MOPs needed after Isolation Transformer

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I have been lurking this great site trying to find an answer to a question. So far I haven't been able to find it, so I thought why not give it a go.

I have been assigned to make an electrical isolation scheme for clarifying the MOPs in our system.

The setup of our device is like this, we have mains connected to a 1:1 isolation transformer. On the secondary side we have several power supply's and a PC with a UL mark. I was wondering, since we have a 2 MOPP isolation transformer, do I also need to state the MOP of the power supply's on the secondary side of the transformer?

If so, where can I find how much MOPs the power supply's (can I find this on the UL website or do I need to ask the manufacturer?)

Hopefully you can help me!


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I would like to quick bump, I have the same question. Do you need medical grade PSU (2 MOPP) if they are connected to medical grade isolation transformer or it is enough to have EN 62368-1 compliant PSU's (2 MOOP provides 1 MOPP) connected to it? The secondary of PSU is separated from CF applied part by other components. Maybe someone does know where I could look up to? I do not find information about separation in this kind of power supply configuration.
In my opinion it should be enough to have medical grade isolation transformer with EN 62368-1 SMPSU's but I it would be great to get confirmation from the experts.
In such case the main consideration left is non-medical grade PSU leakage currents (it should be checked by in house measurements).

Peter Selvey

Super Moderator
The situation: a 2 MOPP isolation transformer with various 950/62368 SMPSU on the secondary side (intermediate circuit).

It's actually a complicated due to the EMC capacitors in the secondary SMPSUs that often make the intermediate circuit live even though it is "isolated". Especially if there are a few PSUs, the cumulative EMC capacitors could easily create a circuit that can supply >>1mA if the intermediate circuit was touched directly, or for example measured using the leakage current test circuit. It's certain that it would be >0.1mA that makes it "live" according to the standard.

However, there are a couple of mitigating factors.

Assuming the capacitors are balanced (L-E, N-E), the open circuit voltage will be half of the transformer secondary. You need to draw up a circuit to see how this happens, but you can also just measure it as well.

Also it is common to use a 110V secondary even though the mains supply is 230V.

The result is that the working voltage of the "live parts" in the intermediate circuit can be as low as 55Vrms.

In which case, the SMPUS needs to be assessed for 2 MOPP @ 55V, which is usually OK if it is certified for 2 MOOP @ 240V.

The other informal point is that having a transformer will reduce the overvoltage transients in the intermediate circuit. So while we are stuck with Cat II in the standard, in the background we should really be using Category I, which would require lower values for clearances.

Upshot is that it is safe but the proof is a little complicated.

An option here is to quietly ignore it all and pretend the intermediate circuit is not live.

In my life as a test engineer, I have written it up properly, but then you need the reviewing engineer and potentially certification reviewer, accreditation auditor on board as well, and the chances of one of them not understanding is high especially if they haven't see the situation before (which is fair enough, it is complicated ...). So ... what they don't know doesn't hurt? These days I might not try to explain in the report but I would put it in the datasheets.
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