A comprehensive 64 page guide to electrical insulation testing containing helpful diagrams, value tables, product information and much more!. over a period of time, and the location and importance of the equipment megohmmeter is connected to the insulation to be tested and is operated for a short. The Dielectric Absorption (or Time-Resistance) test is an extension of the Megger, “A stitch in time – The Complete Guide to Electrical Insulation Testing”, a free.

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Or, what may be more serious, current from an energized line may leak into the apparatus and cause inconsistent readings, particularly if the live line is DC.

Using a Digital Multimeter 4. Understanding Insulation Resistance Testing Why have an insulation testing program? If a kilohm measurement is obtained, insulation has been thoroughly saturated, but may be reclaimable.

A Stitch In Time

Finally, it rises to a high value, as room temperature 20 C is reached. Before proceeding with tests, read the article on Preparation of Apparatus for Test, page Pay particular attention to conductors aa lead away from the circuit being tested and make sure they have been properly disconnected from any source of voltage. Equipment AC Rating DC Test Voltage up to volts and volts to volts and 1, volts 2, volts 1, to 2, volts or higher 4, volts and above 1, to 5, volts, or higher Test voltages used for proof testing of equipment are considerably higher than those used for routine maintenance.

How to Apply the Temperature Correction Factor Any increase of leakage current with time is a warning of trouble, as discussed in the tests described in the following section.

A Stitch In Time | Megger Insights

Any persistent downward trend is usually fair warning of trouble ahead, even though the readings may be higher than the suggested minimum safe values. When using a motor-driven or rectifier-operated instrument, the time intervals are usually 1 minute and 10 minutes from the time the testing voltage is applied. Instruction Manual Option Card for Frequency Divider Deliver this instruction manual without fail to those who actually operate the equipment.


Never connect a Megger insulation tester to energized lines or equipment. The test More information.

If you have a line-operated Megger instrument, you ll get best results by running the test 10 minutes, taking readings at 1- and at minutes, to get the polarization index.

JP Power Technology Inc. The time required to charge the geometric capacitance is very short usually no more than a few seconds and that which causes further delay in reaching full charge is a dielectric absorption effect. As explained in the Safety Precautions section, however, insulation testers should never be connected to energized lines or equipment when operated in any of the Insulation Test or Resistance Test modes.

Likewise the armature can be tested by itself. As the size of your equipment increases, there are also marked economic advantages in DC over AC testing.

In some cases, or with certain equipment, drying out may not be necessary. In summary, here are some general observations about how you can interpret periodic insulation resistance tests, and what you should do with the result: With good insulation, the conduction or leakage current should build up to a steady value that is constant for the applied voltage, as shown in Fig.

Safety Operation More information.

The Complete Guide to Electrical Insulation Testing A STITCH IN TIME 1 – PDF

The trend of the curve may be more significant than the numerical values themselves. The One-Megohm Rule For many years one megohm has been widely used as a fair allowable lower limit for insulation resistance of ordinary industrial electrical equipment rated up to volts, and is still recommended for those who may not be too familiar with insulation resistance testing practices, or who may not wish to approach the problem from a more technical point of view.


In this Investigation you will build circuits More information. November For service and information contact: Figure 6 Typical curve of insulation resistance in megohms with time for the short time or spot-reading test method.

Disconnect from other equipment and circuits, including neutral and protective workmen s temporary ground connections.

After a clean-up, a spot reading with the Megger tester shows 1. The curves of Fig. Time-Resistance Method When using a hand-cranked instrument, operate continuously for 1 minute. Preparation of Apparatus to test 1. If neutral or other ground connections have to be disconnected, make sure they are not carrying current at the time, and that when disconnected no other equipment will lack necessary protection. However, if you ve got salt water, you must first wash away the salt with fresh water.

Figure 9 Typical card plot of a time-resistance or double-reading test. As pin stithc or cracks develop, moisture and foreign matter penetrate the surfaces of the insulation, providing a low resistance path for leakage current.

Insulation resistance should be approximately one megohm for each 1, volts of operating voltage, with a minimum value of one megohm.

The rule may be stated: AC Generators Figure 20 With this connection, the insulation resistance will be that of the generator stator winding and connecting cable combined.

Take care in this respect. What was good insulation has become a partial conductor.