1. PURPOSE
1.1 This document defines the guidelines for insulation resistance (megger) testing on both shielded and nonshielded power cables.
2. Scope
2.1 Insulation resistance (megger) testing shall be performed on power cables for 480 V service or higher in order to determine condition of the cable insulation.
3. Related Documents
3.1 Air Products Engineering Documents
4AEL-620300 Electrical Work
4. Equipment
4.1 The insulation resistance test set is a megohmmeter or a “megger.” Air Products requires all insulation resistance (megger) testing to be performed with a dc megohmmeter per paragraph 7.2.1 of 4AEL-620300. The test results of a megohmmeter give insulation resistance values measured in megohms (M).
4.1.1 For low-voltage power cables (rated 600 V or less), a 1,000 Vdc megohmmeter shall be used.
4.1.2 For high-voltage power cables (rated above 600 V), a 2,500 Vdc megohmmeter shall be used.
5. Applications
5.1 There are basically three versions of insulation resistance (megger) testing with a dc megohmmeter.
5.1.1 Basic acceptance test: This type of insulation resistance (megger) testing is usually performed on low-voltage power cables (rated 600 V or less), with a 1,000 Vdc megohmmeter. This test is performed by applying the dc test voltage until the insulation resistance value stabilizes (usually one to three minutes).
5.1.2 Dielectric Absorption Ratio (DAR) test: This type of insulation resistance (megger) testing is usually performed on either low-voltage power cables (rated 600 V or less), with a 1,000 Vdc megohmmeter, or high-voltage power cables (rated above 600 V), with a 2,500 Vdc megohmmeter. This test is performed by applying the dc test voltage for one (1) minute and
taking insulation resistance readings at both the 30-second time and the one-minute time. The DAR is defined as the one-minute insulation resistance value divided by the 30-second insulation resistance value.
5.1.3 Polarization Index (PI) test: This type of insulation resistance (megger) testing is usually performed on or high-voltage power cables (rated above 600 V), with a 2,500 Vdc megohmmeter. This test is performed by applying the dc test voltage for ten (10) minutes and taking insulation resistance readings at every one-minute interval. The Polarization Index (PI) is defined as the ten-minute insulation resistance value divided by the one-minute insulation resistance value.
5.2 Regardless of which version of insulation resistance (megger) testing is performed, when testing is complete, the stored energy within the cable must be discharged. Grounds shall be applied for a time period adequate to drain all insulation stored charge.
6. Recording Results
6.1 The Insulation Resistance Test Results form on page 24 of Specification 4AEL-620300 was developed specifically to record the results of insulation resistance (megger) testing.
7. Evaluating Results
7.1 There are many factors that influence insulation resistance (megger) test results, making it very difficult to give discrete values of “good” and “bad” power cables. Major factors that can affect the test results are: cable length, cable insulation thickness and type, cable preparation, weather conditions, and megger test set connections.
7.2 The Air Products basic “rule of thumb” is that the minimum acceptable insulation resistance (megger) test results are 1 M per 1 kV of cable voltage rating + 1M.
Example:
The minimum acceptable result for a 5 kV power cable would be 5 * (1 M) + 1 M = 6 M.
7.3 If using either the DAR test or the PI test, the following table shows industry standards of evaluating cable insulation based on the test results.
| Condition of Cable
Insulation |
60-Sec/30-Sec Ratio
Dielectric Absorption Ratio |
10-Min/1-Min Ratio
Polarization Index |
| Dangerous | Less Than 1.0 | Less Than 1.0 |
| Questionable | 1.0 to 1.24 | 1.0 to 2.0 |
| Good | 1.25 to 1.60 | 2.1 to 4.0 |
| Excellent | Above 1.61 | Above 4.0 |
7.4 Again, it must be stressed that evaluating the condition of power cable insulation is very subjective. The methods shown in this document are guidelines only. For additional help in test results evaluation, contact the power cable manufacturer.