C31 and C32.
One cap is common in this position but two in series is equivalent.
https://www.power.com/community/for...capacitor-between-primary-and-secondary-sides

View attachment 266611

Oh yea ! Those two caps. !
My bad.
But tht capacitor is always there in most of the PSU's I troubleshoot had no issues with GND becoming live.
They Y1 caps is small and high voltage. Though they are thru the bridge one cannot say they are isolated.
still most of the SMSP have tht cap. And they are well isolated.

Wait a minute.....!
Last night I fixed a treadmill tht trips RCCB, it happened some wannabe mess with DB wiring and I dunno how but they manage the give 2 phase to that power socket cables where 3 treads were used.
All the 3 tread blew the varistors and after that trips the RCCB. Lot of head banging and found out the that 12V SMSP used to drive the running LED's was dead and causing too much earth leakage.
( The funny part here is the PSU is actually used to boost the LED brightness. The GND was earthed to main earth.
The LEDs were working and it never occurred to me check the PSU or remove it's output for tht matter to isolate earth fault issue. The LED was working thru the arduino supply via main console !!! Go figure )

Swapped the PSU and it works without RCCB tripping.

I think now is a good time to crack open the PSU.

Gimme some time..!

 

It does not matter if the capacitors in series with the mains connection are theoretically "perfect". The hazard is that the capacitance couples the AC current to the output side of the supply. Thus that hazard is intrinsic and not due to capacitor failure, but due to capacitor function.
Using a decent quality (High input resistance) voltmeter, you can measure the AC voltage between either supply output terminal and an "earth ground" and find mains frequency voltage, the exactvoltage will depend on the capacitance. Thus my comment that such supplies shuld only be used for batter charging when the device is not being used.

 

Most product safety standards limit the current allowed to flow through the Y safety capacitors to 0.25mA – which determines the maximum capacitor value for a given circuit; thereby eliminating the potential hazard due to the capacitor function.

 

If there is no other connection to the one side of the mains feed in that circuit shown in post #8, then those capacitors are not safety capacitors at all.
And certainly any who wish to disagree are welcome to grab the output lines of that 5 volt supply and grab a solid ground connection at the same time. The resulting shock will tend to verify that not all circuits are equally safe.

Who wants to go first??
NOTE THAT THIS IS A HAZARDOUS ACTION!!! It is not a safe thing to do!!!

 

#1 It has the CE mark

#2 the input is 40 ma and the output is 600 ma. How could they do that without a transformer?

 

The capacitors are very small value and do not pass enough current to be harmful.
With, for instance, most laptop supplies you can just about feel the current.

ALL flyback supplies (except “medical” ones) have class Y capacitors between rectified mains and output, they return induced hf currents in the secondary back to the primary and are part of the EMC filtering. A maximum current of 700uA is permitted to flow from mains to secondary terminals. However, they must withstand a flash test of 4kV.
It is therefore inadvisable to use a large number of this type of supply in parallel, or with common 0V connection, because the leakage current can build up to a point where itis noticeable.
There is also a possibility that the capacitors can be discharged into the input of equipment connected which occasionally can damage it as the capacitor could have 110V on it, but it is <2nF, so not too much energy.
It is better idea to connect the 0V output to ground.

 

Some small compliant power supplies exist that have the secondary isolated from the primary. The EMI Y capacitor goes to earth when there is a connection to earth. Feedback for regulation is obtained from a feedback winding and the secondary is wound to have minimum electric coupling with the primary. To wit:

 

The circuit shown in post #27 IS CORRECTLY ISOLATED!! No capacitive coupling from either side of the mains to either side of the output. Thus no shock hazard. AND, note that RF1 is a fuse resistor, because things should be fused.
AND, thank You to D.C. for provideing the example.

 

If this danger is true (I have my doubts since they have been used for a long time) shouldn't the mods remove this from the forum site? https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

 

If you look closely at the transformer, you will see an extra parallel primary winding, with one end disconnected. That winding is in phase with the secondary. It serves the same purpose as the the class Y capacitors. The description of how it works is in Power Integrations’ application notes somewhere. It maintains good magnetic coupling between the windings, for high efficiency, but changes the electrostatic coupling, I think.
of course, there is always some capacitance between primary and secondary, a high impedance meter might show the output being 110V from earth, but the current is limited to a lower value than with the usual 2.2nF cap.

 

I see that it is more useful to inform folks of a possibly lethal problem, rather than hide an explanation from them based on the presumption that some will do something dangerous because they do not understand the warning "Caution, Hazard of death" attached to something.
So far most folks observe such a warning, even though it is rather frightening.

 

If this danger is true (I have my doubts since they have been used for a long time) shouldn't the mods remove this from the forum site? https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

It is unlikely that the power supply that is the subject of this thread is dangerous beyond that expected with a 5 volt 3 watt power supply.

 

It is unlikely that the power supply that is the subject of this thread is dangerous beyond that expected with a 5 volt 3 watt power supply.

But those X and Y caps are used in larger things. If they are as dangerous as billy thinks the article onsite should be taken down. I can't believe that if their use is so dangerous, that all of the regulating bodies in the world still recognize their use. It's my opinion this is just another billy fantasy.

 

The power supply in question has the Conformité Européenne mark on it (If it isn't the Chinese Export mark that looks confusingly similar) and that means that it complies with all relevant laws in the country into which it is imported or sold.


You want the one on the left.

 

All who wish to verify the claimed safety of those supplies are quite welcome to firmly connect themselves to the output common line and an adequate mains system ground connection to verify that nothing happens.

BUT!! If it does, recall that "I told you so."

 

All who wish to verify the claimed safety of those supplies are quite welcome to firmly connect themselves to the output common line and an adequate mains system ground connection to verify that nothing happens.

BUT!! If it does, recall that "I told you so."

As every double-insulated flyback converter I have ever seen is designed this way, most people already will have connected themselves between output 0V and earth.