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UL / CSA Approval & non-isolated power supply

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mrmain

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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?

Thanks for any help.
 

mrmain said:
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.
Click to expand...
No offense but it looks like you're prime candidate for a Darwin award...

:!: Anything that's electrically connected to either live or neutral, must be considered dangerous voltage, and isolated such that touching it is impossible for users of the equipment. You should never assume that live wire will always be live, and neutral wire always neutral (only shielding ground like what connects metal outside of a refrigerator to earth, should be touch-safe). Wiring a mains connection as ground reference to other devices (can you touch those electronics?) is reckless, and if someone gets electrocuted that way you can be held responsible. :!:

(in the small chance that the above doesn't apply to your particular design / application: please provide pix of the equipment & how it's used, to convince us that it's safe)

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?
Click to expand...
Regardless of what standards bodies would say about it, depending on a relay to switch things off in case of reversed AC connections, would be dangerous. For starters a relay needs time to switch - time in which you can be shocked & damage done. Also relays can get stuck sometimes, fail to switch or whatever.

The only thing I'd trust relays with safety-wise, is the amount of electrical isolation between controlling side & switched side. And then only if undamaged, sourced from reputable supplier, and specifications don't seem out-of-expected-range by looking at mechanical construction of that relay.
 

Thanks for the response.

None of the conductors in question is accessible, except during installation &/or servicing.

I agree that the BEST option is a fully isolated design.

The relay option is not my solution, but one that I'm required to evaluate.
If a relay were used, I'd prefer it be actuate when the wiring were correct, rather than the proposed "actuate to disable the system when wiring is incorrect". This would be more "fail-safe", although there is still the issue of contacts becoming welded together. The main reason this won't work is that it would require current flow between "AC HOT" and "EARTH" under normal operating conditions. Current flow on earth is not allowed under normal operating conditions, and it would also cause a GFCI breaker to trip, should the equipment be supplied through one.

In the proposed configuration, the relay would configured to actuate when the system were miss-wired (AC HOT & Neutral swapped). The intention here is that the relay would prevent the system from operating, forcing the installer to correct the problem AT INSTALLATION TIME. The issue of relay "failure to operate" (which could be undetected), allowing a miss-wired unit to continue to operate.

I'm trying to get a feel for standard practice for this type of equipment, and I'd like to avoid the Darwin - especially since it would probably be some other poor soul that would pay the price for my award.
 

Tell me the name of your product so that I can buy one for people I don't like. As the previous poster said, this is a TERRIBLE idea. If you're willing to pay for a relay, why not put that money to better use and buy a transformer instead?????
 

Sorry I blasted you earlier, mrmain! :oops: It's just that mains AC voltage safety is not something to be taken lightly - either know what you're doing there, or stay away from it & leave it to people who do (and what you described seemed outright stupid/dangerous).

mrmain said:
None of the conductors in question is accessible, except during installation &/or servicing.
Click to expand...
So people doing installation/servicing are aware that something marked "+5V" might be read as "AC live voltage +5V" :?: And that awareness is guaranteed to be the case X years after equipment is installed, company that installed it has moved on & other folks are doing the servicing now? And physical construction is such that people other than those aware of "high voltage, dangerous" have no way of coming into contact with any of that circuitry, without breaking into a box that says "high voltage, keep out" on it?

Not saying it does, but just supposing that all of that holds true (and will keep doing so), what exactly would be the problem using mains AC live wire as 0V reference point? I mean if circuit as a whole is adequately isolated from outside world (a few kV at least), then the safe-to-touch ground (not the 'neutral' but the safety one, that connects to the planet we live on with very low resistance & high current capability) wouldn't come into play, never carry current & therefore never cause any fuse or whatever to trip, right?

If that doesn't hold true: please explain how current could possibly flow from mains-connected circuit (either live or neutral) to safety ground? Whichever route could be pointed out = what would make above condition(s) fails = what would make the circuit unsafe = what could make you responsible if someone gets electrocuted.

Just the plain fact that chosen reference point might have live voltage on it, shouldn't matter: if it happens to be the neutral wire, the other wire (live) will have 120VAC on it (with respect to chosen 0V point). If reference point happens to be the live wire, the other wire (neutral) will have 120VAC on it (with respect to chosen 0V point). Same thing in both cases, shouldn't affect circuit operation.
 
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Thanks for the response, RetroTechie.

Again, you are correct, the circuit will work fine with the AC HOT & Neutral swapped, (except for the latent safety issue).

The analysis re: lifetime safety is also bang on. However, give a guy a screwdriver, and you can certainly get inside an old TV - where there were KILOVOLTs just waiting to discharge.

If the requirement for safety is "a screwdriver is required to access the dangerous part" then the system as designed is no less safe than a TV (well, an old CRT one).

Anyway, it looks like this design is migrating toward the desired end result of full isolation.
Sanity prevails.

Thanks for your insights.
 

mrmain said:
Thanks for the response, RetroTechie.

Hello Retro Techie
I have used a design of non isolated power supply (120/220) to 12V (refernce design from ST micro for the viper12 chip) and from the 12V I generate 3.3V using LM317. My ground is the neutral, and I have a 3.3V pin sticking out, are you suggesting that this pin is dangerouse? (I am lucky it didn't kill me yet)? My poower supply can be plugged into the wall one way, there will be no swapping HOT and NEUTRAL

Do you know or do think UL/CSA would approve a product with such a power supply?

Thanks
Click to expand...
 

Welcome, sivaley!

sivaley said:
I have used a design of non isolated power supply (120/220) to 12V (refernce design from ST micro for the viper12 chip) and from the 12V I generate 3.3V using LM317. My ground is the neutral, and I have a 3.3V pin sticking out, are you suggesting that this pin is dangerouse?
Click to expand...
Any mains connection is dangerous for a human, and plus or minus a few volts doesn't change that. Also a low-voltage regulator IC like LM317 in between makes no difference, as it'll just behave like a wire when >50V wants to push through it.

My poower supply can be plugged into the wall one way, there will be no swapping HOT and NEUTRAL
Click to expand...
That's assuming inside wall sockets, hot & neutral are always wired to the same pins.

  1. That's a very optimistic assumption.
  2. Which might not always be true. Even if it's true at your place (have you checked all wall outlets in your home?), it might not hold @ other people's places. And
  3. If untrue, should still NOT be a safety issue.
  4. Furthermore that's assuming the plug going to your device is never changed into another one that does allow plugging in reverse orientation.
Bottom line: don't make this assumption, and certainly don't rely on it for safety.

Do you know or do think UL/CSA would approve a product with such a power supply?
Click to expand...
I wouldn't, and I'm just a hobbyist. Expect a professional to look even closer.

But that doesn't even matter... what do you care about: whether it's safe, or what organizations like UL/CSA would say about it?
 

Thanks retro. As Iam sutre you know, the neutral bar of a plug has a wider tip than the hot bar (0.315" vs. 0.25") and all sockets even those with a ground, have a longer slit (0.34") for the neutral and 0.28" for the hot, so there is no way it will be plugged backwards. Assuming sockets are correctly wired is a very valid logical, ethical and legal assumption that is why there are standards and codes, only fools will buy a fool proof product.

I asked because I thought you knew, my bad
 

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