Mains on Patient with Part Attached to Protective Earth in Patient Area

MThomas

Starting to get Involved
#1
In the Mains on Patient SFC, 110% of maximum line voltage is conceptually applied to the patient, the patient is well-insulated from protective earth, and the current through applied parts and patient connections is measured and verified to be below some SFC limit.

Now let us suppose there is a piece of metal in the patient area that is protectively earthed. The metal is part of the MEDICAL EQUIPMENT.

If the patient touches this metal, is this a double fault condition (while mains are on the patient)? Do all protectively earthed conductors in the patient area and part of the ME without documented insulation between the metal and patient become a problem for 240 volt mains?

The patient is fully aware, conscious, has mobility, and can sense pain and react, although physiology suggests that current from 264VAC will inhibit motion. Applied parts, with and without patient connections, are in contact with the patient for 30 minutes typically.
 
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Benjamin Weber

Trusted Information Resource
#2
To start with: External voltage (usually mains) is only applicable for F-type applied parts.

The SFC "PE interrupted" (switch S7 open) is NOT applied during this test (see fig. 16 of IEC 60601-1). The situation "external voltage on F-type applied parts" is neither is SFC, nor ist NC. It is "special test condition". condiser it beeing a half fault condition. The voltage on the applied part is not caused by a fault in the ME equipmment, that's why it's not SFC. But of cource this situation is not normal, so it's not NC.

The additional requirement for F-type applied parts is, that all patient connections shall be separated by 1 MOPP for max. mains voltage from all other parts (see cl. 8.5.2.1). This includes all PE connected parts of your ME equipment. But this applied to the patient connections, not really the patient him/herself.

I recommend to evaluate the chance of the patient being at mains voltage potential and touching PE connected parts of your ME equipment.
 

MThomas

Starting to get Involved
#3
Ok to be clear there is an f type part with a patient connection on the patient.

It is my understanding that all accessible surfaces have to be evaluated to see if these accessible surfaces need to be treated as f type applied parts based on probability of contact, patient condition, and length of contact.

Contact with a protectively earthed surface of any duration in any patient condition is unacceptable risk with mains at 240. So we must ensure probability of contact with a PE part is neglible or add documented insulation of 1 Mopp or 2 Mopp to Protectively earthed surface?
 
#4
Contact with a protectively earthed surface of any duration in any patient condition is unacceptable risk with mains at 240. So we must ensure probability of contact with a PE part is neglible or add documented insulation of 1 Mopp or 2 Mopp to Protectively earthed surface?
I'm dealing with this question now. Regarding your comment MThomas, what about Type B applied parts, which can be connected to earth (PE or otherwise)? Type B applied parts can well be in the patient environment, is my understanding (e.g. MRI machines or surgical microscopes), or am I missing something here?
 

Benjamin Weber

Trusted Information Resource
#5
Do you mean, that there is an F-type applied part and another B-type applied part involved? In this case, both applied part types shall be seperated from each other, whoch actually results form the F-type requirement "1 MOPP vs. all other parts for maximum mains voltage". But be carefull!! The standard actually says, that the patient connections of the applied parts shall be separated (and not the whole applied part!).

This means, that the design of the applied parts and the corresponding patient connections (which might be only a small portion of the applied part) shall be such, that the separation from all other parts (PE, other patient circuits, applied parts....) is achieved. The patient being at mains voltage AND earthe at the same time is not within the scope of the test!!! The source of the mains voltage in the test is not the ME equipment (this would obviously be a SFC!), but other (non)-ME equiptment in the patient environment, let's say a broken luminaire touched by the patient. In this situation the ME equipment under test with F-type applied part shall prevent the patient from being earthed via the device itself! But of course the ME equipment cannot prevent the patient from touching other earthed parts.
 
#6
Thanks Benjamin. What I meant with my comment was in the scenario where only a Type B applied part was involved.

Basically, it was to me confusing that it is deemed safe to have a Type B applied part that is earthed in the patient environment (which can then connect the patient to earth), but for a type BF applied part there was the additional separation requirement due to the possibility of earthing the patient.

I acknowledge that the nature of the application of the two parts is different (type BF is supposed to be for a longer term application to the patient and not that easy to remove), but still, that's why I shared the same question/confusion of the original question.
 

MThomas

Starting to get Involved
#7
I'm dealing with this question now. Regarding your comment MThomas, what about Type B applied parts, which can be connected to earth (PE or otherwise)? Type B applied parts can well be in the patient environment, is my understanding (e.g. MRI machines or surgical microscopes), or am I missing something here?
Hello all unfortunately this thread has become more complicated than it needed to be...sorry for not being prompt.

1. Forget applied parts for this discussion
2. The risk of a patient touching an accessible surface must be subjected to an analysis to determine if the surface needs B or BF protection.
3. The philosophy of the mains on patient test is that a foreign piece of ME fails and places mains on patient. Lets overlook the scenario where the ME under investigation puts mains on the patient. This is not possible if there are two means of protection.
4. Example 1- metal bed rail. Patient is unconscious, or cannot understand pain, or cannot move. This should be obvious unless somehow there are no mains or 60VDC or less. ? Alpine rescue litter?
5. Example 2- metal column on cart attached to mains. Some design team members want to earth the column as a means of protection from a certain electrical failure in cart. Other design team members are concerned that patient can touch metal column. All design team members have to reach consensus before putting plastic around the column is considered.

The only right answer to #5 is to conduct a credible risk analysis because there are risk analysis can lead to two answers.

Please feel free to provide feedback.
 
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