Is the reading falling, even though it still passes?
One megohm reading says whether a circuit passes today. A series of readings says where it is heading. A slow decline can be caught and repaired long before the value reaches the minimum.
Insulation-resistance trend analysis compares the readings taken on the same circuit at each test over the years, instead of judging each one alone. A single reading only shows whether the circuit is above the minimum today. The trend shows whether the insulation is steady, slowly wearing out or has just been damaged, so work can be planned before the circuit fails or an earth fault grounds the loop.
1Start with a baseline
The first point on the chart is the reading taken at commissioning, when the cable and joints were new and dry. Record it with everything needed to repeat the test: the circuit and section, the test voltage and the time to reading, the tester and its calibration date, the temperature and the weather. Every later reading is judged against this baseline, so a circuit with no baseline can only be compared with the minimum.
2Make each reading comparable
A trend is only as good as the conditions behind it. A change in the method looks like a change in the cable. Keep these the same each time:
| Keep the same | Why it matters |
|---|---|
| Test voltage and time to reading | Insulation resistance rises with time under voltage, so a 10-second and a 1-minute reading are not comparable |
| Circuit section and test point | A different length of cable has a different resistance |
| Tester, in calibration | Instrument error would show up as a trend |
| Reference temperature | Resistance falls as the cable warms, so correct readings to one temperature |
| Weather and cable-end condition | Damp soil, wet joints and dirty terminations lower the reading without changing the cable |
3Correct for temperature
Insulation resistance falls as the cable gets warmer, so a summer reading is lower than a winter one on the same sound cable. Correct each reading to a reference temperature, often 20 °C, before it goes on the chart. A common rule of thumb for many insulation materials is that resistance roughly halves for every 10 °C of warming:
4Plot it and read the shape
Plot the corrected reading of every test against time, one line per circuit, on a log scale. Insulation resistance can span several powers of ten and tends to fall by a similar proportion each year, so a steady decline shows as a near-straight line that can be extended toward the minimum. Then read the shape:
| Shape | What it usually means | Response |
|---|---|---|
| Flat, small seasonal swing | Sound insulation; the swing is temperature and moisture | Keep the routine interval |
| Steady slow decline | Ageing, moisture ingress or gradual damage | Retest sooner, plan the repair before the alert level is reached |
| Sudden step down | A new fault: a damaged cable, a failed joint or water in a connector | Split the loop at the pits and find the section |
| Readings scatter up and down | The method is changing, or the cable ends are wet or dirty | Fix the method, clean and dry the ends, repeat |
5Set an alert level above the minimum
The minimum is the value below which the circuit is not acceptable, and it comes from the specification. Waiting for it is waiting for the failure, so set an alert level higher up that starts action early. A simple form is a fraction of the baseline:
The alert fraction and the minimum are set by the airport's maintenance plan and the specification, so the numbers above are only illustrations.
6What to do when the trend turns
A falling trend is a reason to test that circuit more often and to find where the loss is: isolate the circuit, split the loop at the pits and test each section, then use fault location methods if one section stands out. Treat a cable that is already trending down with care. A hi-pot test is a stress, so run it only where the specification calls for it, and do not use it to "see if it holds" on weak insulation. After a repair, take new readings and mark the change on the chart so the line is not misread later.
7Records and review
Keep the chart with the circuit's records and review it at every test, not only when a reading looks wrong. Record the measured value, the corrected value, the temperature, the conditions, the tester and the date. Continuous insulation monitoring and ground-fault detection on the CCR can add early warning between tests, but the periodic readings are what give a like-for-like history.
Frequently Asked Questions
Q1Why trend the readings if the circuit already passes?
A pass only says the reading is above the minimum today. Two circuits can both pass while one is steady and the other is falling every year. The trend shows which is which, so the falling circuit can be found and repaired while it is still working, instead of after it fails.
Q2Why do readings change with the seasons?
Insulation resistance falls as the cable warms and as moisture gets into joints, cable ends and the soil around the cable. So a circuit can read lower in a hot, wet season and recover in a cool, dry one. Recording the temperature and the weather, and correcting to a reference temperature, separates this normal swing from real deterioration.
Q3Why plot the readings on a log scale?
Insulation resistance is measured in megohms and can span several powers of ten. It also tends to fall by a similar proportion each year, not by a fixed amount. On a log scale that steady fall shows as a near-straight line, which is easy to extend toward the minimum, and a sound circuit shows as a flat line.
Q4How often should insulation resistance be measured?
The interval comes from the airport's maintenance plan and the specification. A flat trend can stay on the routine interval. When the trend is falling toward the alert level, test that circuit sooner, and keep every test under the same conditions so the new points can be compared with the old.
Q5What is the difference between a slow decline and a sudden drop?
A slow decline usually points to ageing, moisture ingress or gradual damage along the cable, and gives time to plan work. A sudden step down between two tests points to a new fault, such as a damaged cable, a failed joint or water in a connector. Split the loop at the pits to find which section it is in.
Q6Do readings need to be corrected for temperature?
Yes, if the readings are to be compared over time. A common rule of thumb is that insulation resistance roughly halves for each 10 degrees C rise, but the right correction depends on the cable type. Use the correction table in the specification or from the cable maker, and record both the measured and the corrected value.
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