Experience indicates there are two main causes for insulation failure at high frequency: thermal and corona. Both of these are influenced by the higher frequency of the test waveform and the effects may not be well appreciated for the test engineer more familiar with mains frequency testing.
In addition to high voltage, electrosurgery devices also operate at a relatively high frequency of around 400kHz. At this frequency, surprisingly high currents can flow in the insulation - roughly 8,000 times higher than those at mains frequency. For example, a 50 cm length of cable immersed in saline solution tested at 1000Vrms/400kHz can easily have over 200mA flowing through the insulation, creating a sizeable 200VA load.
Although this VA load is predominately reactive or apparent power (var), insulators are not perfect and a portion will appear as real power in watts. This is determined by the dissipation factor (δ) of the material. Some materials like PVC have a high factor (δ around 0.01-0.03), which means that 1-3% of the total VA load will appear as heat. In the example above, if the test sample was PVC insulation with δ = 0.02, it means the test would create 4W of heat (200VA x 0.02). That heat can be enough to melt the insulation, causing dielectric breakdown.
The theory is discussed in more detail in this article from the original MEDTEQ website. As the article indicates, key factors are:
the rms test voltage, with heat is proportional to Vrms squared
thickness of the insulation, with heat inversely proportional to thickness squared
material dissipation factor
the test set up (availability of heatsinks such as liquids, electrodes)
While it is obvious the authors of IEC 60601-2-2 are aware of the thermal effects, the test in the standard seems to be poorly designed considering these factors. First, the standard concentrates on peak voltage and has a fairly weak control of the rms voltage. Secondly it is expected that thickness is not constant, so testing multiple samples may make sense, particularly for thin insulation. And thirdly the heatsink effect of the set up should be carefully considered.
In critical situations, good manufacturers will opt for low dissipation factor materials such as Teflon, which has δ ~ 0.001 or 0.1%. This ensures safety in spite of the weakness in the standard. Even so, thin insulation in the order of 100µm can still get hot - keeping in mind heat density is a inverse function of thickness squared (1/t²), which means that half thickness is four times as hot.
Conversely, very thick insulation can be fine even with high dissipation factors. Thick PVC insulation on a wiring to an active electrode can often be an acceptable solution, and concerns over the test set up need not be considered.
A test developed by MEDTEQ is to enclose the sample in 50µm copper foil (commonly available) with a thermocouple embedded in the foil and connected to a battery operated digital thermometer. The foil is connected to the negative electrode (prevents any damage to the thermometer), with the active electrode connected to the high voltage. During the test the thermometer may not read accurately due to the high frequency noise. However, immediately after the test voltage is removed, the thermocouple will indicate if any significant heating occurred. For 300V PVC insulation tested at 1000Vrms/400kHz, this test routinely creates temperatures in the order of 80-100°C, demonstrating that the material is not suitable at high frequency. A similar test can be done by coating the foil in black material, and monitoring with an IR camera.
This test has been useful in discriminating between good and bad materials at high frequency. It is a common rookie mistake for those seeking to break in to the active electrode market to reach for materials with high dielectric strength but also high dissipation factors.
The potential for high temperatures also raises a separate point overlooked by the standard: the potential to burn the patient. It may be that real world situations may mitigate the risk, but it would seem theoretically possible that insulation could pass the test while still reaching temperatures far above those which could burn the patient. This again supports a leaning towards low dissipation materials, verified to have low temperatures at high frequency.
Corona is caused when the local electric field exceeds that necessary to break the oxygen bonds in air, around 3kV/mm. In most tests, the electric field is not evenly distributed and there will be areas of much higher fields typically close to one or both electrodes. For example, electrodes with 2kV separated by 3mm have an average field of just 666V/mm, well below that needed to cause corona. However if one of the electrodes is a sharp point, the voltage drop occurs mostly around that electrode causing gradients around the tip above 3kV//mm. This creates a visible corona around that electrode, typically in the form of a purple glow, and ozone as a by product.
In the context of dielectric strength, corona is not normally considered a failure. Standards including IEC 60601-2-2 indicate that it can be ignored. This makes sense for normal electrical safety testing: firstly, corona is not actually a full breakdown, it is a local effect and an arc bridging both electrodes rarely occurs. Secondly, dielectric strength is intended to test solid insulation, not the air. In particular most dielectric strength tests are many times larger than the rated voltage (e.g. 4000Vrms for 2MOPP @ 240V in IEC 60601-1). This high ratio between the rating/test is not a safety factor but instead an ageing test for solid insulation. In that context, corona is genuinely a side effect, not something that is representative of effects that can be expected at rated voltage.
Unfortunately this is not true for high frequency insulation. Firstly, the test is not an ageing test which means the ratio between the test voltage and rating is much closer, just 20%. That means that corona could also occur at real voltages used in clinical situations. Secondly, corona can damage the surface of the insulation, literally burning the top layer. In many applications the active electrode insulation needs to be thin, such as catheter or endoscopic applications. If for example the insulation is only 100µm thick, and corona burns off 50µm, the dielectric strength or thermal limits of the remaining material can be exceeded, leading to complete breakdown. Analysis and experience also indicates that the onset of corona for thin insulation is quite low. Finally, there is reference in literature and anecdotal evidence that the onset of corona is lower at high frequency by as much as 30% (i.e. ~2kV/mm).
From experience, corona is the most common cause of breakdown in actual tests for thin insulation. But the results are not consistent: one sample may survive 30s of corona, while another breaks down. The onset of corona is also dependent on external factors such as temperature and humidity, and the shape of the electrodes. In tests with saline soaked cloth, corona occurs around the edges and boils off the liquid, leading to variable results.
Practical experience suggests that the wire wrap test is the most repeatable. This creates corona at fairly consistent voltage, and consistent visible damage to the surface of the insulation. Breakdown is still variable suggesting that a number of samples are required (e.g. 10 samples). Temperature and humidity should be controlled.
Currently IEC 60601-2-2 states that corona can be ignored, allows tests on a single sample and uses the wire wrap test for certain situations only. In the interests of safety, manufacturers are recommended to consider using the wire wrap test for all active insulation, and to test multiple samples, or consider regular samples from production.