Ground Rules by John Cadick: The Equipment is in Service - Now What? (Part 8)

Source: Cadick Corporation
The eight-step electrical preventive maintenance program

Insulation Testing – Part 2

By John Cadick, P.E., Cadick Corporation

Contents
Introduction
Measuring Insulation Resistance
Results Evaluation: Insulation Resistance and Polarization Index
Test Instruments
Care and Use of Test Instruments
Summary

Introduction (Back to top)
Last week we discussed the common ways that insulation is field-tested. This week we will look a little more deeply into the actual procedures and expected results of using DC methods. You may want to open or print a copy of last week's column for reference to the drawings and diagrams.

Measuring Insulation Resistance (Back to top)
Since Ic and Ida disappear in time, a true insulation resistance can be read by applying the DC and then simply waiting long enough for them to be gone. A minimum of one minute should be used to make any resistance or current reading. This approach is summarized below:

Measuring Insulation Resistance

  1. Apply the DC voltage (The test voltage is applied with either a megohmmeter or high potential test set. Both of these units are explained later in this column).
  2. Wait one minute for the charging and absorption current to disappear.
  3. Measure the remaining current (Il) or the insulation resistance.
The insulation resistance value is temperature sensitive and must be corrected to a common temperature value, usually 20 °C. The resulting temperature corrected result is then compared to previous readings and/or field standards.

Polarization Index
An even better way to evaluate insulation quality is to compare the insulation readings at different times. The ratio of insulation readings taken at ten minutes (R10) and one minute (R1) is called the "polarization index" or PI. To measure polarization index, do the following:

Measuring Polarization Index

  1. Apply the DC voltage (The test voltage is applied with either a megohmmeter or high potential test set. Both of these units are explained later in this column).
  2. Wait one minute and take an insulation reading.
  3. Wait nine more minutes (total of ten from start) and take another insulation reading. (R10)
  4. Calculate the polarization index from the following formula: PI=R10 / R1
The polarization index reading is very valuable since it is not affected by temperature. In many cases the polarization index is the only reading that is used. On smaller equipment a measurement similar to the polarization index called the "dielectric absorption ratio" is sometimes used. The dielectric absorption ratio is the one-minute to 30-second ratio instead of the ten minute to 1-minute ratio.

Results Evaluation: Insulation Resistance and Polarization Index (Back to top)
Insulation Resistance
Many "rules of thumb" have been developed over the years for evaluating insulation resistance readings. The most common one is given by the following formula: I. R.min = 1 megohm/kV + 1 megohm

For example: If you are measuring insulation resistance in a 5 kV circuit, the minimum insulation resistance reading should be (5 x 1) + 1 or six (6) megohms.

More rigorous standards are published by The InterNational Electrical Testing Association in their Maintenance Specifications Standard.

An even better way is to keep accurate records of your test results and compare them. Such trending is the heart of a good Condition Based Maintenance (CBM) program and will be discussed in depth in later columns.

Whatever standard you use, you should investigate any equipment whose insulation values fall below the standard. Locate the problem and correct it.

Polarization Index
The higher the polarization index, the better the insulation. The table to the right may be used to provide a general evaluation of equipment.

Note however that if a PI is greater than 4 by more than about 20%, you should investigate for the possibility of dry, brittle insulation.

Test Instruments (Back to top)
Both the insulation resistance and polarization index test can be performed with either of two different types of equipment – the megohm meter (MEGGER), and the High Potential Test set (HIPOT).

Megohm meters
A megohm meters is a device that produces DC test voltage, either with its own internal generator or electronically. The ones that have their own internal generator must be cranked either by hand or by an internal motor. The instrument measures the current produced by its voltage and, internally, converts it to resistance to be read by the operator. Cranking a hand generator for ten minutes (polarization index) can be a tough job so motor driven generators and electronic devices are used in most modern applications.

Megohm meters are arguably the most commonly used electrical instruments except for multimeters. The scale reads directly in megohms or ohms, and some very sensitive instruments read in teraohms Common voltages employed in the megohm meter are 500, 1000, 2500, 5000, and 10,000 volts. Both insulation resistance and polarization index may be performed using a megohm meter.

High Potential Test Sets (HIPOTS)
The high potential test set is similar to a megohm meter in many respects. It supplies a high DC voltage, usually from electronic means. The voltage capabilities are generally much higher ranging from 5000 Volts to as high as 500,000 volts. Because of the higher voltages, HIPOTS are usually metered with milliammeters and microammeters. Thus the polarization index calculations are made by dividing the one minute reading by the ten minute reading.

CAUTION:

    Because of the capacitive and dielectric absorption charges, insulation will hold a DC charge for a long time after the test set is removed. When using a DC HIPOT, you must apply a ground wire to the insulation system for at least the same amount of time as the test voltage was applied. Generally a ½ hour minimum is recommended.
HIPOTS are also used to proof-test insulation. To do this a very high voltage (up to three times the insulation rating) is applied in steps. Current is monitored and recorded at each voltage step to identify when and if the insulation starts to fail. A graph is made of leakage current vs. voltage and kept for reference.

Care and Use of Test Instruments (Back to top)

  1. All instruements with analog meters should be level when read.
  2. Needle type meters should be read directly above the meter. Some have mirrors behind the needle path to help in this.
  3. All instruments should be handled and stored with care.
  4. Any instrument should be calibrated against a correct meter at least once a year.
Summary (Back to top)
The last two columns have introduced you to the concept of insulation testing. We have discussed both AC and DC test equipment, and briefly described the methods, analysis, weaknesses, and cautions of each type. In later columns I will discuss the actual application of these tests to various types of test equipment. Circuit breakers, switchgear, transformers, and a variety of other equipment will be discussed.

A registered professional engineer, John Cadick has specialized for three decades in electrical engineering, training, and management. In 1986 he created Cadick Professional Services (forerunner to the present-day Cadick Corporation), a consulting firm in Garland, Texas. His firm specializes in electrical engineering and training, working extensively in the areas of power system design and engineering studies, condition based maintenance programs, and electrical safety. Prior to the creation of Cadick Corporation, John held a number of technical and managerial positions with electric utilities, electrical testing firms, and consulting firms. Mr. Cadick is a widely published author of numerous articles and technical papers. He is the author of the Electrical Safety Handbook as well as Cables and Wiring. His expertise in electrical engineering as well as electrical maintenance and testing coupled with his extensive experience in the electrical power industry makes Mr. Cadick a highly respected and sought after consultant in the industry. (Back to top)