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Safety Requirements for non-isolated AC/DC power supply

2248 Views 10 Replies 5 Participants Last post by  Big John
Hi,
I'm new to the forum and this would be my first question.
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
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...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?
You can't do that because, like you said, this is not isolated from the grounded AC side. Every time the negative diodes are forward-biased it will simply be creating a hard short-circuit to ground.
2) Do I need to worry about GFCI on the DC side (for personnel safety)?
What's the application? If you put GFCI protection on the AC side it will automatically cover the DC side because you don't have a separately derived system.
 

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Discussion Starter · #5 ·
Thanks everyone.
The power supply would be used for lighting applications.
Big John,
I thought the AC side cannot detect the ground fault because of the rectifier which is connected to both line and neutral. Isn't that a valid assumption?
If not so you think AC GFCI would be enough for this application?
Thanks
 

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...I thought the AC side cannot detect the ground fault because of the rectifier which is connected to both line and neutral. Isn't that a valid assumption?
Not if this is just a plain-jane rectifier. There's no isolation from AC to DC through a diode. It's no different than turning a relay on and off 120 times a second; the current that passes through the bridge is still referenced to the line-side grounded AC system.
...If not so you think AC GFCI would be enough for this application?
If you need GFCI protection and this is truly not an isolated DC supply, it then AC side GFCI protection will protect the whole circuit.

But you haven't provided enough information to determine why you need GFCI protection
 

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Discussion Starter · #7 ·
Technically my original assumption was that since the regular GFCI on the AC line outlet cannot protect the DC side so I would need GFCI protection. But if the AC side actually DOES detect ground faults on the DC side, then there would be nothing to worry about.
Just out of curiosity, if the DC side was isolated and the negative leg was connected to earth, was there any safety regulations to force the circuit to have GFCI on the positive leg on the DC side?
 

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...Just out of curiosity, if the DC side was isolated and the negative leg was connected to earth, was there any safety regulations to force the circuit to have GFCI on the positive leg on the DC side?
I don't know Canadian code, nor do I know the application where you're using this, so I can't say.

In the states there's no code requirement to GFCI protect anything like this. And I don't know if there even exists any DC ground-fault detection equipment that's ETL listed for use as personnel shock protection.
 

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there are voltage limitations in the NEC for luminaires in dwellings, and listing requirements for dc luminaires, don't know about CEC tho. however, since the OP is sparce on info, I doubt he'll get accurate info anyway.
 

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I don't know Canadian code, nor do I know the application where you're using this, so I can't say.

In the states there's no code requirement to GFCI protect anything like this. And I don't know if there even exists any DC ground-fault detection equipment that's ETL listed for use as personnel shock protection.
Bender probably does, most of there stuff is UL/cUL.
 
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