Safety Requirements for Bioimpedance Measuring Device Power via USB Cable

eezud

Inactive Registered Visitor
I want to design a a battery powered device which measures bioimpedance. It is intended to be powered only by an external battery pack (5V) via a USB cable. I want to make sure that the device is safe and fulfills the IEC 60601 safety requirements. Although the intended power supply is only 5V, I see that the use of USB cable increases the risk of misuse. e.g., the user can connect it to different USB power source. Also, the external battery pack and its charger are not medically approved.
1. Is there any special requirements for USB power supply in 60601? Or, do I have to treat it as the mains supply (240V)?
2. Will using a medically approved isolated dc-dc converter inside my device be enough for the isolation requirements?
 

Peter Selvey

Leader
Super Moderator
If the equipment bioimpedance, then it needs a Type BF isolation barrier which means the measurement circuit is isolated from all other parts by a 1.5kV/4mm barrier (isolated dc/dc power supply, optocouplers etc, visible 4mm separation between tracks on the PCB etc).

Apart from this there are probably no special concerns.

Other parts of the equipment can be treated as operator accessible and only need to comply with IEC 60950-1 requirements.

If the equipment needs to be connected to a PC for normal use (e.g. for display of data or setting parameters), then if the PC is not specified it is up to the user to verify the PC is IEC 60950-1 compliant. You might want to consider risk that the user cannot practically do this, as has been discussed in other threads. In that case a possible risk control could be to increase the BF isolation barrier to 4kV/8mm. Another option may be to advise the use of well known trustworthy brands.
 

eezud

Inactive Registered Visitor
Hi Peter,
I would like to understand more clearly. The standard says that ME shall have 2 Means of Protection. I checked the table 3, and it states the test voltage for reference voltage:
212 < U < 354V, 1 MOPP = 1.5kV and 2 MOPP = 4KV

and table 8, says
for U < 250ac, 1 MOPP = 4mm, and 2 MOPP = 8mm

I guess your suggested values are from table 3 and 8. Does it mean that if other operator accessible parts of the ME comply with 60950, I just need 1 MOPP?
 
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