Wiring Ground to Negative?

XmasFiend

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Dec 30, 2019
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Hi ACL,

I have seen several people wiring the GND to the -V on PSU's...

1578971605823.png

I didnt just want to do it because it seems like the norm, I would like to know the logic behind it?


Cheers
-Mick
 
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algerdes

Al Gerdes
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Though earth ground is supposed to be "0", I have found it to run several volts (depending on where the ground rod is) and have wondered about this practice as well. Even more important is if the neutral comes loose any where in the circuit, this means the earth ground can carry a full return voltage (until a breaker trips).

Our SOP is that the AC and DC sides should never meet. This violates that.

As asked, is there any logic to doing this?
 

David_AVD

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Personally I have never connected the V- terminal to the mains earth terminal.

One reason to do it could be to reduce EMI (interference) caused by the power supply, but how effective that might be would vary greatly.

An issue you may have is when multiple power supplies earthed that way that are plugged into power circuits with differing earth potentials.

If the V- terminals of those power supplies are also interconnected (via controllers, PI cables, etc) you may get a current flow in the V- wiring.

Al mentions a possible issue with a missing neutral causing issues, but I don't agree. With the Australian "MEN" system, a fault like that can't really cause an issue. If it does the RCD will trip

Still, I would not connect V- to mains earth at the power supply in a typical Christmas lights installation.
 

XmasFiend

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Thanks guys, yeah i think ill steer clear of this GRN to Negative setup.
 

algerdes

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Interesting. Haven't heard of the "MEN" system. (Would like to learn about it.) I agree that the RCD should shutdown the whole thing, but what timing is on that?

Either way, I have never installed that jumper and won't without good cause.
 

scamper

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Al mentions a possible issue with a missing neutral causing issues, but I don't agree. With the Australian "MEN" system, a fault like that can't really cause an issue. If it does the RCD will trip
The RCD SHOULD trip.
Unfortunately there are always exceptions, also if you have an older house, you may not even have rcd's installed yet.
I recall when I was a kid getting a "tingle" everytime I went for a shower off the taps, this got worse until finally my dad called the electric company and found a faulty neautral wire at the meter box.
There was also just last year a girl that got electricuted from touching the garden tap.
So SHOULD is the appropriate word.

Now as far as the wiring the dc to ac. Not something that I would do. First as we all know there is resistance and therefor voltage drop over any length of cable, This applies to the earth side also. AC by it's nature creates ripples or harmonics which is introduced into the earth and neutral (or be it small) Now if you add this to the dc negative and are trying to run data as we do at very small voltages, then any harmonic introduced into the dc side as far as I am concerned is going to be a problem if you are trying to fault find some weird data issue.
 

David_AVD

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Connecting the low voltage side of the power supply to mains earth is often done in appliances, although double insulated is more common these days. It's not always bad thing and certainly not against the electrical regulations. You may need to consider the side effects of doing so though.
 

Katekate

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Interesting. Haven't heard of the "MEN" system. (Would like to learn about it.) I agree that the RCD should shutdown the whole thing, but what timing is on that?

Either way, I have never installed that jumper and won't without good cause.

RCD are supposed to trip in 30ms or less, if 30mA of current goes missing.
 

algerdes

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RCD are supposed to trip in 30ms or less, if 30mA of current goes missing.
Thank you for this information. Learning how others do things is valuable, to be sure.

I equate the M.E.N connection to a hard-wire from the neutral to the ground buss at the sub-panels, with local ground rod installed. (If it is an actual device, and not just a jumper, please let me know.) We do this now, with boxes that are a specified number of feet from the main panel (usually out buildings such as a shed/garage/workshop.)

One thing to note: Back in USAF electronics school, I had an instructor that put rods into the ground at a distance of about 80 feet. We strung a line between them, elevating it so it couldn't pick up anything from the ground itself, and inserted a meter. Nothing else connected. Amazingly we found a significant voltage on the line. This is where we first learned about "ground not being 'ground'".

In any case, I thank you again for this lesson. I'll keep it in mind.
 

Rickras85

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RCD are supposed to trip in 30ms or less, if 30mA of current goes missing.
the type of rcd’s used in domestic installation ie 30ma type only have to trip within 300ms to comply not 30ms. If 5 x times the trip current (150ma) it needs to trip within 40ms.

Commoning zero volts dc to earth is done more in industrial applications for reasons mentioned above and for testing purposes. None of these should apply in our Christmas display.
 

seano

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The RCD SHOULD trip.
Unfortunately there are always exceptions, also if you have an older house, you may not even have rcd's installed yet.
I recall when I was a kid getting a "tingle" everytime I went for a shower off the taps, this got worse until finally my dad called the electric company and found a faulty neautral wire at the meter box.
There was also just last year a girl that got electricuted from touching the garden tap.
So SHOULD is the appropriate word.

Now as far as the wiring the dc to ac. Not something that I would do. First as we all know there is resistance and therefor voltage drop over any length of cable, This applies to the earth side also. AC by it's nature creates ripples or harmonics which is introduced into the earth and neutral (or be it small) Now if you add this to the dc negative and are trying to run data as we do at very small voltages, then any harmonic introduced into the dc side as far as I am concerned is going to be a problem if you are trying to fault find some weird data issue.
That girls house got demolished because they could not be sure that it was safe
 
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