IEC 60601-2-2 Dielectric heating

This article has been transferred from the original MEDTEQ website with minor editorial update.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The mechanism of breakdown at high frequency is different to normal mains frequency dielectric strength tests - it can be thermal, rather than basic ripping apart of electrons from atoms. Also, for mains frequency tests, there tends to be a large margin between the test requirement and what the insulation can really handle, meaning that errors in the test method are not always critical. In contrast, the margin for HF insulation can be slight, and the test method can greatly affect the test result.

HF burns, in part due to insulation failure, continue to be a major area of litigation. Of particular concern is the high fatality rate associated with unintentional internal burns which may go unnoticed.

For those involved in designing or testing HF insulation, it is absolutely critical to have a good understanding of the theory behind HF insulation and what causes breakdown. This article looks into the detail of one of those mechanisms: thermal effects. 


Theory

All insulating materials behave like capacitors. With an ac voltage applied, some current will flow. At 230V 50/60Hz, this amount of current is very small, in the order of 20uA between conductors in a 2m length of mains cable. But at 300-400kHz, the current is nearly 10,000 times higher, easily reaching in the order of 10mA at 500Vrms, for just short 10cm of cable.  

All insulating materials will heat up due to the ac electric field. This is called dielectric heating or dipole heating. One way to think of this is to consider the heating to be due to the friction of molecules moving in the electric field. Microwave ovens use this property to heat food, and dielectric heating is used also in industrial applications such as welding plastics. These applications make use of high frequency, usually in the MHz or GHz range.

At 50-60Hz the amount of heating is so small it amounts to a fraction of a fraction of a degree. But again at 300-400kHz the amount of heating can be enough to melt the insulation.

The temperature rise caused by dielectric heating can be estimated from:

dT = 2π V2 f ε0εr d t / H D d2      (K or °C)

Although this is a rather complicated looking formulae, it is mostly made up of material specific parameters that can be found with some research (more details on this are provided below). To get some feel for what this means, let's put this in a table where voltage and thickness are varied, for a frequency of 400kHz, showing two common materials, PVC and Teflon:

Predicted insulation temperature rise @ 400kHz

 

Voltage PVC Insulation Thickness (mm)
(Vrms) 1 0.8 0.6 0.4 0.2
  Temperature rise (K)
200 0.7 1.1 1.9 4.3 17.3
400 2.8 4.3 7.7 17.3 69.3
600 6.2 9.7 17.3 39.0 156.0
800 11.1 17.3 30.8 69.3 277.3
1000 17.3 27.1 48.1 108.3 433.3
1200 25.0 39.0 69.3 156.0 623.9

 

 Table #1: Because of a high dissipation factor (d = 0.016), PVC can melt at thicknesses commonly found in insulation. 
For broad HF surgical applications, a thickness of at least 0.8mm is recommended 

 

Voltage Teflon Insulation Thickness (mm)
(Vrms) 0.5 0.3 0.1 0.05 0.03
  Temperature rise (K)
200 0.0 0.1 0.5 2.1 5.9
400 0.1 0.2 2.1 8.5 23.7
600 0.2 0.5 4.8 19.2 53.4
800 0.3 0.9 8.5 34.2 94.9
1000 0.5 1.5 13.3 53.4 148.3
1200 0.8 2.1 19.2 76.9 213.5

 

 Table #2: Teflon has a far lower dissipation factor (less than 0.0002), so even 0.1mm is enough for broad HF surgical applications. 
However, because of Teflon's superior qualities and high cost, insulation thickness is often reduced to around the 0.1mm region

For PVC insulation, these predicted values match well with experimental tests, where a small gauge thermocouple was used as the negative electrode and the temperature monitored during and after the test. For insulation with thickness varying between 0.3mm and 0.5mm, temperatures of over 80°C were recorded at voltages of 900Vrms 300kHz, and increasing the voltage to 1100Vrms resulted in complete breakdown.


Practical testing

As the formulae indicates, the temperature rise is a function of voltage squared, and an inverse function of thickness squared. This means for example, if the voltage is doubled, or the thickness is halved, the temperature rise quadruples. Even smaller variations of 10-20% can have a big impact on the test result due the squared relation.

Because insulation thickness varies considerably in normal wiring, it is possible that one sample may pass while another may not. Although IEC 60601-2-2 and IEC 60601-2-18 do not require multiple samples to be tested, good design practice would dictate enough samples to provide confidence, which in turn depends on the margin. For example, if your rated voltage is only 400Vrms, and your thickness is 0.8+/- 0.2mm, then high margin means the test is only a formality. On the other hand, if your rating is 1200Vrms, and the thickess if 0.8+/-0.2mm, perhaps 10 samples would be reasonable.

Test labs need to take care that the applied voltage is accurate and stable, which is not an easy task. Most testing is performed using HF surgical equipment as the source, however, these often do not have a stable output. Also, the measurement of voltage at HF is an area not well understood. In general, passive HV probes (such as 1000:1 probes) should not be used, since at 400kHz these probes operate in a capacitive region in which calibration is no longer valid (see here for more discussion) and large errors are common. Specially selected active probes or custom made dividers which have been validated at 400kHz (or the frequency of interest) are recommended.   

Perhaps the biggest impact to the test result is heat sinking. The above formulae for temperature rise assumes that all the heat produced cannot escape. However, the test methods described in IEC 60601-2-2 and IEC 60601-2-18 do not require the test sample to be thermally insulated. This means, some or most of the heat will be drawn away by the metal conductors on either side of the insulation, by normal convection cooling if the sample is tested in an open environment, or by the liquid if the sample immersed in fluid or wrapped in a saline soaked cloth.  

This heat sinking varies greatly with the test set up. The test in IEC 60601-2-2 (wire wrap test) is perhaps the most severe, but even something as simple as the test orientation (horizontal or vertical) is enough to substantially affect the test result.

Because of these three factors (variations in insulation thickness, applied voltage, heatsinking) bench testing of HF insulation should only be relied on as a back up to design calculations. Test labs should ask the manufacturer for the material properties, and then make a calculation whether the material is thermally stable at the rated voltage and frequency. 

The above formulea is again repeated here, and the following table provides more details on the parameters needed to estimate temperature rise. The temperature rise should be combined with ambient (maybe 35°C for the human body) and then compared to the insulation's temperature limit.

 

dT = 2π V2 f ε0εr d t / H D d2      (K or °C)

 

 

Symbol  Parameter Units Typical value Notes
V Test voltage Vrms 600 - 1200Vrms

Depends on rating and test standard. Note that ratings with high peak or peak to peak values may still have moderate rms voltages. Under IEC 60601-2-2, a rating of 6000Vp would require a test with 1200Vrms. 
 

f Test frequency  Hz 300-400kHz Depends on rating. Monopolar HF surgical equipment is usually less than 400kHz1 
ε0 Free space permittivity  F/m 8.85 x 10-12 Constant
εr Relative permittivity  unit less ~2 Does not vary much with materials
δ Dissipation factor unit less 0.0001 ~ 0.02 Most important factor, varies greatly with material. Use the 1MHz figures (not 1kHz)
t Test time s 30s IEC 60601-2-2 and IEC 60601-2-18 both specify 30s
H Specific heat J/gK 0.8 ~ 1 Does not vary much with materials
D Density  g/cm3 1.4 ~ 2 Does not vary much with materials
d Insulation thickness mm 0.1 ~ 1 Based on material specification. Use minimum value

 

 1Dielectric heating also occurs in bipolar applications, but due to the significantly lower voltage, the effect is much less significant.   

IEC 60601-2-2 201.8.8.3 High Frequency Dielectric Strength

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. 

Thermal

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 

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.