Insulation requirements of HV pulse circuits

william guo

Starting to get Involved
#1
Hello everybody,
The device is a Shock wave balloon catheter system which is intended to treat calcified artery tissue in vivo, and classified as type CF applied part. The balloon is inflatable with a liquid and contains miniature electrodes which are connected to external HV pulse generator.
The HV pulse has characteristics: peak voltage: 7000VDC; RMS: about 10V; single pulse width 2us; pulse interval: 1s; 10 pulses per trigger.
Is table 6 of IEC 60601-1 dielectric strength test voltage based on peak working voltage appropriate for this kind circuits? Because HV pulses are not continuous.
Note 3 of table 12 “It is recognised that the values in this table do not take into account waveforms with low r.m.s. and high PEAK.”, which working voltage (peak or rms) shall be used for determination CL and CR? CL and CR will be huge different for RMS and Peak working voltage.
Thanks
 
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Peter Selvey

Leader
Super Moderator
#2
This is a special case and if required, should be handled under alternative solutions (Clause 4.5, I think).

As a reference, IEC 60601-2-2 (ESUs) can be used which also deals with high voltage and high frequency. This allows testing at +20% for 30s at high frequency followed by a test with the peak the same as normal +1kV at either mains frequency or dc. The 20% itself is fairly weak as a safety factor so the main safety factor is in the test duration which being 30s is several times longer than normal ESU activations of only a few seconds. The low frequency test at 1kV is mainly intended to detect if the high frequency test caused any insulation damage, so it is important to do this sequentially (after the HF test).

So, using something similar in your case it would be the same 2us pulses adjust the system to produce 8.4kV (+20%) with say 60 pulses instead of 10, followed by either 8000Vdc or 5660Vrms for the low frequency test. Obviously the parts should have undergone any sterilisation and other processing prior to the test as well.

CL and CR should be again special justified cases.

Normally, CL is mostly based on overvoltage transients in the supply which are usually >> working voltage hence the need for some big spacing even for relatively low voltages. In your case the 7000V is likely to be >> supply transients, so you will be designing for the 7000V basically plus a safety margin. For example design for 10kV, do some research for the true CL not based on transients. IEC 60664-1 might be a good reference.

CR is based on long term stress (think 240V for 10 years, continuously energised), and is not relevant in this case due to relatively short time use. Even more so if the actual catheter might is single use. Basically no CR limit is OK.

A side point to note is that oscilloscopes and high voltage probes are unspecified for high frequency and can have errors in excess of 20% even if labelled as "calibrated". Take care to make sure the probe compensation is adjusted as best as possible (the 1kHz square wave adjustment). But even then, peak measurements may have errors of 5-10%. Most scopes are only 8 bit resolution which is pretty lame if you think about it, and can lead to an uncertainty of around 5% just due to lack of resolution. HV probes have non-flat frequency response in the pass band, because it's impossible to have a single divider that works from dc to 100MHz. The actual implementation is a patch work of compensation systems with an undeclared specification of ±5% and typically ±3% as long as the compensation is adjusted correctly. Anyway, calibration at dc or 50/60Hz has nothing to do with the measurement system for a 2us pulse. I raise this as it will feed into decisions for testing (e.g. CL), and can lead to conflicts with test labs (especially as the lab might have errors of 20% in their probe/scope system), or just internally between the tests by design and QA departments, or even chasing your tail in design, one day you measure 7.2kV, and the next it's 6.1kV, freak out about instability, but it turns out to be just because you used a different probe or channel in the scope, nothing to do with the design. You might guess I have been there, done that!
 

william guo

Starting to get Involved
#3
Hi Peter,
thanks very much for your detailed inputs. The frequency of HV pulses is not as high as ESU - pulse width 2us; pulse interval: 1s, about 1Hz<<1kHz. Therefore, rationale of ESU insulation requirements may not be appropriate. Our team member propose use IEC 60601-2-4 as reference, however, requirements of IEC 60601-2-4 are based on energy and insulation resistance.

Can we use IEC 60664-1 as reference to set insulation requirements between HV circuits and earth/enlcosure/other secondary circuits, with some safety facors such as 1.2 applied for MOPP and addressed these in cl4.5 of IEC 60601-1? BTW, The plastic parts of balloon catheter cannot be designed as insulation for protection against electrical risk, but only for function purpose because of compromise to the intended clinical use.
The challange is how the evidence will be provided to the reviewer/auditor that these MOPs are reasonable.
 

Peter Selvey

Leader
Super Moderator
#4
I would consider the output "high frequency" due to the 2us pulse width, or more specifically the high rise/fall time for such a pulse. It is important to test using the same waveform as there are stresses at high frequency that do not occur at lower frequencies.

If you consider for example just the insulation in the wires to the transducer, assume these will have about 200pF due to the long length (ball park). Also assume the pulse has a rise/fall time of 200ns (0.2us). Then based on Q = It = VC, during the rising edge the current in the insulation will be I = VC/t = 7200*200p/200n = 7.2A. That's an awful lot of current and will cause the insulation to heat up, and is one of the cause of insulation failure at higher frequencies (dielectric heating). The same current will flow on the falling edge, so in normal use there will be 20 of the 7.2A pulses that the insulation has to withstand. This does not occur when testing at dc or mains frequency.

Corona (ionisation of air) is more intense at higher frequency as well, and will almost certainly be occurring at 7.2kV/200ns rise. Corona causes heating and insulation damage.

It is agreed that the low pulse repetition rate means the HF stresses are likely to be less than ESU, but the HF stresses will still occur. I am not suggesting to use IEC 60601-2-2 waveforms. I'm just suggesting to use the same waveform as your device normally produces, just bump up the voltage a bit and give a few more pulses in the pulse train, to make sure the insulation can handle these high frequency stresses with some margin.

Above it is mentioned that the plastic parts of the balloon catheter cannot be designed as insulation because of clinical needs. I'm not sure this makes sense. The insulation may need to be thin because of clinical needs. If so, it is important not to overtest. Many test in IEC 60601-1 is overkill to keep it simple. So IEC 60601-1 test is wrong in this case. The test can be reduced to the minimum needed. But there should still be an insulation system with minimum specification and properly tested. It is not possible to say, due to clinical needs, there is no insulation.

Due to the high frequency aspect (2us pulses), there may be negligible risk for stopping the heart, fibrillation, pulmonary arrest. However, there will still be a risk of burns or perforation (of the artery, along the catheter path). In this case, it is also important to know the amount of energy per pulse the system can supply in fault condition. If this energy is very low, it could help as part of risk evaluation and deciding minimum specification for the insulation system. Do you know this energy/pulse?
 

william guo

Starting to get Involved
#5
thanks for your expertise, peter. I attached insulation diagram and photos of balloon.
Max. energy/pulse is about 3.5J. In the insulation diagram, 1: 2MOPP/working voltage; 2: 2MOPP/working voltage, 1MOPP mains voltage; 3: 2MOPP/working voltage, 1MOPP mains voltage; 4: 2MOPP/mains voltage; 5: 2MOOP/mains voltage; 6: 2MOOP/mains voltage. How about 7, balloon catheter canbe treated two parts, the in vivo insert part with ballon and catheter, and in vitro part which includes operator handle and cable connected to HV pulse generator. Is 7 necessary assumed that insulation 1, 2, 3 are working well, or if 7 canbe designed as 2MOPP/working voltage, 1MOPP/Mains, and there are another insulation path between generator enclosure/earth and outer part of balloon catheter, the 2 and 3 canbe designed as 2MOOPs/working voltage? If we working for insulation 7 throught balloon catheter, the balloon as attached can only withstand about max. 1000VAC/1min.
So, how to set insulation specification for these? It's a little chaotic, sorry for that...
 

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Peter Selvey

Leader
Super Moderator
#6
This is a very specialised case so might be difficult to cover in a forum like this, especially it may involve confidential material as well. But anyway for now I will try.

Due to the HF nature (fast rise time), it is not possible to have a truly "floating" output, so some insulation will be needed in the catheter and around the transducer. Small stray capacitances in the transformer, cabling etc will act to "earth" the output. In the best case scenario, the capacitance will be nicely balanced between each output wire and earth, so that the voltage to earth for each wire will be half of the pulse voltage (e.g. one wire +3.5kVp, the other -3.5kVp). However, that is also difficult to realise and control. In practice it will probably be something random like +5kV for one wire and -2kV for the other, i.e more than 3.5kV but less than 7kV. Even so that is a possibility that could be explored.

Another issue is whether the actual transducer really gets 7kV. As previously discussed, the insulation in the catheter will draw a lot of current, which could be as high as 5A or even 10A. But your transformer may not be able to deliver such high current. It's possible that while the transformer is designed to output 7kV open circuit, by the time it reaches the transducer (balloon) the voltage is significantly lower.

These two effects could be investigated to help reduce the insulation requirements around the transducer. It's possible ... maybe ... that the voltage in each wire and earth is as little as 1kVp at the actual catheter.

You could also look at the insulation around the transducer itself, this may be enough to avoid relying on the balloon as insulation. It sounds like if the balloon material is failing at 1kVrms/1min it is not really useful for "safety insulation".

Another idea is to simulate not having the balloon (or a broken balloon) and see what happens. For example test in some kind of artificial set up simulating artery, tissue etc, simulate balloon insulation failure, see 7kV/3.6J can cause burns, perforation etc. If it does not, then investigate why. It may be some other reason why it's not as dangerous as it sounds even though it is "7kV 2us pulses".
 

william guo

Starting to get Involved
#7
thanks peter. I'm not sure the "insulation around tranducer" , there is no tranducer design in this system, shock wave generated in the balloon by high voltage spark acting upon fluid within balloon. We will go to a deep analysis based on your interpretation.
 

Peter Selvey

Leader
Super Moderator
#8
Interesting! In the end it looks like you might need to model the system including stray components like cable capacitance, wire inductance, water impedance and so on, in order to understand what is actually happening under normal condition, which will then allow you to consider possible failure modes and effects.

This is different to the normal way which we just assume worst case and build a solid barrier around the dangerous stuff (e.g. thick insulation, spacings designed for 7kV). Although this is a very blunt tool it is effective to make the analysis simple. Since that option is not available, it needs more accurate modelling and failure analysis, which hopefully will show that the balloon insulation is enough, for whatever reason.
 
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