Pre-Conditioning Leakage Current

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Roland chung

Trusted Information Resource
#12
Sorry Peter, it is difficult to get your points in this topic.

It is also worth to note that most EMC capacitors have a negative temperature co-efficient. In some cases the leakage can be up to 20% higher in cold condition due to this effect. It has nothing to do with the equipment absorbing moisture. So even if you don't run the humidity treatment, make sure to measure the leakage in cold condition.
I agree with the leakage in cold condition is higher than leakage in warm-up state. However, the same situation also exist in leakage after humidity treatment. We are talking about the leakage before and after humidity BUT not different states (i.e. cold AND warm-up) in same period (before humidity OR after humidity).

Again, the increase in leakage current after humidity conditioning is most likely due to the negative temperature co-efficient (NTC) in EMC capacitors.
I really want to know what is the relationship between humidity treatment and cold condition/warm-up you mentioned.

Again, the temperature in humidity chamber is same as ambient temperature (cold condition) in order to avoid condensation. Therefore, the temperature in humidity chamber never influence EMC capacitor unless you switch on the equipment when perform the humidity test so cause the equipment to warm up.

The fact is that the leakage current after humidity is 10% higher than leakage current before humidity. I believe it is because of moisture.
 

Peter Selvey

Staff member
Super Moderator
#13
Just for reference, the standard requires two measurements, one at normal operating temperature/normal humidity, and one cold condition/93% RH. The IECEE CB scheme protocol uses the terms "before" and "after" to distinguish these two measurements. So, when looking at the numbers in a test report (or talking to an IEC test engineer), "before" and "after" means "hot" and "cold". If the leakage is primarily due to EMC capacitors, the "after" readings will normally be higher than "before" due to the NTC parameter.

But you may be referring to "before" to mean a special measurement, not required by the standard, in cold condition before placing the device in the humidity chamber (i.e. one taken for reference purpose only).

In this case I would be suspicious if there is any increase due to the basic physics. Water itself is not a very good conductor, rather it aids conduction by being a good supporter of ions in solution (and the ions do the conducting). Most insulating materials only absorb trace amounts of water and do not release any ions into solution.

However, as mentioned before, some insulating materials such paper (used in some larger transformers) and powder (used in high power heaters) do become highly conductive in the presence of water. These are protected by sealing, and the humidity test is important to verify the effectiveness of sealing. If the sealing is not effective, the presence of water will normally cause catastrophic breakdown, not a gentle increase in leakage.

If leakage currents do really vary by 10% due to moisture, then more investigation seems reasonable, as it suggests some material is not only hygroscopic but also ions are becoming free, which is significant.

Consider repeating the test at 40C (as there is more than 2 times available water than at 23C) and test only neutral open condition which would also ensure repeatability as there is no power to the device (no heating) and have the highest reading to minimize set up errors. You could also test each isolating component separately and see which is causing the increase.
 
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