Capacitive Coupling / Parasitic Capacitance Questions 2

[Buckshott00]

Hello All,

I have (2) questions regarding Capacitive Coupling. I am pretty out of my league here as I am ME, and been a long time since I've had a circuits class. Any help is appreciated

I am designing a handle for monopolar surgical instrument. Now the handle itself has quite the irregular shape; additionally, there is an electrode running through parts of it to charge the tool for the desired electrical cutting and cauterization. The handle is also metal, as I mentioned it is well insulated against direct coupling, or arcing, but as I understand it there is no way to avoid some level parasitic capacitance or induction charging on that metal handle. Additionally, the longer the device is used the more charge it builds potentially discharging in unintended places.

1st question: Since I have the CAD model and geometry of the components, the material properties of the handle, including the dielectric properties of the insulator components and I know the power settings of the electrode, is it possible to estimate the capacitance of the handle? We don't want surgeons accidentally zapping themselves.

2nd question: Is there anyway to accurately measure that? I mean without discharging? So if I put an electro-generator on a prototype is there some sort of probe or dmm I could use to accurately and precisely measure the capacitance, and I could record it over time?

Thank you!

 

[itsmoked]

Well I stand corrected! A picture is worth a bazillion words. That's not at all the style handle I was picturing. Thanks for the cool pictures. All of them! I see what you're up against now.

Seems silly the originators didn't make the probe a small gap with the ground immediately associated with the tip. Like tweezers, so no current has to flow through the patient.

Back to your issue. I was not kidding when I said wrap it in foil. That would be like the most intimate hand grip a user could manage. That foil would be the maximum a hand could ever couple with. Then, measure the potential, preferably with a fast scope, to the associated grounding mat. This setup will show you the maximum that could be capacitively coupled into a gripping hand. You can also add a 10K high-voltage resistor to stand in for the human's skin resistance if desired. That won't change the potential but it would change the current seen exiting thru the simulated user.

If you don't have a high voltage scope probe definitely use a resistor, as mentioned, but stack it up as a resistor divider to reduce the voltage the probe would see. The voltage divider equation will allow you to calculate exactly what was applied to the top of the divider by the 'foil hand'.

If you have a scope some foil and a handful of resistors you could be testing in minutes.

One point. Don't use wire-wound resistors as they will spoil things with their inherent inductance.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

Going back to the top of the thread: "Additionally, the longer the device is used the more charge it builds potentially discharging in unintended places."

That description does not apply to AC capacitive coupling. AC doesn't 'build up' a charge, at least not without a diode. Therefore, the assumptions upon which these Capacitive Coupling / Parasitic Capacitance Questions are based seem to be incorrect. This is assuming that the waveform really is AC as described.

It might be worth backing-up a bit and rechecking the assumptions of what's the root cause (or causes).

 

[Buckshott00]

@itsmoked
Thanks! I figure I am going to have to pick up some equipment to test this! So these recommendations really help! Thank you! All I have at my disposal right now is an old hi-pot machine and a cheap harbor freight DMM. So I really appreciate the advice, knowing how to best setup the test is going to save money in the test environment.

@VE1BLL
I don't understand. It could be my own incompetence on these systems work, please help me. Is it the phrase "builds up"? Are you saying that as soon as the electrode is charged anything nearby that conducts is already at its max capacitance? If you could clarify a bit it would really help me understand. Otherwise, I would have to guess that something is at least acting like a diode because at least from what I read, the longer the surgery goes on and the more the Sawbones lays on the foot pedal, the more likely the patient is to catch a stray burn.

 

[Buckshott00]

@itsmoked

What you're describing sounds a lot like bipolar electrosurgery, which seems all together safer to me!

I found this about the generators, although the source is old: "Most modern electrosurgical generators are isolated from earth ground, and have fault monitors that will disable the machine and sound an alarm if the ground electrode circuit is not intact. While these features minimize the incidence of alternate pathway burns, several measures can be taken to make them even less likely to occur."

This is type of arcing is odd to me. I wish I understood it better. They reported it as far back as the 70's but it was initially being reported as something that was theoretically possible but unlikely. But then you have photographic evidence of it happening and at a rate that correlates to the incident rate.

These are current arcs right? I think my understanding of capacitors is insufficient, I keep thinking of cameras and tasers. These seem like low voltage systems, but I don't know.

 

[IRstuff]

"I don't understand. It could be my own incompetence on these systems work, please help me. Is it the phrase "builds up"? Are you saying that as soon as the electrode is charged anything nearby that conducts is already at its max capacitance? If you could clarify a bit it would really help me understand. Otherwise, I would have to guess that something is at least acting like a diode because at least from what I read, the longer the surgery goes on and the more the Sawbones lays on the foot pedal, the more likely the patient is to catch a stray burn."

This type of capacitor does not have "max capacitance" as if it were changing; it's not changing. More likely, it's a bad circuit design, and the machine is changing its output voltage or frequency or both as duty cycle increases or usage duration increases.

Your photos are indeed showing arcing, but I think that's just poor design or poor material. That level of arcing is showing a complete lack of isolation at the point of arcing, and the insulation is essentially acting as if it were non existent. I would seriously look at that particular probe to see if the insulation is either damaged or degraded. It's possible that something the process of sterilization or other preparation is damaging the material.


TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Buckshott00]

@IRstuff

The output voltage and frequency are controlled by the ESG. " the machine is changing its output voltage or frequency or both as duty cycle increases or usage duration increases" So something IS happening during duration. You're saying that it's likely voltage, frequency or both. If the generator is supplying the selected voltage at the selected frequency where is this increase coming from?

You're unknowingly repeating the same lines that we're stated in much earlier white papers, and were later deemed inaccurate. I know it seems unlikely but the scenario is playing out as I have described. I can assure you, that insulation is checked both before and after each use. Insulation failure or degradation is a real but different failure mode and it is accounted for. This is different. It seems like you're thinking of it more along the lines of direct coupling. It does seem like the most likely failure mode, but in these situations it isn't happening that way. If the insert were uninsulated and it touched something and arc'd it would be obvious, if the insulation were cracked or degraded it would be obvious. This is something else. The insulation checks good and yet there is arcing from the charged/hot/electrified(term?) insert through the insulation through the air through the patient back to the return electrode.

That was my understanding of why it was called capacitive coupling because the insulation and the air are acting as the dielectric part of the capacitor.

 

[VEBill]

An AC waveform will charge and discharge capacitors with equal vigor; there's nothing to break the symmetry. At 1 MHz (for example), this charge and discharge cycle will occur every microsecond.

So, if the description is that the problem is building up over a much longer period, then that's a strong hint that the root cause is something other than a fixed characteristic such as 'parasitic capacitance'.

Note - this logic is precariously balanced on the information provided and assumes that I've not missed anything.

 

[IRstuff]

I don't know who or how these things were checked, but if the probe was shielded as the manufacturer suggests that it is, only a shield AND insulation failure can produce those arcs. Even relatively small currents would require 10x the capacitance that could possibly exist in the entire probe AND a complete failure of insulation AND a complete failure of the shielding.


"AORN recommends active electrode monitoring (AEM) as a way to minimize the risk of stray electrosurgical burns due to insulation failure and capacitive coupling. Under normal operating conditions, AEM technology delivers 100 percent of the power to the surgeon’s intended site. Capacitively coupled energy is safely drained to the generator via a protective shield built into 5mm AEM instruments. If primary insulation fails, or the level of capacitively coupled current becomes too much, AEM technology shuts down the generator, protecting the patient from a potentially life-threatening burn, and alerts the perioperative staff."

So, why are you attempting to do something different?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Buckshott00]

@VE1BLL

That could be my own misunderstanding, but Capacitance is stored electrical charge. What I have to go on is the scientists and engineers who deal with this everyday have called it Capacitive coupling.

 

[Buckshott00]

@IRstuff

"Capacitive Coupling occurs whenever two conductors are separated by a non-conductor. An electrostatic field is created between the two conductors. As a result, a current in one conductor can, through this electrostatic field, induce a current in the second conductor. An "inadvertent capacitor" may be created by the composition and placement of surgical instruments:

Electrode (conductor) + Electrode Insulation (non-conductor) + Metal Cannula (conductor) = CAPACITOR

If we consider a metal cannula system, a capacitor occurs whenever a non-conductor separates two conductors. The conductive active electrode is surrounded by nonconductive insulation - in turn surrounded by a conductive metal cannula. An "inadvertent capacitor" may be created by the surgical instruments. If we consider a plastic cannula system, capacitance cannot be entirely eliminated with an all plastic cannula. The patient's conductive tissue completes the definition of a capacitor; capacitance is reduced, but is not entirely eliminated. Finally, if we finally consider a hybrid cannula system, this is the worst combination and occurs when metal cannula held in place by a plastic anchor, or when a metal reducing tube is placed through a plastic cannula. The metal cannula or reducer still creates a capacitor with the active electrode. However the plastic port prevents the current from dissipating through the abdominal wall. Capacitively coupled current may exit to adjacent tissue on its way to the patient return electrode can cause significant injury.

I'm not trying to do anything different. In fact, all of that is fine, what I'm concerned about is what is now going to happen in "Zone 4" The front part and what's going on in the patient, I'm not concerned about, I've got it covered, I'm looking at the back end of this. So if a surgeon is touching this newly designed handle, that he doesn't act like the new grounding pad.

BTW I agree with AORN but only 1 company actually makes tools like that right now, and they're the ones selling the AEM system. Meaning they've identified th

 

[Buckshott00]

realized I never posted the zones pic please see below

 

[VEBill]

Yes, capacitive coupling can cause things to become charged. But the Universe tends to be symmetrical. So for one-half cycle the other plate (the handle) can be charged positive, and then for the other one-cycle cycle, it can be charged negative. Lacking a diode to break the symmetry, ones is as good as the other. Rinse and repeat at 1 million time per second.

The axis of time is a rich vein, in terms of providing evidence. If something is building up over a period of minutes, then you need to find a mechanism that matches that time period. Capacitive coupling of 1MHz AC isn't it.

Usual disclaimer applies.

 

[Buckshott00]

@VE1BLL

I get what you're saying I really do, so I'm looking for explanations. All I can tell you is the data that I have gathered or has been presented to me as best I understand it. I don't understand how it could be charged enough to discharge at half a micro second. In fact, I don't think that is what is going on, but it's also not what I am concerned with. Whatever is going on over in zones 2 and 3, it's real and it's happening, the people seeing it might not be describing it right, but I'm seeing instruments with sidewalls blown out and patients with burns on their insides.

All that aside, I want to make sure it doesn't happen in zone 4

 

[Buckshott00]

As I said before all of these instruments undergo insulation testing before and after surgery

Here is an FDA event revolving the manufacturers of that double insulated AEM instrument. I'm not trying to create a new system, I'm not worried about zones 2 and 3 although it seems there is plenty of reason to be concerned. I want to know: If I have this standard electrode, and I have my insulator piece, and the handle around the insulator pieces is conductive, how do I test for capacitance accurately?

http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfMAUDE/detail.cfm?mdrfoi__id=448203

 

[Buckshott00]

Clarification: Side walls blown out was from insulation failure not from the capacitive coupling, I reread what I wrote, and what I wrote and what I was trying to convey don't match. I have seen the sidewalls blown out, with weak or degrade insulation. I know what that looks like, and results in burns. I have seen instruments that look good but have discharged through insulation that tested good and the burns those instruments have caused.

 

[Buckshott00]

Best explanation I have seen so far
CAPACITIVELY COUPLED CURRENT

The phenomenon of capacitively coupled currents is more difficult to understand. It is the inducement of current through the intact insulation of active electrodes to surrounding cannulas or instruments.
Note: The term induced/inducement technically relates to inductive coupling, not capacitive coupling. However, the term will be used in this paper for the purpose of teaching the concept of the transfer of current through a nonconductor.

What is capacitance? It is a stored electrical charge. Capacitors are defined as two electrical conductors separated by an insulator. During minimally invasive procedures incorporating the passage of electrosurgical energy into the abdomen, a capacitor can be created. The active electrode (a conductor) is surrounded by insulation (a nonconductor) which is often passed through a metal cannula (conductor). A capacitor has been created. A capacitor can induce an electrical current into the metal cannula through the process of capacitance. This capacitively coupled current wants to complete the circuit by finding a pathway to the patient return electrode. The electrical charge will be stored in the metal cannula until either the generator is deactivated or a pathway to complete the circuit presents itself. As we begin our discussion of capacitively coupled current, it is important to remember that capacitively coupled current is increased with a high voltage coagulation waveform, as opposed to a low voltage cutting waveform. Additionally, activating the generator in open circuit greatly increases the level of capacitance that will occur. Also, by isolating the outer conductor (metal cannula or instrument) from the abdominal wall by a plastic nonconductor, the possibility for injury is increased. The level of capacitance is increased when the cannula is small (i.e. 5 mm vs. 10 mm) and the electrode is long.

The amount of electrical energy capacitively coupled onto the metal cannula will be dependent on a number of variables--first, when the generator is activated in open circuit. This, as previously described, causes the generator to go to maximum voltage. The higher the voltage, the more current that will be produced. Conversely, capacitance will be minimal if the generator is activated in closed circuit. This means that the active electrode is touching or in near proximity to the target tissue. This will help ensure that the generator’s output will travel through the target tissue on its way to the patient return electrode. Voltages will remain lower and the amount of capacitance will likewise remain low.

The second variable is the waveform selected by the surgeon. Since the cut waveform uses significantly less voltage, the amount of capacitive current is less. The cutting waveform can be used for both vaporization (cutting) and desiccation. The high voltage coagulation waveform should be reserved for fulguration. When using the coagulation waveform, the surgeon should be careful not to activate the generator in open circuit.

The type of cannula/trocar system chosen also plays an important role in the amount of capacitance produced and whether this current can be safely dispersed. An all metal system is appropriate in that any capacitive currents will be safely dispersed through the greater surface area provided by the chest or abdominal wall, thereby reducing current density. In normal circumstances, this surface area will be adequate to safely dissipate any current buildup on the cannula without significant heat production or tissue damage.

An all plastic cannula system is also appropriate. When using an all plastic system the definition of a capacitor has been eliminated. Instead of two conductors separated by a nonconductor, there is now the conductive electrode, covered by nonconductive insulation, surrounded by the nonconductive cannula. A capacitor no longer exists and, therefore, concerns over the capacitively coupled current have also been eliminated.

The big problem with capacitance occurs when a metal cannula is held in place by a plastic anchor (hybrid trocar/cannula system). (This complies with the definition of a capacitor through the metal portion of the cannula.) However, the coupled current is not able to disperse safely through the chest or abdominal wall because the cannula is held in place by a nonconductive plastic anchor.

Consequently, the current that is coupled onto the metal cannula can only complete the circuit by discharging to tissue that it may encounter within the cavity (Figure 3). The unknown variable is the amount of tissue that is in proximity to the metal cannula. If it is large, it is unlikely that the discharging current will cause any damage due to the principle of low current density. However, a small contact area will create high current density and potentially a significant injury. Often this occurrence is outside the field of vision and, therefore, is not detected as a problem until several days postoperatively.

 

[IRstuff]

If everything is working you shouldn't be able to measure the capacitance, because there should be a shield in-between.

The last picture you posted is weird, in that I don't understand if the upper electrode tip is intentionally uninsulated and unshielded, in which case, the zap to the bowel is almost a given. But, if that "electrode tip" isn't being used, why is it powered, and if it's not, why isn't it grounded or insulated?

It seems to me that most of what I'm reading here is a consequence of bad decisions in the OR, rather than an inherent design issue, possibly. Why would someone be using this metal cannula if this is a known problem?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Buckshott00]

Hopefully this sheds some light on how this is happening with AC voltage.

So worst case scenario High Power Generator, Coagulation Open circuit, device is known to be a inadvertent capacitor. How do I measure how much of a capacitor it really is, and determine where it is likely to discharge?

LoL maybe should tell the surgeons just wear really thick gloves

http://www.dremed.com/catalog/documents/Lforcefx.pdf

 

[IRstuff]

The capacitive discharge as shown in that picture is specifically possible because of the metal cannula, since what the metal does is to put all of the capacitance in parallel. A probe without a metal cannula cannot discharge in this fashion, since the charge that could be generated cannot move to the point of closest approach, since the material is non conducting. Since the metal cannula is not participating in anything, I don't see why just a plastic sleeve wouldn't eliminate this problem.

As I stated before, a capacitance meter would be used to measure capacitance, but without a metal cannula, you would get meaningless measurements.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Buckshott00]

@IRstuff

That I get. Unfortunately the metal cannula does have other uses so getting rid of it isn't really an option (on this design anyway). Plus as I have stated before, I'm not really worried about zones 2 and 3 What I have is much more interesting. So in zone 4 (the handle) we have the "bovie post" connector going through Radel insulation pieces which are housed in a big 17-4SS shell.

After talking with you guys, I'm sure it's a capacitor; I'm trying to determine how bad it is. If I do like kevin / itsmoked said and wrap the handle in foil where is the worst case challenge to place the other probe?