IEC60601-1: Impedance and current-carrying capability (8.6.4)

[cjlark]

I am working with a medical drill system consisting of two main subsystems.

1.The Footswitch which receives AC power from a wall outlet and through a Certified internal power supply converts the AC power to DC power (36V)
2.The handpiece (drill) which connects to the Footswitch and receives the DC power signals to commutate its motor.

While at out 60601 test house we have been told that any accessible metal parts on the handpiece need to pass the testing laid out in 8.6.4a. To me, this doesn't make sense. I can understand why the Footswitch would need to pass this test as there is a reasonable scenario where Mains voltage could come in contact with its metal enclosure. I do not see how this could possibly happen on the handpiece since it is downstream from the AC Mains voltage and only receives DC voltage outputted from the certified internal power supply.

Could someone help me understand why our Handpiece needs to meet this requirement? Is the rationale that it doesn't matter how the handpiece gets energized? We have run some tests on very similar predicate devices and they are not able to meet the 100mOhm requirement. I am wondering why.

Thank you.

 

[Peter Selvey]

According to the standard the test is not applicable.

Clause 8.6 is for protective earthing which if the definitions are read closely are intended for low impedance to the earth of the mains supply for protective purpose. Any part which is 2MOPP/2MOOP away from mains parts is out of scope for 8.6.

However, it may be that connection of the frame to the 0V line of the 36V circuit is used for safety, for example if the motor circuit shorts to the frame of the handpiece, this is picked up by overcurrent protection and the output shuts down with no safety issues. This is similar to protective earthing and tested in a similar way, by inspecting the cable AWG, checking for reliable connections and if needed testing with a current that represents the maximum current of the power supply plus a safety margin. The main difference is in the current used for analysis/testing. For example if the power supply limits the current to 2A, verify the wiring/connections/test are based on say 5A rather than 25A.

Also, it should be noted that in the final installation, there may be installation and periodic tests. Sometimes it is easier to connect everything metal to PE terminal with 16AWG wiring just in case, since the person performing the test won't know the internal construction and whether it needs to be protectively earthed or not. Otherwise, the instructions for use/installation need to clearly state where the protective earth test needs to be done.