Endoscope Applied Part (Electrical and Temperature)

[scopeus]

We are developing a single-use endoscope with additional internal electronics including a small solenoid in the handle.
There is no electrical isolation between the insertion tube and the handle and the display unit. The display unit has 2 MOPP from mains due to our selected power supply and 2 MOPP from a USB port on the display unit.
1) Based on this, do we consider the entire system to be a BF applied part, even though only the insertion tube necessarily comes in physical contact with the patient during normal use?
2) For temperature limits, we have concerns over the solenoid causing small areas of the handle, not normally touched by the operator, to exceed the 43 degree C temperature limit when a single-fault condition occurs where the controlling mosfet is damaged (shorted) or the MCU is damaged (pin stuck high). This assumes a contact duration of over 10 minutes, even though these areas would not even contact the patient and only be touched by the operator unintentionally. Would we be able to apply less strict temperature requirements for these parts that do not come into contact with the patient and the operator despite it being considered part of the same applied part electrically speaking?

Any help on this matter would be greatly appreciated.
Best regards

 

[Tidge]

Does 60601-2-18 offer any clarification?

For the temperature... if it takes as long as 10 minutes for harm to occur, and harm is only to user (obviously patient cannot respond) I am inclined to believe that the risks might be acceptable, depending on the benefit.

 

[Peter Selvey]

Type BF insulation is in principle between the applied part and "everything else" including accessible parts that the operator can contact, it's not just isolation from mains. Imagine the applied part is inserted in say saline solution, and the operator accessible parts are wrapped in foil which is also connected to ground, metal frames, SIP/SOPs and so on, and then apply 1500Vrms between the saline and the foil/ground.

In reality for things like endoscopes there may be contiguous areas where one end is inside the patient and the other end outside and operator accessible, which is often given a free pass on the Type BF (or CF) isolation, on the basis that it's just not practical. However this free pass has limits and it would not extend to for example a display unit. Generally you have to decide where the "applied part" ends and the "operator accessible parts" begins, and keep it reasonable, don't push the "applied part" too far.

For the temperature, as Tidge said, the limit should be much higher for incidental operator contact, see Table 23. I guess the reference to 43°C is based on the applied part limits (Table 24). Again, it's necessary to clearly define which part is applied part and which is operator.

 

[scopeus]

Thank you both for your replies.

As a little background, we require the additional electronics in the bronchoscope for a built-in suction generation and control system.

It is not feasible to isolate the signals from within the scope until it reaches the display unit. Electrical isolation does not occur in the handle nor in the cable going to the display unit. In most currently marketed single-use bronchoscope systems, I don't think it is even isolated in the processing boards inside. Only isolated from the mains, and SIP/SOP ports. I believe this to be a common practice with the single-use scopes as is evidenced by the BF applied part marking on the Glidescope display units (Glidescope Core example here). This, to me indicates, it was evaluated as a BF applied part system with the connected endoscopes. However, I agree it seems odd to consider the whole system as an applied part, especially for temperature purposes. Surely, the temperature limits would be reduced to what is considered for accessible parts for parts of the endoscope handle and display unit not typically touched?

 

[Peter Selvey]

It's possible to have the display/controller be "operator accessible" while the internal electronics is considered a "patient circuit" (effectively, an electrical extension of the applied part). For this the outer plastic case forms the Type BF isolation barrier. The photo shows IP54 so I presume it is a pretty solid barrier between the inside and outside parts. It works well because it's a fairly small device. The larger the system, the more interfaces with the outside world (touch screens, control knobs, buttons, connectors, case screws etc etc the more difficult it gets to design and maintain the BF insulation between the internal electronics and accessible parts. Keep in mind it is not just the type test, but also need controls to make sure it can be maintained in regular production, and with design changes. But it can be done.

 

[scopeus]

It's possible to have the display/controller be "operator accessible" while the internal electronics is considered a "patient circuit" (effectively, an electrical extension of the applied part). For this the outer plastic case forms the Type BF isolation barrier. The photo shows IP54 so I presume it is a pretty solid barrier between the inside and outside parts. It works well because it's a fairly small device. The larger the system, the more interfaces with the outside world (touch screens, control knobs, buttons, connectors, case screws etc etc the more difficult it gets to design and maintain the BF insulation between the internal electronics and accessible parts. Keep in mind it is not just the type test, but also need controls to make sure it can be maintained in regular production, and with design changes. But it can be done.

Okay, this makes sense. We are now considering the outside of the handle and the display unit to be accessible parts, rather than applied part as the insulation will provide 2X MOPP from the internal circuitry that is for the applied part.

I have a follow-up question regarding the temperature requirements. The base of the handle and the top part of the handle are not going to be touched while in use, at least not intentionally and if so for a short period. Does that mean I should look at the requirements of Table 34 for those areas or does the entire handle, regardless of whether the user would be touching it during use have to comply with the requirements of Table 23. Can we provide a mapping of where on the handle the user is intended to touch during use (gripping portion) and where not (rest of the handle)?

 

[Peter Selvey]

Table 34 is for abnormal/fault condition and in principle should be fairly rare conditions, like 0.001 events per year. If the condition that could cause high temperatures is not normal but could occur more frequently e.g. motor overload due to occasional friction in the endoscope or operator stuff up, the limits in Table 34 would not make sense. I also think the limits in Table 23 for t<1s are a bit unrealistic, mainly because it's hard to be sure that someone would only touch the surface for <1s and the limits are quite high as well, 74°C metal or liquid will burn pretty quickly. I think the limit for 1-10s are better to use for accessible parts that are not required to be contacted, but are fairly easy to contact anyway.

The other factor is that operation manuals usually specify room temp range say 10-40°C but in reality the use would typically be 23°C and the max maybe 27°C. If the IFU says 10-40°C room and the accessible surface is measured at say 50°C in a 23°C room (which is totally fine), this ends up being 67°C in the report, which is for me is dangerous, but not a real number because no one would ever use it in a 40°C room. So a manufacturer might say it's <1s as a workaround, so they can pick up the 74°C limit. That's standards for you .

Anyway, rather than designing for the limit, design for common sense and then check that also fits into the limits.

 

[scopeus]

Okay, thank you Peter.
The condition where one component may get hot is under a rare fault condition so it makes sense to apply table 34 in that scenario and use the table 23 values otherwise for that section of the handle's surface. I really appreciate your help.
Have a good weekend!