Understanding DC Patient Leakage Failure from 0.3VDC
Can someone help me understand DC leakage current limits.
We have a computer display in the Patient Area. We changed form Mains AC to 12VDC SELV power to enhance safety. However the long DC power cable drops 0.2V along the ground cable, floating the display's functional earth, exposed at the VESA mount, to 0.2VDC above mains ground.
Now, measured Touch Current of 200uADC (~0.2V/1KOhm) exceeds the limit <100uA. Also by Cl 4.6, we have to comply with Type B Patient Applied part leakage limit (Operator could touch VESA mount and patient), which is 100uA for AC, but a minute 10uA for DC !
Clearly, electrocution by 0.2V is hardly terror inspiring. In fact 0.2V can not stimulate the heart in any way.
The standard explains on P254 that "PATIENT AUXILIARY CURRENT…Lower values are given for d.c. to prevent tissue necrosis with long-term application.". and on P252 "PATIENT LEAKAGE CURRENT…The 50 μA current allowed in SINGLE FAULT CONDITION is not likely to result in a current density sufficient to stimulate neuromuscular tissues nor, if d.c., cause necrosis."
Also, the risk of VF from DC is 1/5 that for AC; on p250 the standard says: "e) The RISK [of VF] is highest and approximately equal for frequencies in the 10 Hz to 200 Hz range. It [risk of VF] is lower, by a factor of nearly 5, at d.c.".
So, for risk of VF from DC current, 5 times the leakage current limits might be more applicable; the imposed 1/10 limit, being for risk of necrosis, surely should be reserved for the relatively rare chronic application of general MEE.
Anyway, my immediate problem is: can I rationalise this 15x over limit DC leakage current from exposed 0.2VDC, by stating it is from brief contact with 0.2VDC? Ho do others handle it - surely this must be a fairly common problem, since an 11mV drop on a DC power cable will fall foul of the standard. Or do I have to spend money trying to insulate the 0.2VDC on the VESA mount into a Type BF part?