Hello
I am looking for advice from the emc group on the ESD / EMC design
of an RS-232 port device that is a patient connected medical device.
I am one of those troublesome technicians that has to test and
certify our company's products, and then report back the results to
those in charge. Esd testing is going badly because I am breaking
things at low discharge levels.
First off, our product requires 510K approval. During normal of the
device, the patient is capacitively coupled to the device, other times
the patient can be directly coupled to an ECG connection. It is roll
around device the plugs into a 15 amp outlet, In most installations,
the unit will be connected to an external modem (supplied locally by
that country, meeting their reg requirements) for external service
diagnostics. Our RS-232 cable design has an full outside shield
(for EMC) which connects the sub-d backshells together, but the
mounting screws are exposed metal. The RS-232 connection on
the back of our product has been designed to float the backshell
and mounting hardware. I presume the design reason here was to
have an extra layer of isolation from the patient.
I see 2 problems (maybe more) with this design:
1) This design negates the nice 360 degree EMC shielded cables
by isolating the sub-d backshells and mounting hardware from
ground.
2) PCB layout needs lots of trace isolation, high breakdown RS-232
transceivers and opto isolators to standing off the ESD discharge
to ground. The nice Maxim parts survive but the optocouplers
(2.5 kV RMS) aren't so lucky. A side effect of the design is that the
isolated circuity floats and maintains the ESD discharge for a while.
I would like to ground the sub-d backshell and mounting hardware
at the bulkhead and be done with it. Comments???
g GE Medical Systems
_____________________________________________
David Garnier
Senior Technician
Ultrasound Engineering
TEL: 414-647-4286 FAX: 414-647-4090
EMAIL: [email protected]
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