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!

 

[benta]

It's been many years since I worked with medical electronics, but how about earthing?

Benta.

 

[IRstuff]

Q1. Yes, that should be doable, since it's essentially the thickness and value of the dielectric
Q2. Yes, you get a capacitance meter

Nevertheless, if you are worried about capacitance, then you have insufficient shielding, i.e., as benta stated, grounding. However, you've not even stated whether the voltages are AC or DC, or what frequency, if the former.

TTFN
faq731-376

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[VEBill]

Disclaimer: This is not my area so all advice given should be carefully checked and double-checked.

Depending on the signal / power you're sending down the cable to the tool, perhaps the tool handle could be effectively an extension of the transmission line in terms of electrical properties. If so, parasitic capacitance is simply part of the transmission line, along with series inductance.

Note: there can be complications depending on how the currents flow.

The handle should be firmly grounded, perhaps via the coaxial feed cable, so there should be no chance of shock. This might even call for redundancy, so perhaps an additional grounding wire. Or triaxial cable.

 

[Buckshott00]

I've attached a picture regarding the load of these devices and how they're used. The issue with conductive capacitance is that something could build up enough charge to burn the patient or surgeon unintentionally.

So for surgeries using a monopolar device: The device is attached to an electrosurgical generator. The settings can vary depending on what the surgeon is trying to do cut or coagulate etc. However, their general response is to crank up the voltage(AC). The generators can vary quite a bit but have 3 ranges low power 50-100watts mid power100-200 and high power 300-400 watts. The frequency also changes depending on how the surgeon wants to cut, but on the high end It's a high frequency ~1MHz.

So when things are working the way they're supposed to the electrodes cut taking the path of least resistance. However, with all that induced charge patients can get burned in other areas. So the device is split up into zones so as to classify zone 1 is the intended cut area, zone 2 and 3 are parts of the instrument that are most likely to discharge in conductive capacitance, but our design has proven valid to oppose this on earlier model. However, now we have a new dilemma we're introducing more metal where typically the handle would be a composite. AKA Zone 4, the zone that the surgeon manipulates. The concern is that during prolonged surgery.

That's how the monopolar instrument works, the return pad (aka grounding pad although not a true ground) is the electrode placed underneath the patient in this type of surgery. So adding a ground line from the handle would take care of it, but it generally is not accepted in hospitals.

The worry is that the composite (Radel 5000 grade) will wear (legitimate concern) and the handle will induce a large enough charge to arc to the electrode or through the insulation, and burn the surgeon.

 

[Buckshott00]

Capacitive coupling: The condition that
occurs when alternating (AC) electrical current is
transferred from one conductor (an electrode), across
intact insulation, into adjacent conductive materials
(tissue or skin) or another metal surgical instrument.
Capacitance is stored electrical charge.

 

[itsmoked]

400W? You're not going to have any problem. You can wrap the handle in tin foil and then put a scope between the foil and local ground to see just how paltry any capacitive charge is going to be.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LiteYear]

But according to the load profile this device can manage 200W into 1000ohms. That's several hundred volts oscillating around 1MHz. You wouldn't need much capacitance to couple a significant voltage to the handle. Probably not enough to kill anyone (provided the power supply continues to limit current as it should) but possibly enough for a jolt or a burn.

To address your questions:
1) Yes, in theory. For a first pass estimate, can you approximate the handle to a cylinder with a concentric conductor? If you can, you can get a ballpark estimate using the capacitance of concentric conductors forumla. For example, plug your numbers into the first one on this page.

2) Accurately measuring parasitic capacitance is tricky. Stray fields and the effect of the measuring device can be significant. Devices do exist and might be worth a shot. For a complex shape like the tool you describe it might be easier to take operational voltage measurements instead (be sure to load your measurement to quantify the source impedance of the coupled voltage).

But to echo earlier suggestions, it's a Good Idea to ground the enclosure of a device with a metal enclosure, containing live circuits to 100's of volts. Off the top of my head it solves three problems at once - it reduces the capacitively coupled voltage you're describing; it bleeds off any static build-up; and more fundamentally, any breakdown of the insulation will be detected by a ground trip. You wouldn't need a grounding line, just a shield in the cable that runs between the power supply and the tool.

 

[VEBill]

The handle is firmly grounded for safety? Right?

If not, then why not?

If it is, then how could anyone get a shock from it? *


* There is "magic" with RF (not really), but at 1 MHz the wavelength is 300m (or a bit less in a transmission line), so you won't stumble into any unexpected one-quarter wavelength lines in a ten foot cable. One-quarter wavelength is where a chunk of metal can be grounded at one end and high voltage at the other. At least until the FCC shows up and requests that you turn it off.

 

[Buckshott00]

@itsmoked
The handle itself is 17-4, admittedly capacitive coupling is more serious inside the body

 

[VEBill]

The cable from the "ESU" to the handle is not simply a single conductor? It's at least a coaxial cable, right?

 

[VEBill]

In the latest picture, which way is that arc going?

If the handle is grounded, perhaps it's ground seeking current headed back? Taking a short cut back to ground?

How can we know?

 

[Buckshott00]

@ everyone
I don't know if it has a shielding wire to the handle electrode but I guess so, it still doesn't solve the issue. Otherwise there wouldn't be the need for companies to come up with stuff like active electrode monitoring for these devices. You guys know better than me about that stuff. I put a hyperlink below so you can see the electrodes and cords. The idea of the surgery is to purposely create a controlled arc for the purpose of cutting or coagulating or desiccating etc. The electrode is hot, it works through the insert and creates an arc trying to complete a circuit to the grounding pad. The issue at hand is when these are charged they can sometimes discharge in unexpected places, like through the insulation. So the insulation handles direct coupling, but as far as I can tell short of actually adding another ground wire to the handle there is no way to avoid it building charge. Adding an additional grounding wire is something for Bovie or Coviden or someone to work on, I'm just trying to determine if there is anyway to legitimately test for this in a prototype (incidentally it might as well be a big block metal) The electrode is insulated by a few mm of radel, and a few mm of air.

@liteyear
inside the body it has caused death, but generally it's cuts or burns in unexpected places. perforated bowels are seriously painful and have lots of complications. But you nailed it, a jolt or burn to the hand of a surgeon or nurse and game over.

So as to answer everyone's question: The surgical instrument has to be take apart for cleaning. This generally means that the handle and the piece that is actually grasping tissue or cutting are separate items. In order to take advantage of the electrosurgical properties the insert has to be energized from the electrode the electrode has to stay embedded in the handle. However, the handle still has to actuate the "hot" insert. The way this usually addressed is to have a non-conductive handle. The make or break of this new design is if you can have a conductive handle but insulate it enough or adjust it somehow to not discharge into the doctor's hand.

So if I was to design a test, I would test worst case scenario, like the handle's been through an autoclave a bunch of times and the surgeon has the settings cranked to the max and has been laying on the foot pedal for a ton of time with an open circuit. If can determine that it won't discharge into someone's hand I'm super happy. I just was thinking if I could quantitatively capture the increase in capacitance I could demonstrate such a test would be valid and that even if the handle is building a charge, it's not enough burn someone

 

[Buckshott00]

if this helps there are output characteristics at around page 40. This would be a low power model

 

[Buckshott00]

@VE1BLL

I think so. So imagine you're on your back on a surgical table. Underneath you is a ground pad electrode. So it might be taking a shortcut back to the ground, or it could be with the charge it's built up that the difference in the potentials has narrowed to the point that it's taking multiple paths back to the ground? I really wish I knew the electrical behavior better. I mean I know the old adage that it will always take the path of least resistance only holds true if there is big difference in the electrical potential difference right?

The part that's circled is the part displaying the undesired behavior but if you look at the rest of the picture you can see that the instrument is cutting in the desired location as well. I put a link to where the cables are, I think they are shielded because the plugs into the generator are multi-pronged, but I really don't know.

 

[VEBill]

Words like "charged", etc. imply DC and usually apply to static electrity problems. This system is apparently using AC / RF, so that vocabulary isn't quite right.

In general, metal things (especially handles) around high voltage are supposed to be grounded. I don't understand how any other design approach could ever be contemplated, except all plastic double insulated. Is this system Approved?

Actuating the arc could be by an integrated low voltage control embedded in the handle. Trigger squeeze to control power.

You may wish to review the design concepts of modern MIG/TIG welding and Plasma Cutting handles. They're way ahead of where this design has reached.

The ESU and handle designs should be integrated. I think that you hinted that they're separate. If so, that's a problem.

The whole system could be improved if the return path were better controlled. An isolated/balanced system with a return path paddle and plenty of saline splashed around. As it is, the return path and its current density is a matter of luck.

 

[VEBill]

If the handle is grounded, as it should be, those arcs may be ground seeking from the patient to the grounded handle. You should rule this possibility out before worrying about parasitic capacitance.

The whole design concept should address all these safety factors from the start.

"Path of least resistance" is untrue. Current and voltage follows Kirchoff's laws and distributes themselves accordingly. A complication is arcing where the ionized path provides a lower voltage drop and hogs more of the current than expected. Kirchoff's laws apply at any voltage, noting any nonlinearities.

 

[IRstuff]

" So adding a ground line from the handle would take care of it, but it generally is not accepted in hospitals."

There is absolutely no reason the probe couldn't have a shield that runs back to the box in the same 2-conductor cable that is currently being used.

TTFN
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Need help writing a question or understanding a reply? forum1529

 

[Buckshott00]

@VE1BLL

I get that, it's the first thing that popped into my mind when I was informed of this potential failure mode. Unfortunately, there is nothing I can do about it. The handle is separate and has to use a standard post electrode that works with standard cables and generator units. This system isn't anything new. It has been in frequent use since the early 90's. I'm not redesigning the system. The electrical components are all standard equipment. The only thing new here is that instead of a composite handle body or dip coating a metal handle for insulation; a metal handle is shelled out and insulated with composite pieces. What I'm looking for is to better understand how to realistically measure and induce the capacitive coupling so as to test for it and not have it happen to a surgeon or nurse.

@IRstuff I really don't know. I didn't design the cable, it probably is. I posted a link to some of the most popular cables and the electrodes. They're standard uniform accessories, and my hands are bound to using standard connectors/fasteners. What is there is there, so the question is will the handle work in those conditions or won't it. I'm trying to figure that out theoretically first, and then try to quantitatively measure those issues in prototype testing.

 

[Buckshott00]

@VE1BLL

Also, Thank you. What are the right terms for an AC system? All I can think to call them otherwise is conductors...

I mean this was my introduction to this phenomenon, I attached the picture earlier showing the arc through the halar tube.

Capacitive coupling: Capacitive coupling occurs in the presence of a capacitator and is created when two conductors are separated by an insulator. It can occur many times during a laparoscopic procedure, as tissue, trocar and instrument (itself consisting of active electrode and primary insulation) are in close proximity. Capacitive coupling can transfer current to non-target tissue through intact insulation, thereby causing stray electrosurgical burns. There’s nothing a surgeon can do to prevent the latter two causes of electrosurgical burns. What’s more, in an American College of Surgeons survey, 49 percent of surgeons said they had not heard of capacitive coupling. That’s why it’s especially important that monitored monopolar instruments are in place.

- See more at:

http://encision.com/news/electrosur...from-laparoscopic-burns/#sthash.5oFS5On7.dpuf