BEFORE MEASURING

In order to obtain meaningful insulation resistance measurements, technicians should carefully examine the system being tested. The best results are achieved when:

• The system or equipment is taken out of service and disconnected from all other circuits, switches, capacitors, brushes, lightning arrestors, and circuit breakers.
• The temperature of the conductor is above the dewpoint of the ambient air.
• The surface of the conductor is free of carbon and other foreign matter that can become conductive in humid conditions.
• Test voltage will not exceed the equipment manufacturer’s maximum test voltage.
• The system under test has been completely discharged to ground.
• The effect of temperature is considered. It is recommended that tests be performed at a standard conductor temperature of 20 degrees C (68 degrees F). To measure the conductor temperature, use a noncontact infrared thermometer (such as the Fluke 65).

Megohmmeters that have automatic voltage measurement capability can warn you when a conductor is unexpectedly live. An auto-discharge feature helps technicians avoid painful shocks from the voltage stored on equipment capacitance. Agency approvals from UL or others, and an IEC CAT rating of 600 V CAT III or better, will ensure that the instrument will stand up in a tough electric environment.

INSULATION RESISTANCE AND LEAKAGE CURRENTS

During the testing procedure, high dc voltage generated by pressing the test button will cause a small (microamps) current to flow through the conductor and the insulation.

For a fixed voltage, the higher the current, the lower the resistance (E = IR, R = E/I). The total resistance is the sum of the internal resistance of the conductor (small value) plus the insulation resistance in MWs. The value of insulation resistance read on the meter will be a function of following three independent subcurrents.

Conductive leakage current (IL) is a small (microamp) amount of current that normally flows through insulation, between conductors, or from a conductor to ground. This current increases as insulation deteriorates and becomes predominant after the absorption current vanishes.

Capacitive charging leakage current (IC) occurs when two or more conductors are run together and act as a capacitor. This flows as dc voltage is applied and drops out after the insulation has been charged to its full test voltage.

Polarization absorption leakage current (IA) is caused by the polarization of molecules within dielectric material. In most equipment, the capacitive current and polarization current are relatively high and decline over time. This will show up as a low initial megohm reading that steadily increases over time.

WHEN AND HOW TO TEST

When testing the insulation resistance in generators, transformers, motors, and wiring installations, you can employ any of the following predictive maintenance tests. Whether you choose the spot-reading, step-voltage, or time-resistance tests depends on the unit under test and the reason for testing.

Proof testing ensures proper installation and integrity of conductors. The proof test is a simple, quick test used to indicate the instantaneous condition of insulation. It is commonly used as part of the “commissioning” process for a new installation. It provides no diagnostic data and is not very useful for predictive maintenance.

A proof test is performed with a single voltage, usually between 500 and 5,000 V, for about 60 sec. It is common to stress the insulation above normal working voltages in order to detect subtle weaknesses in the insulation.

Predictive maintenance tests can provide important information about the present and future state of conductors, generators, transformers, and motors. Examining the collected data will aid in scheduling diagnostic and repair work, which can help reduce downtime from unexpected failures.

During spot-reading/short-time resistance tests, the megohmmeter is connected directly across the equipment being tested and a test voltage is applied for about 60 sec. In order to reach a stable insulation reading in that time, the test should only be performed on low-capacitance equipment.

The step-voltage test involves resistance testing at various voltage settings. In this test, you apply each test voltage for the same period of time (usually 60 sec), graphing the recorded insulation resistance. By applying increasing voltages in steps, the insulation is exposed to increased electrical stress that can reveal information about flaws in the insulation such as pinholes, physical damage, or brittleness.

The dielectric-absorption/time-resistance test compares the absorption characteristics of contaminated insulation with the absorption characteristics of good insulation. The test voltage is applied over a 10-min period, with the data recorded every 10 sec for the first minute and then every minute thereafter. The interpretation of the slope of the plotted graph determines the condition of the insulation.

The polarization index (PI) test is another method for determining the quality of insulation. It is particularly valuable for uncovering moisture and oil ingress, which have a flattening effect on the PI curve, causing current leakage and eventually shorted windings.

The polarization index is the ratio of two time-resistance readings: one is taken after 1 min and the other is taken after 10 min. With good insulation, the insulation resistance will start low and get higher as the capacitive leakage current and absorption current get smaller. Results are obtained by dividing the 10-min test value by the 1-min value. A low polarization index usually indicates problems with the insulation.

Testing insulation in electrical systems is a critical step in providing safety and longer system life.

Pereles is with Fluke Corp. For more detailed information, visit www.fluke.com (website) to download the Insulation Resistance Testing application note.

Publication date: 03/11/2002