Electrical Surgical Tool with Big Metal Applied Part

  • Thread starter ThisCharmingMan
  • Start date
T

ThisCharmingMan

#1
I am looking to design an electrical surgical tool that will have a metal Applied Part but the attached handle, held by the surgeon, is also metal and at the same potential as the Applied Part. In normal use the handle would not necessarily touch the patient.
I know that it is acceptable, although not ideal, for the AP to be protectively earthed and Type B. However if instead it were type BF, would the handle be considered as:
i. Effectively an Applied Part (see clause 4.6) or..
ii. An accessible part (in which case definitely a no-no e.g. Mains on AP)
I would really appreciate any thoughts on this.:thanx:
 
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Peter Selvey

Staff member
Super Moderator
#3
There is an risk based exception allowed in 8.5.2.2 which would allow an unearthed Type B rated applied part without isolation from the operator. The rationale uses an example of a dental handpiece.

The actual test in 8.7.4.7 d) makes no mention of the exception, but it is clear from 8.5.2.2.

The use of risk to justify the exclusion opens up some questions as to why the "mains on unearthed accessible parts" test is really necessary in the first place, and if it is, why a dental handpiece can be excluded.

The assumption of a external, low impedance un-insulated mains voltage floating in the patient environment seems unrealistic as it would expose not only the patient but the operators and visitors to an unacceptable risk.

At least you could argue the situation is unlikely for a handpiece, as the operator is likely to be shocked if not electrocuted in the process, and so would not get around to applying the applied part.
 
M

MIREGMGR

#4
(...) as the operator is likely to be shocked if not electrocuted in the process, and so would not get around to applying the applied part.
"Patient Safety Via Assured Clinician Death Prior To Device Application" certainly is a cheerful regulatory approach. :D
 
T

ThisCharmingMan

#5
Peter, Thanks very much for your reply. Do you think that the "exclusion" mentioned in the guidance (8.5.2.2) could also be applied to Type BF? The two "exclusions" immediately above it refer explicitly to Type BF & Type B respectively, whereas the final one just says "Applied Part".
 

Peter Selvey

Staff member
Super Moderator
#6
Unfortunately the clause (8.5.5.2, including the title) specifically refers to Type B.

The theory behind a Type BF applied part is equally dodgy in terms of justification, since again it implies that there is a low impedance mains voltage somehow floating around in the patient environment. I have never seen any evidence that this has ever occurred in the real world.

Nevertheless, BF is a claim, and if it is claimed ideally you should meet the requirements. BF isolation can also be useful for other reasons (e.g. prevention of interference between different medical devices).

CF is a different matter, as you do not need to have a low impedance mains source, a leakage (capacitively coupled) current is enough. And it is not unreasonable to expect to some leakage current floating around, e.g. due to a broken earth lead or just using double insulated (Class II) devices. The leakage current can be small to be not really noticed by the operator, but still enough to cause fibrillation if applied directly to the heart.
 
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