specimen is considered to have passed the test.
IEEE std 400.2-2013
IEEE Guide for Field Testing of Shielded Power Cable System Using Very Low Frequency (VLF) (Less than 1HZ)
Table 3- VLF withstand text voltages for sinusoidal and cosine-rectangular waveforms (see note 1)
Cable System Rating
(phase to phase) 		Installation
(phase to ground) 		Acceptance
(phase to ground) 		Maintenance2
(phase to ground)
(see Note 2)
kV kV rms kV peak kV rms kV peak kV rms kV peak
5 9 13 10 14 7 10
8 11 16 13 18 10 14
15 19 27 21 30 16 22
20 24 (note3) 34 (note3) 26 37 20 28
25 29 (note3) 41 (note3) 32 45 24 (note3) 34 (note3)
28 32 45 36 (note3) 51 (note3) 28 38
30 34 48 38 54 29 (note3) 41 (note3)
35 39 55 44 62 33 (note3) 47 (note3)
46 51 72 57 81 43 61
69 75 106 84 119 63 89

NOTE 1 - If the operating voltage is a voltage class lower than the rated voltage of the cable, it is recommended that the maintenance test voltages should be those corresponding to the operating voltage class.

NOTE 2 -IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) The maintenance voltage is about 75% of the acceptance test voltage magnitude.

NOTE 3 - Some existing test sets have a maximum voltage that is up to 5% below the values listed in the table. These test sets are acceptable to be used. However, there is a risk that the cable may be “undertested” due to a combination of lower test voltage and allowed uncertainty of the measuring circuit.

VLF ac voltage testing methods utilize ac signals at frequencies in the range of 0.01 Hz to 1 Hz. The most commonly used, commercially available VLF ac voltage test frequency is 0.1 Hz. VLF ac test voltages with cosine-rectangular and the sinusoidal wave shapes are most commonly used. While other wave shapes are available for testing of cable systems, recommended test voltage levels have not been established.

Fault Current Calculations

Short circuit analysis of the system model is done using SKM's A_FAULT module, which determines the maximum fault levels at each bus; typically, this would be inside switchgear and panels. The module automatically compare these values against manufacturer short circuit current ratings.

A_FAULT Module

The module provides calculated fault values, complies with ANSI/IEEE C37 standard for fault currents calculations. The software also uses the standard's evaluation factors and ratios required for high- and low-voltage device short circuit duty evaluation.

short circuit analysis
Electrical System Short Circuit Analysis

General

Very Low Frequency (VLF)cable testing is a widely used method for assessing the integrity and reliability of high voltage cables, such as power cables used in electrical grids.

NETA Test Procedure, Medium Voltage
Cables
Medium- and High-Voltage

NETA ATS (Acceptance)

7.3.3 Cables, Medium- and High-Voltage
A. Visual and Mechanical Inspection:
  1. Compare cable data with drawings and specifications.
  2. Inspect exposed sections of cables for physical damage.
  3. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.3.3.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torquewrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform a thermographic survey in accordance with Section 9.
  4. Inspect compression-applied connectors for correct cable match and indentation.
  5. Inspect shield grounding, cable supports, and terminations.
  6. Verify that visible cable bends meet or exceed ICEA and manufacturer’s minimum published bending radius.
  7. *Inspect fireproofing in common cable areas.
  8. If cables are terminated through window-type current transformers, inspect to verify that neutral and ground conductors are correctly placed and that shields are correctly terminated for operation of protective devices.
  9. Inspect for correct identification and arrangements.
  10. Inspect cable jacket and insulation condition.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 7.3.3.A.3.1.
  2. Perform an insulation-resistance test individually on each conductor and each shield with all other conductors and shields grounded. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.
  3. Perform a shield-continuity test on each power cable.
  4. Perform cable time domain reflectometer (TDR) measurements on each conductor.
  5. In accordance with ICEA, IEC, IEEE and other power cable consensus standards, testing can be performed by means of direct current, power frequency alternating current, very low frequency alternating current, or damped alternating current (DAC). These sources may be used to perform insulation-withstand tests, and baseline diagnostic tests such as partial discharge analysis, and power factor or dissipation factor. The selection shall be made after an evaluation of the available test methods and a review of the installed cable system. Some of the available test methods are listed below.
    6. Dielectric Withstand
    1. Direct current (dc) dielectric withstand voltage
    2. Very low frequency (VLF) dielectric withstand voltage
    3. Power frequency (50/60 Hz) dielectric withstand voltage
    4. Damped alternating current (DAC) voltage
    Baseline Diagnostic Testing
    1. Power factor/ dissipation factor (tan delta)
      • Power frequency (50/60 Hz)
      • Very low frequency (VLF)
    2. DC insulation resistance
    3. Partial discharge
      1. Online (50/60 Hz)
      2. Off line
        • Power Frequency (50/60 Hz)
        • Very low frequency (VLF)
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Bolt-torque levels should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
  3. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  4. The minimum bend radius to which insulated cables may be bent for permanent training shall be in accordance with Table 100.22.
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated.
  3. Shielding shall exhibit continuity.Investigate resistance values in excess of ten ohms per 1000 feet of cable.
  4. TDR graphical measurements should clearly identify the cable length and characteristic should be consistent with other phases.
  5. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the test, the test specimen is considered to have passed the test.
  6. Based on the test methodology chosen, refer to applicable standards or manufacturer’s literature for acceptable values.

NETA MTS (Maintenance)

7.3.3 Shielded Cables, Medium- and High-Voltage
A. Visual and Mechanical Inspection:
  1. Inspect exposed sections of cables for physical damage and evidence of overheating and corona.
  2. Inspect terminations and splices for physical damage, evidence of overheating, and corona.
  3. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.3.3.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torquewrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform a thermographic survey in accordance with Section 9.
  4. Inspect compression-applied connectors for correct cable match and indentation.
  5. Inspect shield grounding and cable support.
  6. Verify that visible cable bends meet or exceed ICEA and/or manufacturer’s minimum allowable bending radius.
  7. *Inspect fireproofing in common cable areas.
  8. If cables are terminated through window-type current transformers, inspect to verify that neutral and ground conductors are correctly placed and that shields are correctly terminated for operation of protective devices.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter in accordance with Section 7.3.3.A.3.1.
  2. Perform an insulation-resistance test individually on each conductor with all other conductors and shields grounded. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.
  3. Perform a shield-continuity test on each power cable by ohmmeter method.
    4. Due to the various cable testing methods commercially available, the following section does not denote “optional” or “required” tests. It is only after careful analysis of all circuit parameters between the testing entity and the cable owner that a preferred testing method should be selected.
  4. In accordance with ICEA, IEC, IEEE and other power cable consensus standards, testing can be performed by means of direct current, power frequency alternating current, very low frequency alternating current, or damped alternating current. These sources may be used to perform insulation withstand tests, and diagnostic tests such as partial discharge analysis, and power factor or dissipation factor. The selection can only be made after an evaluation of the available test methods, manufacturer’s published data, and a review of the installed cable system.
    5. Dielectric Withstand
    1. Direct current (DC) dielectric withstand voltage
    2. Very low frequency (VLF) dielectric withstand voltage
    3. Power frequency (50/60 Hz) dielectric withstand voltage
    Diagnostic Tests
    1. Power factor/dissipation factor (tan delta)
      1. Power frequency (50/60 Hz)
      2. Very low frequency (VLF)
      3. Damped-alternating current (20 to 500 Hz)
    2. DC insulation resistance
    3. Partial discharge/li>
      1. Online (50/60 Hz)
      2. Off line
        1. Power frequency (50/60 Hz)
        2. Very low frequency (VLF)
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value. (7.3.3.A.3.1)/li>
  2. Bolt-torque levels should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.3.3.A.3.2)
  3. Results of the thermographic survey shall be in accordance with Section 9.(optional)
  4. The minimum bend radius to which insulated cables may be bent for permanent training shall be in accordance with Table 100.22.
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Insulation-resistance values should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated.
  3. Shielding shall exhibit continuity. Investigate resistance values in excess of ten ohms per 1000 feet of cable.
  4. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the test, the test specimen is considered to have passed the test.
  5. Based on the test methodology chosen, refer to applicable standards or manufacturer’s literature for acceptable values.
NETA ATS / MTS
TABLE 100.5
Neta Table 100.5
NETA ATS / MTS
TABLE 100.12
Neta Table 100.5
Neta Table 100.5
NETA ATS / MTS
TABLE 100.22
Neta Table 100.22-1
Neta Table 100.22-2