Safety Requirements for non-isolated AC/DC power supply

[kami7jun]

Hi,
I'm working on a non-isolated AC/DC power supply with universal AC line input and 380VDC 3kW output. The output is for sure not isolated from the input.
This power supply is supposed to supply some lighting applications and for safety purposes all the metal equipment in the load are connected to the AC safety ground. Now I have two questions:
1) Can I connect the DC output negative leg to the AC safety ground (considering the fact that it is not isolated.) If not, why is that not possible?

2) Do I need to worry about GFCI on the DC side (for personnel safety)? I searched a lot for this issue and I couldn't find anything on this. There is ground fault protection on PV arrays but that is solely for fire hazard protection and not human protection.

I'd appreciate it if we can brain storm here about the issue to find a solution.
Thanks

 

[mtwieg]

For 1), no you definitely can't tie the the output to earth/safety ground. If you do, then it can no longer function as a safety ground, because is it carrying load current all the time. Same reason you're not allowed to short neutral to ground in an appliance.

 

[chuckey]

380v is not a mile away from 440V AC, look up three phase generator sets and see what they do.
Frank

 

[schmitt trigger]

"This power supply is supposed to supply some lighting applications..............."

No lighting application can use 380 VDC directly. There must be a secondary conversion process, which most likely will use a transformer based topology (forward, flyback, pushpull, bridge). I sincerely hope they are doing that, and not attempting to do it with a non-isolated topology.

 

[Dan Mills]

Plenty of arc ballasts use what amounts to a non isolated PFC stage followed by non isolated buck converter.
Consider a long arc krypton lamp or such, maybe 150V or so, and maybe a few KW, the supplies are usually non isolated (And you can tell when the cooling water is contaminated because it shorts the ignition pulse).

Not everything in the world is a flea power flyback.

Regards, Dan.

 

[schmitt trigger]

"Not everything in the world is a flea power flyback."

I'm perfectly aware of that fact.

But I'm also aware that a long arc krypton lamp is a extremely specialized, very niche lighting application.

The immense majority requests in this forum, or any other forums, are related to LED lightning, or perhaps CFL.
My error was to assume that the OP was looking for a solution for either LEDs or CFLs. In those instances, my comment applies.

 

[Dan Mills]

3KW @ nearly 400V seems more then a little over the top for most LED applications however.

Still I would say that if the OP is asking that kind of question about a non isolated power supply at this level the answer should probably be 'get someone else to design the thing', seriously there are no second chances with that kind of DC.

Regards, Dan.

 

[schmitt trigger]

"seriously there are no second chances with that kind of DC."

I fully agree with that.

People who have not worked with high power DC don't realize that a high power DC short is not self-extinguishing like AC.
There is not the benefit of zero crossings, where the current actually goes to zero. A DC short has to be blown away.