Routine insulation measurement and leakage current

Why routine measurement of insulation is necessary

Insulation is the part of electrical equipment that ages fastest. In service it is attacked by heat, moisture, dirt, oil vapour, vibration, corona and voltage surges, and its resistance and strength fall slowly without any outward sign. Routine measurement is needed to:

• Detect deterioration early (moisture ingress, contamination, cracks, ageing) before it leads to breakdown.
• Prevent unplanned outages and costly damage to transformers, motors, cables and generators.
• Ensure safety of people against leakage currents and shocks.
• Build a trend record: comparison with earlier readings shows the rate of ageing, which is more useful than a single value.
• Plan maintenance (drying, cleaning, rewinding, replacement) and check equipment after repair or long shutdown before energizing.

Interpretation from leakage current

When a DC test voltage is applied, the total current has three components:

 I
 |\
 | \  capacitive charging (dies in seconds)
 |  \
 |   \__  absorption (decays over minutes)
 |      \_____
 |            ------- conduction/leakage (steady)
 +------------------------------------> t

Insulation resistance $R = V/I$. Results are read as follows:

1. Current versus time (constant voltage)

• Current falls steadily with time and settles to a low value: absorption dominates, insulation is clean and dry.
• Current stays nearly constant from the start: leakage dominates, insulation is wet or dirty.
• Current rises with time or jumps erratically: serious defect, possible imminent breakdown.

This is expressed as the polarization index $PI = R_{10\,min}/R_{1\,min} = I_{1\,min}/I_{10\,min}$ (and dielectric absorption ratio $DAR = R_{60\,s}/R_{30\,s}$):

2. Current versus voltage (step-voltage test)

• Leakage current rises linearly with voltage (constant $R$): insulation is healthy.
• Current rises faster than voltage, with a "knee" in the curve: weak spots, cracks, moisture or contamination; the test is stopped before breakdown.

3. Comparison

• Compare with previous readings, with phases of the same machine and with similar equipment; a sudden fall in $R$ (rise in leakage) is more significant than the absolute value.
• Correct readings to a reference temperature (insulation resistance roughly halves for every 10 °C rise), since leakage current increases with temperature and humidity.