I already started the RMA process after calling in...I'm curious to see what others might think.
 
I had this power supply hooked up to one of my systems that I didn't use much. It was working fine, until last night when I powered it on, pretty sparks and smokes came shooting out of the 24 pin connector. Upon hooking up another power supply, the computer no longer boots.
 
I stripped the computer and found that the motherboard is fried, you can see burnt spots under the 24 pin connector port where the action was coming from. I have no idea what caused this exactly, but I did not want to test the power supply with any of my other hardware for obvious reasons...I also do not know as of yet if any of my other parts are fried as well, cpu, ram, gpu, nvme, until I get another mini itx Z390 motherboard to test.

 
The gentleman on the phone was pretty quick to rush me off of the phone without telling me anything else on what would happen next, if this power supply is responsible for frying my motherboard and potentially other parts? Nothing. He originally insisted on just sending me a new 24 pin cable and to test it with that and I heavily disagreed and told him I am NOT going to do that as we do not know what caused this and if there's a problem with the PSU itself, why am I gonna risk frying another motherboard/potentially other parts? Not to mention some simple searching regarding this PSU brings up a bunch of problems that people state with it of which I don't know is related or not.
 
ASRock Z390m-ITX/AC motherboard.
EVGA 850 G3
Intel 9400 CPU
Corsair 16gb RAM LPX 3000 ram
Zotac 1660ti
Samsung 970 nvME
 

It’s very strange that this would happen without there being a cause. I would suggest sending the 24pin cable back with the power supply so it can be tested as well.

Since you said z390, I would assume most of the components are relatively new.

How was the motherboard set up? Was it in a case, and could anything, to possibly include an insect, have gotten in behind the motherboard?

Could the problem have started at another component, and cascaded back to the 24 pin? Usually the power supply would go into over current protection shutting everything down, and then problems typically start as the power supply is attempting to boot. While I don’t see motherboards having issues like this as often, graphics cards have been known to do something like that when a component is failing.

Could you provide more pictures of the motherboard and area that were damaged, as well as the back of the motherboard? It won’t help much here, but I am just very curious and like to look. I would suggest contacting Asrock about the motherboard, and talk to them about getting a replacement. Don’t immediately say, “yeah, the power supply killed it” because they aren’t going to help, but if they will look at the board, they may find a different cause that we can not see.

Cool GTX
Dang, Sorry for your Loss.  Its Never fun to have an equipment failure
It would take a lot of investigating to see if a conclusion could be reached - regarding the Root Cause of the failure
 1) Was it a power surge ?
 2) Was it a short on the MB or device installed on the MB ?
 3) Was it the PSU itself ?
 If the 24-pin cable is Not damaged & you have the "test start Cap" - then you could test the PSU --> Outside of the PC
 These questions will come up so I will ask:
 Was the PC plugged into a power strip & was it a surge suppression type or did you use a UPS ?
 Were Only the factory supplied cables used to connect the PC ?
 

No power surges, also plugged into a surge protector. 
The motherboard specifically, as shown in the picture.
Factory cables that came with the PSU only.
 
I do have the test PSU clip that I was going to test, but ran out of time last night.
I'll try that when I get home...but the 24 pin cable reeks of burnt electronics.
 

the_Scarlet_one
It’s very strange that this would happen without there being a cause. I would suggest sending the 24pin cable back with the power supply so it can be tested as well.
Since you said z390, I would assume most of the components are relatively new.
How was the motherboard set up? Was it in a case, and could anything, to possibly include an insect, have gotten in behind the motherboard?
Could the problem have started at another component, and cascaded back to the 24 pin? Usually the power supply would go into over current protection shutting everything down, and then problems typically start as the power supply is attempting to boot. While I don’t see motherboards having issues like this as often, graphics cards have been known to do something like that when a component is failing.
Could you provide more pictures of the motherboard and area that were damaged, as well as the back of the motherboard? It won’t help much here, but I am just very curious and like to look. I would suggest contacting Asrock about the motherboard, and talk to them about getting a replacement. Don’t immediately say, “yeah, the power supply killed it” because they aren’t going to help, but if they will look at the board, they may find a different cause that we can not see.

That's exactly what I intend to do, send in the main PSU unit along with the 24 pin cable.
The components are as new as late november/early December, the board/cpu, ram and gpu some months back along with the PSU itself some months back.
Motherboard in a case, no insects, and I thoroughly inspected all components.
Unfortunately, I already packed up the board and sent it back and having it replaced and once I get that, I will use a different PSU and see if my other components are OK or not.
Though as I said, I thoroughly inspected everything and the picture I provided is the only part that visibly shows damage.
 

CraptacularOne
Looking at your picture it looks like the 12v rail overloaded. 
 What Cool GTX suggested is worth a try too. You can either use the power cap tester that should have come with the PSU or a paper clip and short the green wire to any of the black wires. Then try just plugging in a fan and see if it works. 
 Here is a pinout of a 24pin ATX connector. The arrow is pointing to where the green wire is, you can short that to any of the black wires with a paper clip and test the PSU yourself by simply switching the power button on and off on the PSU itself. 

While briefly after the smoke and sparks show, and almost immediately, the fans that were hooked up were still spinning before I yanked the power. I didn't try turning it back on after that though, so I'm not sure, and ran out of time to test the PSU by itself of which I'll try tonight.
 
Thanks everyone.

in the past, motherboards melted the 24 pin when there was too many devices loaded into the PCI lanes, like 3 or 4 GPU at full load. Usually the problem was the motherboard not being able to support the amount of current being drawn.

If the power supply itself doesn’t show signs of damage, it makes me curious if the power supply was the issue.

Please do keep us up to date with what you find out.

Not the first, and won't be the last. I had an EVGA 1600 T2 kill two of my gigabyte x99 soc champion motherboards. The psu went in the trash, and I bought a seasonic psu. Not really evga's fault though since they don't make the psu themselves.

Tested PSU with detection tool, no fan spin, just a single clicking noise, eco mode off and toggled, no fan spin at all.

Update:
New motherboard received, other components tested, CPU/RAM/M2/GPU/AIO, all working fine. Phew.
I sent the PSU along with the 24 pin cable to EVGA a couple of days ago, will report back once they receive it and determine if anything is wrong or not.

That is Good News

thanks for the update

I might buy PSU tester before you try the 24pin connecter. I had to RMA my Supernova G3 550. Out of the box plugged it into to my good mobo and immediately began to smell burning. It was subtle so I thought it was just current heating up some plastic in the connectors. After the board didn't post I unplugged the cables and tried an old dell PSU I knew worked with that mobo. After that it wouldn't even power up with the old PSU. Still not sure what was at fault doing troubleshooting I plugged it into my Dell mobo and that was much more dramatic. As soon as I switched it on a puff of smoke came up from the mobo and a strong smell of smoke. I bought a thermaltake Dr Power II and tested the PSU which it failed the tests. I got my replacement mobo and a interim PSU lent to me from a friend and it turns out all 4 of my HDs that were connected when the mobo fried wouldn't spin up anymore which is from HD PCB damage usually. Last week they said the PSU tested good but they still shipped out a new G3 650 to me. I got the PSU today and the PSU and all cables tested good. I went and tested the old cables and the old 24 pin mobo cable failed on the tester.