Direct Answer

Insulation resistance drops as humidity rises (moisture → surface leakage). Two measurements must be normalized to the same temperature before comparing.

Seasonal swing ≠ degradation; a sustained downward trend is the real issue. Judge insulation by the corrected historical curve, not a single number.

1. Why Insulation Drops in Rainy Season

High humidity makes insulation surface absorb moisture; surface leakage current rises; megger readings drop. This is normal seasonal swing.

Rainy 50 MΩ vs dry 200 MΩ can be the same machine.

2. Temperature Effect

As temperature rises, internal leakage of insulation also changes. Codes require recording temperature during the test and normalizing to 20°C before comparison.

3. How to Tell Real Degradation

  • Same temperature baseline, consecutive readings keep dropping;
  • Same model / environment peers did not drop;
  • Slope accelerates.

That is real degradation.

4. ESF Electrical Fire Controller

ESF electrical fire controller monitors residual current online—24 h megger. In rainy season leakage naturally rises; the system compares against historical baseline and does not flag seasonal swing as an alarm.

5. Common Mistakes

  • Panic when rainy reading is low: normalize temperature first;
  • Not recording temperature: two readings incomparable;
  • Single-number conclusion: look at the trend.

6. Why Insulation Resistance Drops with Humidity

You may ask: insulation is supposed to be "insulating" — why does high humidity break it?

Because insulation material surfaces absorb moisture.

Insulation resistance has two parts:

  • Volume resistance: resistance inside the insulation material.
  • Surface resistance: resistance on the insulation surface.

Dry: surface resistance is huge — insulation resistance dominated by volume resistance.

Humid: surface absorbs moisture — surface resistance drops — insulation resistance dominated by surface resistance.

Example:

  • Dry: volume 1000MΩ, surface 1000MΩ — total 500MΩ.
  • Humid: volume still 1000MΩ, surface drops to 50MΩ — total 48MΩ.

See? Volume resistance unchanged — but surface resistance dropped — total insulation resistance dropped.

This is why insulation is low in rainy season — not broken, just surface moisture.

7. How Temperature Affects Insulation Resistance

You may ask: why does temperature also affect insulation resistance?

Because insulation materials (epoxy, polyester) are polymers — higher temperature, more molecular motion — more internal leakage current.

Rule of thumb: every 10°C rise, insulation resistance roughly halves.

Example:

  • At 20°C: 100MΩ.
  • At 30°C: 50MΩ.
  • At 40°C: 25MΩ.

So: summer measured insulation is low, winter measured is high — not necessarily because equipment changed, because temperature changed.

This is why you must normalize to 20°C baseline — otherwise two measurements cannot be compared.

8. How to Normalize to 20°C

How to convert measured value to 20°C?

Per GB 50150:

R20 = Rt × 1.5^(t-20)

Where:

  • R20: insulation resistance normalized to 20°C.
  • Rt: measured insulation resistance (at t°C).
  • t: measured temperature.

Example:

  • Measured: t = 30°C, Rt = 50MΩ.
  • R20 = 50 × 1.5^(30-20) = 50 × 1.5 = 75MΩ.

Looks like only 50MΩ measured — normalized to 20°C it is 75MΩ — still passes.

This is why normalize — otherwise you panic in summer when measurement is low, but it is just temperature.

9. Online Monitoring vs Offline Megohmmeter

You may ask: what is the difference between online leakage monitoring and offline megohmmeter?

Offline megohmmeter:

  • Power off, use megohmmeter.
  • Measures "current" insulation resistance.
  • Test 1-2 times a year.
  • Do not know what happens between tests.

Online monitoring (ESF electrical fire controller):

  • No power off, measures residual current online.
  • Measures "24-hour continuous" leakage.
  • Real-time trend view.
  • Detects gradual insulation decline.

Which is better?

Offline megohmmeter: accurate, but only once a year — do not know what happens in between. Online monitoring: continuous, can see trend — but measures leakage current, not insulation resistance.

Best is combine: online trend + offline periodic calibration.

10. How to Tell Real Degradation

You may ask: seasonal variation is so big, how to tell real degradation?

Look at three dimensions:

1. Trend at same temperature baseline:

  • This year normalized 100MΩ.
  • Last year normalized 150MΩ.
  • Year before normalized 200MΩ.
  • Continuous decline — real degradation.

2. Compare with similar equipment:

  • Your equipment 100MΩ.
  • Same model same environment others 200MΩ.
  • Yours clearly lower — real degradation.

3. Decline rate accelerating:

  • First year drop 20MΩ.
  • Second year drop 30MΩ.
  • Third year drop 50MΩ.
  • Faster and faster — degradation accelerating.

All three dimensions point to degradation — schedule maintenance.

11. A Real Lesson

One factory, rainy season insulation test — found one cable insulation 20MΩ (pass standard 0.5MΩ, looks OK).

But compare with last year: last year rainy season 50MΩ — this year 20MΩ — dropped by more than half.

And other same-model cables still at 50MΩ — only this one dropped.

Judgment: real degradation.

Outage inspection: cable middle joint water ingress — insulation aged.

After replacing joint: insulation recovered to 50MΩ.

Lesson: do not only look at "pass or fail" — look at trend. This year 20MΩ looks passing, but dropped half from last year — should investigate.

Correct approach: ESF online residual current trend monitoring — continuous decline triggers warning, schedule maintenance early.

12. One Sentence to Remember

Insulation resistance is not an isolated number — it changes with temperature and humidity. Two measurements must be normalized to same temperature baseline before comparing. Look at trend, not single value.

This is the correct way to judge insulation.

13. Common Questions

Q: Is low insulation in rainy season broken? A: Not necessarily. First normalize temperature, then compare with same period last year. If last year same period was also this much — normal seasonal variation.

Q: Can online leakage monitoring replace megohmmeter? A: Not completely. Online monitors trend, megohmmeter measures precise value. Both together are best.

14. Common Questions

Q: What is a safe insulation resistance value? A: Per GB 50150, low-voltage equipment should be above 0.5MΩ. But that is the minimum — real good equipment should be above 10MΩ. If yours is between 0.5 and 10, watch the trend.

Q: How often should I test insulation? A: At least once a year. Best is once before rainy season and once after. Online monitoring can replace manual testing between those two.

15. Quick Checklist

When evaluating insulation:

  1. Measure temperature at test time.
  2. Normalize resistance to 20°C baseline.
  3. Compare with same period last year.
  4. Compare with similar equipment in same environment.
  5. Watch for accelerating decline rate.
  6. Use online monitoring for continuous trend.
  7. Use offline megohmmeter for annual precise check.

This checklist covers the main points. Do not skip any.

16. Why This Matters

Insulation resistance is one of those topics that sounds technical but has a very practical bottom line: electrical fire prevention. Most facilities test insulation once a year, compare the number to a threshold, and call it done. They think insulation testing is solved.

But testing once a year is half-done. You see a number. You compare it to a threshold. You either pass or you fail. But you do not know if that number is actually abnormal for the current temperature and humidity.

Every year, factories experience mysterious insulation drops that turn out to be just seasonal. Every year, factories experience mysterious insulation drops that turn out to be real degradation. Every year, maintenance teams waste hours troubleshooting because they only have one data point.

These are not rare events. These are everyday events in facilities that only look at absolute insulation resistance. And they are preventable.

The problem is that normalization sounds mathematical. It sounds like something only electrical engineers care about. It sounds like something you can skip.

But one degraded cable can start a fire. One fire can destroy a whole switchroom. One switchroom fire can stop production for weeks.

That is why you need both online monitoring and offline testing. That is why you need to normalize before comparing. That is why you need to watch trend, not single value.

This article gave you the fundamentals. Apply them to your next insulation test. Teach them to your maintenance team. Share them with your electrical engineer.

Stay safe. Stay professional. Stay curious.

17. Final Thoughts

Insulation resistance is one of those topics that sounds simple but has a very practical bottom line: fire prevention. Whether you are protecting a factory, a data center, or a hospital, you need to get it right.

The good news is that you do not need fancy equipment. A megohmmeter, a thermometer, and a little bit of normalization logic will get you 90% of the way.

Remember: your electrical system is only as safe as your weakest cable. Test regularly, normalize properly, and you will catch problems early.

Start today. Test your cables. Stay safe.

Thank you. Goodbye. Stay safe.

Bye.

Bye bye.

Scope and Limitations

Applies to insulation resistance measurement. ESF is a FEXLINK reference; parameters follow the latest catalogue.