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I am looking at that Schurter's AC power entry modules to choose a AC filtering module and at page 5 I noticed the differences between Standard version and Medical version:

medical standard version AC

Their only difference is the resistor R in paraller with Cx. So the medical version only cares more that someone can get shocked by the AC being left charged due to Cx?

(I checked the rest of the datasheet, L1, 2, 3 are the same on both versions. Same with Cy.) The only difference seems to be the R resistor.

My question is, is there any other difference to those two versions? I could not spot any, and from my zero experience in mains filtering, it's hard to understand why placing this resistor makes the circuit suddenly "medical rated".

Null
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Christianidis Vasileios
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    Yes, "So.. the charge in Cx?". I fear applying jobs in medical stuff, as what you implied in your question: "only that?". And yes it is, most likely. But, I recommend you to read the specification of the "Medical standard". There are fine details that you must agree, and makes sense as well. That R can come to compensate leakage in Cx, too, since Medical stuffs should have tighter margin. Without reference to the exact section in the standard, not many would be able to answer. – jay Nov 19 '21 at 15:51
  • I just added the question in a more clear way. @jay okay I think your comment covered me there. – Christianidis Vasileios Nov 19 '21 at 18:37
  • @ChristianidisVasileios , you are demonstrating a good understanding of Mains. You just need to get used to it. :) Thanks for having me to help! ... My PADS crashed on me, it took, along with, all the work I have done today.. When it gets sluggish, I should have saved. – jay Nov 19 '21 at 18:41
  • Interest only: I have received a shock from the pins of a device after unplugging it. In my case it was only a nuisance. Worst case it could be worse. – Russell McMahon Nov 30 '21 at 03:38

1 Answers1

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Note: this first part of the answer is secondhand info, I was involved with a medical research project for a few years and did some work on a prototype device but i'm far from an expert and I moved on from the project a few years ago.

AIUI modern versions of the medical standards make a distinction between "patient protection" and "operator protection" with the latter having requirements similar to the commercial standards while the former has stricter requirements, but there are still some areas where the standards for even "operator protection" are stricter than those in commercial standards.

IIRC the medical standards require you to consider "touch current" under "single fault conditions" and for portable equipment they point out that what is earth leakage current under "normal condition" is likely to become touch current under "single fault condition". This means your earth leakage budget can become quite constrained. I do not know if standards for other types of equipment have any similar requirements since I have not read them but I suspect they either don't have them or they are significantly more lenient.

The medical standards do have requirements about capacitor discharge but I don't remember what they are and I don't know if the standards for other types of equipment differ.

Getting back to your datasheet, I notice the following differences between the "medical" and commercial versions.

  • The medical versions have a much lower earth leakage specified on the first page.
  • The standard versions have a 2.2nF Cy, the medical "M80" versions have the option a 0.47nF Cy and the medical "M5" versions have no Cy at all. Presumably this is at least part of how the much lower leakage specifications are achieved.
  • The medical versions as you have noticed have a drain resistor to drain the Cx capacitors and presumably also any other capacitance inside the device that may be exposed to the input terminals.

Strangely there is a diagram for a "M80 with ground line choke", but there don't seem to be any inductor or capacitor values specified for it, nor does it seem to appear in the list of order numbers.

Peter Green
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