MOPP in battery powered device with metal enclosure

F

fehtr

#1
Dear All,

We are developing a battery powered wearable device. The device has a metal enclosure that is type B applied part. The enclosure is connected to circuit reference (battery minus terminal) for electromagnetic compatibility.

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The problem is identifying needed MOPPs

If we don’t provide any MOPP we need to consider all of the following as normal condition (according to 8.1):
  • short circuit of any or all insulation that does not comply with the requirements of 8.8;
  • short circuit of any or all CREEPAGE DISTANCES or AIR CLEARANCES that do not comply with the requirements of 8.9;
But short circuit of insulation (internal circuit ↔ enclosure) will cause operation of protective devices. It seems there is no electrical hazard in this situation.

Leakage current will be exceeded if the following occurs simultaneously:
  • separation (electrically) of the enclosure into two parts while the first part is connected to circuit reference and the second part becames floating
  • short circuit of energized internal circuit to the second part of the enclosure
But this situation seems highly unlikely. It can be considered as acceptable risk.

Nevertheless IEC 60601 requires two MOPP. The only way to show 2MOPP is to separate enclosure from circuit reference and provide 4/2 mm creepage/clearance.

Is there a way to comply with the standard when enclosure is connected to circuit reference?
Can this design be considered as safe?

Sorry for my English
Thanks in advance
 
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