I have had this PSU for a while now, and never got it to power up when I bought it.
 
But I bought one of the PSU testers and the screen says everything passes.  I have tried on 2 mainboards:
 
EVGA X58 and MSI X470 Gaming Plus Max.
 
Both will not power up with the PSU, but I put it on the tester, and the fan spins up and all of the readings look normal:
 
-12v: 12.1
+12v: 12.1
5vsb: 5.1
PG: 260ms
+5v: 5.1
+12V1: 12.0
+3.3v: 3.3
 
Anyone have any ideas why it will not power up the system?

Hi ObsidianPhoenix,
 
I am sorry the PSU is not working with the motherboard.  I do see here it passes with a PSU tester, so it is probably an installation issue with the board setup.  You can try making sure the Jumpers are connected from the Motherboard to the PC case Power and Reset button, and they are installed according to the motherboard manual.
 
Regards,
Peter N

They are, on both mainboards.  The thing is, I can use a Xclio Greatpower 850 with both mainboards just fine.  It is only the EVGA PSU that will not let them power up.
 
They are both hooked up the exact same way, and both are partial modular boards.  The Jumpers for the power switch and reset switch are hooked up the same way in both systems.  I set both up with the EVGA PSU, and nothing will power up.  Just no reaction.  I remove the EVGA PSU and hook up the Xclio PSU the exact same way, and both systems will power up just fine.

Does the PSU tester (which one?) you used check the 24pin Motherboard and the 4+4pin CPU power connectors too?

Yes it has both the 24 pin and the 8 (4+4) pin connections.
 

And you're not using any cables, such as SATA power, from another PSU?..You're using only the cables that came with the 850 B2?

It is possible I have one from another PSU.  I would have to compare all of the cables pretty closely.  But I will check it out.
 

ObsidianPhoenix
It is possible I have one from another PSU.  

 
That could definitely do it.

Ok, I'm an idiot.  I was trying to use one from another PSU...  I could not find my extra cables that came with the original package, and I borrowed one from my other PSU since I had added a couple more SATA Drives and needed power.  
 
Time to tear apart the house to find the cables...
 
Ugh...

Yep yep..Hopefully no SATA drives got smoked, you can feel lucky if they're OK..Cables are not interchangeable between PSUs the vast and overwhelming majority of the time.