120VAC and 12VDC ground

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Rho_HV

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We have several "fused" 12VDC devices in our VFD enclosures powered from a common power supply. The neutral lead output of power supply is grounded to force the fuse to blow in a ground fault situation. 12VDC neutral is connected to the same ground bar as 120VAC input to the power supply.

My question: Can connecting 12VDC neutral to AC ground be a problem?

Thanks,
 
LMAO said:
We have several "fused" 12VDC devices in our VFD enclosures powered from a common power supply. The neutral lead output of power supply is grounded to force the fuse to blow in a ground fault situation. 12VDC neutral is connected to the same ground bar as 120VAC input to the power supply.

My question: Can connecting 12VDC neutral to AC ground be a problem?

Thanks,
Click to expand...
"12VDC" and "Neutral" are mutually exclusive terms. Neutral is a concept that only applies to AC systems.

Neutral in AC does not necessarily mean it is grounded. But Ground always means the same thing, connected to ground.

You are saying you have a grounded 12VDC control circuit (most likely the negative side). Not unusual.
The Neutral of your AC source is also grounded. Not unusual.
What's the different between ground and ground?
 
You can't have a neutral on any two wire system, but you can have a neutral on a 3 wire DC system...exactly like a 120/240 volt single phase AC system.

The grounded conductors of all of the different grounded systems in a building are really connected to the same point, so there should be no problem connecting one conductor of the DC system to the AC equipment grounding conductor.
 
don_resqcapt19 said:
You can't have a neutral on any two wire system, but you can have a neutral on a 3 wire DC system...exactly like a 120/240 volt single phase AC system.

The grounded conductors of all of the different grounded systems in a building are really connected to the same point, so there should be no problem connecting one conductor of the DC system to the AC equipment grounding conductor.
Click to expand...
If you consider it an SDS, that seems OK. But the OP seems to be asking about connecting one DC conductors (which is not connected to chassis ground) to an AC grounded conductor in the same panel. I would say probably not, but cannot support that with a code citation.
 
jumper said:
Are you saying you could have a ungrounded neutral in a AC system?
Click to expand...

It's not what he was saying, but you could under the code. See 250.21. Not that I personally know of any such applications that utilize neutrals.

GoldDigger said:
If you consider it an SDS, that seems OK. But the OP seems to be asking about connecting one DC conductors (which is not connected to chassis ground) to an AC grounded conductor in the same panel. I would say probably not, but cannot support that with a code citation.
Click to expand...

I'm curious how it could not be an SDS in this case and not suffer from a direct short.

don_resqcapt19 said:
The grounded conductors of all of the different grounded systems in a building are really connected to the same point, so there should be no problem connecting one conductor of the DC system to the AC equipment grounding conductor.
Click to expand...

If there's no isolation it's a big problem. Seems like in the OPs situation one is powered from the other, so presumably there's a transformer?
 
160113-2108 EST

LMAO:

There needs to be clarification about words used in your original post.

I would hope that "ground" means something connected to the earth in the context of your question. This also should mean that at the main panel or its equivalent that all ECG (Equipment Grounding Conductors) are connected to a grounding bus that is connected to a grounding conductor to the real earth, and this grounding bus is connected (bonded) to any neutral bus in the main panel. This is the common point where all three entities come together.

At no other point should neutral and EGC be connected together. EGC is normally a non-current carrying conductor.

Neutral is a different animal. It is possibly a normal current carrying conductor, and in a general definition could perform its neutral function without being grounded. This concept originated with Edison in a DC circuit as a means to reduce cost of copper for a given total distribution system power capability.

In a distribution system using a neutral you normally expect there to be a voltage drop along the neutral wire as a result of the load on the circuit associated with the neutral. Only under fault conditions to the EGC should there be any voltage drop along an EGC path. Unfortunately for low level signals there is often times noise on the EGC line.

I would think that in most cases the output of your DC supply is really a SDS (Separately Derived System), and there may be good reasons to not connect its common, whether that is the + or - terminal, to any AC neutral in the cabinet. Rather it is better to connect the DC common to the equipment cabinet, thus, meaning to the EGC. Or to float the DC supply off of the cabinet, but you might reference the DC supply to the cabinet by a resistor divider between + and -.

The reason for not connecting the DC common or reference to the neutral is that you don't want the DC supply referenced to the voltage drop on the neutral relative to earth ground.

.
 
160113-2345 EST

An added comment.

I believe in the vast majority of industrial applications where my electronic equipment was used the input supply was a 480 V ungrounded delta. There was an EGC to the cabinet. This directly provided delta power to 480 V motors, and one phase supplied a 480 V to 120 V control transformer with an ungrounded secondary. My equipment was supplied from the 120 V secondary. Our DC common for +/-5 V, +/-12 V, and +28 V was connected to chassis, to machine cabinet, thru anchor bolts to the cement floor, and from the machine frame and cabinet to the EGC.

None of the DC common wires were were tied to the machine chassis other than via the DC power supply grounding point.

However, the shield wire to LVDTs or straingage transducers was chassis connected at both the electronic equipment end and the transducer end. At one time at Chrysler Eldon Ave this caused a problem when welders put their welding ground connection on the main part of pinion shim machine and welded on a peripheral area where an LVDT and digital indicator were located. Some of the welding current back to the welding ground connection flowed thru the LVDT and its cable burning them up.

.
 
Sorry, my experience is mostly in industrial power systems design so I am not an NEC expert (kind of embarrassing because I am a PE). To clarify, by 12VDC "Neutral" I meant 12VDC return (aka, the negative lead of 12VDC power supply output). It is connected to the ground, just like the single phase 120VAC transformer secondary neutral that feeds the 120VAC:12VDC power supply input.

Will there be a problem?
 
LMAO said:
Sorry, my experience is mostly in industrial power systems design so I am not an NEC expert (kind of embarrassing because I am a PE). To clarify, by 12VDC "Neutral" I meant 12VDC return (aka, the negative lead of 12VDC power supply output). It is connected to the ground, just like the single phase 120VAC transformer secondary neutral that feeds the 120VAC:12VDC power supply input.

Will there be a problem?
Click to expand...

In most "wall wart" DC power supplies the DC negative is not connected to ground, per se, because there is no ground pin on the AC side. I guess it could be connected to the AC neutral, though, which is ultimately connected to ground at the service.
 
GoldDigger said:
If you consider it an SDS, that seems OK. But the OP seems to be asking about connecting one DC conductors (which is not connected to chassis ground) to an AC grounded conductor in the same panel. I would say probably not, but cannot support that with a code citation.
Click to expand...
And I assumed he was connecting one of the lines from the output side of the DC power supply to the equipment grounding bar in the control panel.

That is a common method used in industrial control panels where they want the DC system to be a "grounded" system.
 
I thought he had DC- and an EGC both connected to equipment chassis.
Still scratching head...
 
160114-1317 EST

LMAO:

In your first post you did not make it clear to what you want to connect the negative side of your 12 VDC power supply. You mentioned both the words ground and neutral. Once you leave the main panel where these are tied together (bonded), then they are not expected to be the same potential as that of their termination point in the main panel.

Under normal conditions (non-fault) anywhere along the EGC the potential along the EGC should be the same or close to the potential as at the main panel. This is not expected on the neutral.

You should not want to tie your DC common to neutral, but most likely you want DC common to connect to the electrical cabinet and that in turn should mean it is connected to the EGC (Equipment Grounding Conductor). Hopefully this should mean the potential of the cement floor will be quite close to the potential of the EGC.

.
 
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