Insulation requirements between F-type applied part and enclosure

W

william_N

#1
Hi All,
I am a beginner for 601.
I have a question about the insulation requirements between F-type applied part and enclosure, which the standard states:
1) In the case of an F-TYPE APPLIED PART containing no voltage difference, the insulation
between the PATIENT CONNECTIONS and the ENCLOSURE will only be stressed to the MAINS
VOLTAGE in the case of a fault in other equipment connected to the PATIENT.
This condition rarely occurs; furthermore this insulation is not normally subject to the
transient overvoltages found in the MAINS PART. In view of the above, the insulation
necessary between the APPLIED PART and the ENCLOSURE for the case quoted, need only
satisfy the requirements for BASIC INSULATION.
2) In the case of an F-TYPE APPLIED PART containing parts with voltage difference, the
connection of a PATIENT CONNECTION to earth via an earthed PATIENT (NORMAL CONDITION)
could subject the insulation between other parts and the ENCLOSURE to the whole of the
voltage within the APPLIED PART.
Since this voltage appears in NORMAL CONDITION, even though infrequently, the relevant
insulation should satisfy the requirements for DOUBLE INSULATION or REINFORCED INSULATION.


Can somebody clarify this?
Thanks
 
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Peter Selvey

Staff member
Super Moderator
#2
For the first part (1) there is a historical background, for the second (2) there are some conventions which make the requirement more clear.

The history is that ECG (EKGs) were originally earthed circuits, meaning the patient is solidly earthed (grounded) through the electrodes. This situation was eventually considered dangerous, because one of the reasons that people survive normal electric shocks without any problems was they are not usually grounded. So, if the patient had the earthed ECG electrodes, lent over and touched another device (e.g. TV, radio etc) which had faulty insulation, they could get a serious shock.

This led to the development of F-type isolated patient contact (applied parts). In the 1977 edition of IEC 60601-1 these only had to pass a leakage current test. Two sub-types were BF and CF (body contact, cardiac contact) with "mains on applied part" having limits of 5mA and 0.05mA respectively.

The next edition (1988) recognized the isolation as "Basic Insulation" and thus introduced dielectric strength (1.5kV) and creepage/clearance (4.0/2.5mm) for the insulation barrier.

However, it is important to remember the F-type isolation is based on a hypothetical situation (patient getting a mains shock from other equipment) which rarely exists in the modern world. In truth, it is not important for the original reason. However, there are many other benefits to having an isolated appied part, so it is not something worth fighting.

For the situation (2), it is possible that voltages inside patient circuit (e.g. +/-5V for an ECG circuit) could cause loop currents if the isolation barrier failed and the patient is earthed.

By convention small voltages associated with signal electronics (e.g. 5V) are usually ignored when assessing the F-Type insulation barrier. The reason is that an F-Type insulation must typically pass basic insulation for mains voltage (e.g. 1.5kV, 4.0/2.5mm), which usually exceed the limits for double insulation for the signal voltages.

But, if your design has higher voltages normally associated with the clinical function (e.g. 100V stimulator), then double insulation needs to be properly assessed. For example double @ 100V (3kV, 6.0/3.2mm) would be higher than basic @ 230V (1.5kV, 4.0/2.5mm).
 
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R

raymond02302

#3
Peter gave us a detailed clarification for these situations.
In fact, we just keep this basic principle(At normal condition,there should be 2MOP in place, there shoul be 1MOP in place at single fault condition) in mind, any problem for electirc shock will be simple to understand.
For situation 1, this was defined as B-d in the 2nd edition. The condition is assumed that the patient get a mians from other equipment, which is an fault condition. To prevent the patient form electric shock (from current loop mains>patient>patient circuit>enclosure(in particular, earthed metal enclosure)>mains), there should be 1MOP in place here based on the principle above. Therefore, It should apparently be basic insulation/1MOP.
For situation 2, definded as B-e in 2nd editon, the voltage in the applied part is generally used for physical intention, the correct/intened current loop should be as applied part >patient (specific part of body)> applied part to complete the physical intention.
But, there may be another current loop applied part>enclosure>paitent (not the specific/intended part of body )> applid part. In this situation, there will be an electric shock to patient. To eliminate this conditon (normal conditon), there should be 2MOP/reinforced or double insulation.
 
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