Understanding DC Patient Leakage Failure from 0.3VDC

[michael Cejnar]

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?

[Stijloor]

A Quick Bump!

Can someone help Michael?

Thank you very much!!

[Peter Selvey]

First to address the theory side: 0.2Vdc is absolutely safe in this situation. The body approximates 1kohm impedance only for higher voltages (around 100V), for very large, wet, contact surface areas (~100cm2) or if contact is made directly with the blood stream (e.g. catheters). Experiments with 0.1V found that it was possible to get ~100uA but it required two wet hands firmly pressed face down on aluminium foil, hardly a normal situation.

Next to address the standard:
Under Clause 8.4.2 c) is it possible to have exposed areas in medical electrical equipment up to 60Vdc, provided that risk of those parts being brought into contact with the patient (either directly or indirectly) are acceptable. Although the standard refers to the probability of contact, the final judgement should be based on the probability of harm. Noting that for 0.2V you would need large contact areas for significant current to flow, and that for dc you need long duration for harm, it's pretty clear that the risk is acceptable.

[michael Cejnar]

Thanks Peter
I concur with the 0.2V As a clinical electrophysiologist, I know even with a pacing electrode in the heart, 0.2V is well below the chronaxie for any kind of stimulation.

Yes, you have quoted 8.4.2 c) before, but strictly speaking it specifies a list of exempted parts, which do not include a simple exposed part of an enclosure in question. But if this is the closest exemption, then so be it.

While I have found an easy way to insulate the surface just to be sure, the issue is nevertheless intriguing.