Power brick failure : 500 ohm between phase (or neutral) and earth

[ratatax]

Hi,
I ordered many power supply bricks (230V AC -> 12V DC) from a chinese supplier, and I'm very satisfied with them except that very few units (about 1 of 200) are defective.
I'm ok with that but am curious about what goes wrong in the defective units. Both have the same issue : plugging them immediately triggers the mains breaker. Checking the AC input with an ohm meter shows a resistance (about 500 ohm) between one of the AC pins (phase/neutral) and earth. Resistance only shows up in 1 direction otherwise it's infinite (like a diode).
I haven't opened the plug pack yet since they're welded.
Do you know what common failure in SMPS power packs usually causes this ?

[Zero999]

That's unacceptable. Return all of them, if possible, otherwise junk them. They're not safe. Blacklist that supplier and never buy from them again.

[nightfire]

Could be the bridge rectifier that is faulty. Otherwise there is not much on the primary side of a SMPS that would exhibit such behaviour- maybe also one of the Y-capacitors between L-N or N-PE is somewhat broken too...

[wraper]

Satisfied? You effing got a batch of death traps. Most likely there is an insulation failure after a rectifier. Likely in transformer between windings and output GND is connected to earth. So you are you are measuring a diode in input rectifier through a faulty transformer. Ordering so many PSUs from china without taking one apart and testing insulation at high voltage is insanity.

Do you know what common failure in SMPS power packs usually causes this ?

Only a very severe safety critical failure can cause this. So it's extremely rare in devices that actually passed a real QC and were made according to safety standards.

[Zero999]

What else is wrong?

Have you looked at the PCB? Does it have sufficient creepage and clearances? Have you dismantled the transformer? Does it have sufficient separation between the primary and mains windings? Is the tape strong and heat resistant enough not to melt?

It's not your job to do this. Return them all and never buy from this seller again.

[bdunham7]

I haven't opened the plug pack yet since they're welded.
Do you know what common failure in SMPS power packs usually causes this ?

Please cut open a defective unit and post good photos.  I'm sure you'll get plenty of feedback here.

I have not seen SMPS failures like this at all, and I've poked, prodded and repaired a fair number.  This type of failure should never happen, and I think it is the second worst possible failure to have with signficant fire (as in potentially burning down your house) being the worst.  I don't what you are doing with these, but if you are selling these as a commercial product, your potential liability is extraordinarily high.

[ratatax]

Mmmmh OK. I'll open one and do more investigation. By the way I checked a sample before ordering many units, I'm not that crazy. I opened it and checked the construction looked OK, but I didn't remove the transformer to check its insulation.

If there is a safety issue I really want to take it seriously. I'll open the defective unit and post pictures of it

[ratatax]

OK I cracked it open. It's the faulty one.
To my not-so-experienced eye it looks fairly well constructed, with good isolation distance and not so cheap-looking components.

What should I check first? Transformer? Safety capacitor?

[Monkeh]

Perhaps the obviously crushed earth wire across the live heatsink?

[wraper]

Perhaps the obviously crushed earth wire across the live heatsink?

Yeah.

[ratatax]

Oh wow, you're right. They cut a slot in the heatsink ON PURPOSE to leave some space for it, but for some reason the wire isn't inside the slot. Now I really want to open a working unit to see if the wire is also mispositionned.

[ratatax]

I confirm, the wire was so crushed it had its copper in contact with the heatsink.

[wraper]

Construction looks OK on a first glance. The main safety concern normally would be a transformer. And there is a design flaw which allows earth wire to be jammed into the heatsink if it is dislocated from the cutout. And likely no proper Hi-pot test in production to catch that. However insulation may be pierced also during usage if someone steps on it or it is dropped.

[wraper]

I suspect earth wire was supposed to be shorter so it could not be put outside the slot, or somehow fixed in the slot.

[Monkeh]

It does look vaguely okay for a low cost supply. Some signs of, uh.. cost optimisation.

The total lack of QC and hi-pot testing is a concern. Alternatively, they do both.. and just sell the failures cheap on the grey market to you.

[floobydust]

This Big Clive cheap brick failure is worth watching, because the cause would be caught if safety tested and it's a no-brainer for EE's.
You save nothing as these are powering gear worth 100's of dollars and can electrocute as well.

[Zero999]

I confirm, the wire was so crushed it had its copper in contact with the heatsink.

Is there a burr on the heatsink which is chafing on the cable?

It's good you've learned about this, but you should send them all back and demand a refund.

[ratatax]

I will alert the manufacturer on this issue and check a few working units. Now my stress is at max level, I really hope the working units do not have a crushed wire waiting to fail 

I selected this manufacturer (XINSPOWER) after checking five different PSU's from different suppliers, this one was the most quality-looking and also provided the best efficiency and output ripple compared to the others.

[floobydust]

You want hard proof they have safety approvals, always ask for the product's certificate.
P.S. - where is this thing's fuse? First thing to look for.

[ratatax]

I confirm, the wire was so crushed it had its copper in contact with the heatsink.

Is there a burr on the heatsink which is chafing on the cable?

It's good you've learned about this, but you should send them all back and demand a refund.

What do you mean by burr ? Heatsink looks normal. Doesn't look sharp to cut anything so the pressure against the case during assembly must be huge.

Opened a working one. Wire is correctly positionned. It's held in position with silicone against the capacitor. I guess this is a manual step done in their factory, which is subject to mistakes. Disappointed that they didn't test the units. Unless they tested it and it failed during shipping with vibrations causing just enough damage to the wire to make contact with the heatsink.

[Zero999]

I confirm, the wire was so crushed it had its copper in contact with the heatsink.

Is there a burr on the heatsink which is chafing on the cable?

It's good you've learned about this, but you should send them all back and demand a refund.

What do you mean by burr ?

https://en.wikipedia.org/wiki/Burr_(edge)

If it's not sharp, then it's purely an assembly error, as you've said.

[ratatax]

You want hard proof they have safety approvals, always ask for the product's certificate.
P.S. - where is this thing's fuse? First thing to look for.

I checked it has a proper fuse inside heatshrink tube.
The capacitor between primary and secondary is jd222my1. 400V rated but I don't know if it's a proper safety one with the peak handling of several kV

[ratatax]

I confirm, the wire was so crushed it had its copper in contact with the heatsink.

Is there a burr on the heatsink which is chafing on the cable?

It's good you've learned about this, but you should send them all back and demand a refund.

What do you mean by burr ?

https://en.wikipedia.org/wiki/Burr_(edge)

If it's not sharp, then it's purely an assembly error, as you've said.

Yup! Thank you everyone for the super quick replies on this thread! Not sure about what to do yet with these PSU's, it's quite hard to return anything to China. And if I switch to another manufacturer, it can be worse than that...

[bdunham7]

Now you don't know if the remaining ones are assembled OK or if they just haven't broken the insulation through to make a short yet.  Do you know anyone with an X-ray machine?  If you got the right angle, you might be able to confirm that the wire is cleanly and entirely in the slot.

[srb1954]

I confirm, the wire was so crushed it had its copper in contact with the heatsink.

Is there a burr on the heatsink which is chafing on the cable?

It's good you've learned about this, but you should send them all back and demand a refund.

What do you mean by burr ? Heatsink looks normal. Doesn't look sharp to cut anything so the pressure against the case during assembly must be huge.

Opened a working one. Wire is correctly positionned. It's held in position with silicone against the capacitor. I guess this is a manual step done in their factory, which is subject to mistakes. Disappointed that they didn't test the units. Unless they tested it and it failed during shipping with vibrations causing just enough damage to the wire to make contact with the heatsink.

It might have passed the factory test but with pressure on the wire, and some heat from the heat sink, the insulation on the wire would have flowed away from the pressure point over time leading to the conductor eventually being exposed.

The application of the silicone looks to pretty haphazard to me and shouldn't have been relied upon as additional insulation. I would have expected that where the wire passes through the slot in the heat sink it should have been protected by another insulating sleeve that is both temperature and abrasion resistant.