F-typed isolated barrier of medical electrical equipment

[Onion_Mr]

Hi everyone
Thanks Google bring me to this cove.
We have a medical laser removal device which powered by PSU. You can control the laser beam emit when the touch pins contacted the skin. The device is intended to use in home healthcare environment.

We defined the touch pins as BF applied part. We found there is no isolated barrier between touch pins and main board and the PSU can only provide 2MOOP between PRI and SEC.

My question is
Cl.8.5.2.1 of 601 3rd standard said F-type applied part shall be separated from other parts.
Is 2MOPP provided between PRI and SEC adequate? Or can we provide impedance protection component (4mm/1.5kv) between touch pins and main board?

Look forward to your early reply. TKS

 

[Peter Selvey]

F-Type insulation is a special barrier between the patient and everything else, including earth, general secondary circuits etc. It is in addition to the normal pri-sec isolation.

It is a historical based barrier derived in the 1970s at a time when ECG circuits used to be grounded (earthed). There was a fear that if the patient somehow touched mains (e.g. faulty cheap TV, radio ... remember this was in the 1970s) they would be at heightened risk. Normally our shoes, skin, flooring helps to reduce risk from a direct mains shock, but this is all bypassed if the medical device solidly grounds the patient.

Initially it was a simple requirement (current <5mA if mains applied), but it evolved to a full isolation barrier with 1.5kV/4mm creepage. Of course in the modern era we can consider exposure to direct mains to be negligible probability, so the insulation seems superfluous and the purpose is often confused as being involved with the mains isolation in the equipment.

However it is retained and has other uses such as keeping various patient connections isolated, which limits interference and other problems.

 

[3dBUnderThe Limit]

Thanks? Peter for the background information, but I don?t think that we should under estimate the risks associated with non-medical devices in the patient area. In particular in the home environment where there is virtually no control of devices used by the patient. From personal experience I have felt the gentle tingle from power adapter with failed insulation.

For home use the F-type applied part is also needed for a CLASS I device unless you want to use fixed installation.

The question from Onion_Mr still needs an answer. My opinion would be that there would work with 2MOPP pri-sec, but that would lead to that the entire secondary side would be a part of the applied part, not a practical solution in most cases.

 

[Krisp]

Just like 3dBUnderThe Limit mentioned, if you would rely on the PSU's MOPP, then the whole secondary side will be deemed to be an applied part.

However, you should consider the use of other technologies such as capacitive sensors and infra red sensors that would inherently provide the insulation barrier that you need.

One other option is to use opto-isolators to isolate your touch sensing circuitry from the main board circuitry.