1
\$\begingroup\$

I'm looking at the data sheet for a Meanwell NPF-40 PSU. This is a class II unit and hence does not have an earth connection (see also mechanical specifications, page 6), yet the data sheet specifies a leakage current of <0.25mA @ 277VAC.

I was wondering how a class II unit without earth can even have leakage current. Where does the current leak from and to? This is a similar question, but the accepted solution states that there is leakage from the DC ports to earth, which I don't think can happen with this PSU.

\$\endgroup\$

3 Answers 3

2
\$\begingroup\$

If you look at the datasheet block diagram, there is a capacitor between mains input block and isolated DC output. It is there for EMI suppression. Also even if it was not there, there would be stray capacitance over the transformer coils that can leak AC voltages. Oh and forgot to mention, mains voltage is usually referenced to earth as neutral and earth are connected at some point anyway.

\$\endgroup\$
2
\$\begingroup\$

This spec refers to AC mains safety. They mean leakage current from where it's supposed to be (mains hot and neutral) to anywhere else.

You are correct that there is not a 3rd current path as supplied; however they are talking about an added third current path, e.g. A grounded human touching chassis (or a hot-energized human touching chassis, for that matter). A grounded human touching either of the DC outputs. The unit's Vss output being tied to the chassis of a 2-prong-wired unit that has a ground fault and is floating at 230V for instance. It is a power supply, after all; it will connect to other things.

Specifically, any such leakage will manifest as a difference in current flow between mains hot and neutral. This is relevant because this is how GFCI/RCD protective devices detect ground faults.

So this is a vouch; they are saying you should be able to put 20 of these on a circuit without worrying about a 6ma GFCI trip. And that the leakage current should be non-dangerous.

An example of noncompliance would be a power supply where the DC outputs bounced with the AC sinewave; at the top of the positive bouncing ball the two rails are 170V and 175V, respectively.

\$\endgroup\$
0
\$\begingroup\$

My experience with power adapter SMPS primary AC 240v , Secondary 24v , 4 Amp , DC . If Human body is touched to tip , the leakage is reducing to 12V. Hence I have used 10 nF across -ve to earth , it reduced the undesired ac voltage on secondary side (over required DC voltage) from 49.5v to 34.3 V. Further if 20 nF (10 nF x 2 parallel) is used, ac leakage reduced to 29.1V. Hence if we connect 22nF or 100 nF it may be reduced to negligible. However relevant isolations , proper installation , protections / insulations to be taken care for the above job.

[AC leakage current path]

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.