CSA / UL Approval & NON-isolated power supply

[mainr]

We have a design that uses a NON-ISOLATED power supply. By this I mean that we generate 5VDC from 120VAC, but the local "ground" reference for 5VDC is the AC neutral.
This works fine when wired properly (120VAC HOT wired to the "HOT" terminal, neutral connected to the neutral terminal).
However, should the unit be mis-wired (HOT & Neutral swapped), the ground reference could wind up at 110VAC potential to earth.
The ground reference signal is wired to other devices outside the main box.

Would it be acceptable to the standards bodies (CSA/UL) to provide a relay on the input, such that if hot & neutral were swapped, the relay would open, and kill power to the circuit?

Moving to a fully-isolated design would be the best solution, but would be expensive, since there are multiple signals that must cross the isolation gap. At least two of these signals are analogue.

Thanks for any help.

 

[jghrist]

You should make the ground reference for 5 VDC the ground, not the neutral. The neutral should only be connected to ground in one location. An equipment grounding conductor (EGC) should be run to the equipment.

 

[LionelHutz]

It should not be acceptable no matter how you try to protect against miswiring. The neutral is not a wire that can be connected to components people can touch.

 

[mainr]

Thanks for the responses.

These conductors are normally accessible only during installation &/or servicing. Presumably this would only be done by licensed electricians.

The intention is not to connect AC Neutral & "EARTH" (AC "Ground") at a second location, but to reference the 5V supply to AC Neutral (AC Neutral becomes the "GROUND" reference for the 5V supply).

 

[DRWeig]

Are you trying to get Class 2 listing for this supply? If so, you'll need to isolate.

Look at UL1310. You'll need to arrange protection from reverse polarity on the input even if isolated.

There's a leakage current test from each output terminal to the grounded supply conductor (neutral), which you won't pass if one of the output terminals is connected to neutral.

Many other reasons in UL1310 why not to do this.

If not trying for Class 2 listing, you'll be looking at UL508 if an industrial control panel, or some other standard that applies to the end-use product. Let us know what that end-use product is, and we may be able to provide more of an answer...



Good on ya,

Goober Dave

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