Page 86 - Basic PD Theory

References

Figure 63.
Types of currents for epoxy-mica
insulation with a relatively low
surface leakage current and no
conductance current.

Temperature
Resistance is directly related to winding temperature. For some insulation systems the resistance will double for every 10°C
decrease. The impact of temperature is a property of the insulation and can range from 5° to 20°C for the doubling effect.
Since temperature can have such an overwhelming effect, it is recommended that trend tests be conducted at the same winding
temperatures. If this is not possible, then multiple tests should be conducted at various temperatures to determine the
temperature correction value, K T. K T can also be approximated for insulation resistance halving for a 10°C rise in winding
temperature.

Voltage
Your choice of voltage for testing will have a significant impact on testing. Since the idea behind testing is to locate problems
before they become catastrophes it is recommended that you use the highest voltage in the range. Select a voltage that you are
comfortable with and then stick with it for every subsequent test. The test instrument must be capable of providing a well-
regulated constant voltage for the entire time-frame. It is not recommended that you use a DC hi-pot test set for IR testing,
since the output of it is usually not well-regulated. You are testing a large capacitor and any small changes in the applied voltage
will lead to significant changes in the current. In some situations, the AC power supplied may fluctuate during the test interval.
In these cases you should use an isolating transformer between the AC supply and the test instrument.
Guidelines for DC Voltages to be Applied During Insulation Resistance Test
Winding Rated * Voltage Insulation Resistance Test Voltage
<1,000 V 500V
1,000-2,500 V 500 - 1,000 V
2,501-5,000 V 1,000 - 2,500 V
5,001 - 12,000 V 2,500 - 5,000 V
>12,000 V 5,000 - 10,000 V
* Rated line-to-line voltage for 3 phase machines, line-to-ground voltage for single phase machines and rated DC voltage for dc
machines or field windings

Humidity
When a machine is allowed to cool below the dew point, it is extremely likely that condensation on the surface of the winding
will lead to low insulation resistance readings due to moisture Contamination. Therefore, it is highly recommended that ALL
tests be done above the dew point. If this is not possible, then caution should be observed during the interpretation process to
consider the effects of humidity on the test results.

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