Can an ac and a dc circuit share a ground point?

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Murf said:
There's been a little bit of a debate between coworkers in my office... Can an ac and a dc circuit share a ground point?
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the short answer is "yes".

The long answer requires a lot more information, including what you mean by "ground point". Are you referring to a grounding electrode? Or perhaps a ground stud on a panel?
 
generally speaking all the ground wires are required to be connected together somehow.

if you have a ground stud there is no reason you can't have more than one ring tongue terminal on it. the electrons do not really care if the circuit conductors are AC or DC. The green wires are not circuit conductors and will only see current flow if there is a fault.
 
i not sure. a lifted N on the AC side could be hazardous to the DC side.
why would you not keep the AC and DC "gnd" isolated?
to me it sounds more like bonding AC egc to DC gnd (or vice versa). but why?

if the gnd side of bridge rectifier is tied to the chassis (for DC gnd purposes) do you typically see the AC egc wire feeding that DC power supply also tied to the chassis? if AC egc is on the chassis i usually see the DC gnd isolated to its internal circuitry. a quick check, take any cots 120/240 50/60hz dc power supply (metal case) and ohm from ac egc to the dc gnd terminal. i checking mine right now. my desktop 20lb dc ps has ac egc tied to chassis, the ac egc is not tied to dc gnd.
 
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Besoeker said:
Ignorant Brit strikes again........:)
What would you ground on the DC side?
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It is not uncommon to find grounded DC systems. If you don't ground the DC you need to use fuses in both the positive and negative, if you ground one side, you only need a fuse for the other leg.
 
FionaZuppa said:
i not sure. a lifted N on the AC side could be hazardous to the DC side.
why would you not keep the AC and DC "gnd" isolated?
to me it sounds more like bonding AC egc to DC gnd (or vice versa). but why?

if the gnd side of bridge rectifier is tied to the chassis (for DC gnd purposes) do you typically see the AC egc wire feeding that DC power supply also tied to the chassis? if AC egc is on the chassis i usually see the DC gnd isolated to its internal circuitry. a quick check, take any cots 120/240 50/60hz dc power supply (metal case) and ohm from ac egc to the dc gnd terminal. i checking mine right now. my desktop 20lb dc ps has ac egc tied to chassis, the ac egc is not tied to dc gnd.
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If the DC system is a grounded system, I don't know of any code compliant way to have the DC grounding system isolated from the AC grounding system.

I would not expect to see either the positive or negative side of the power supply to be connected internally to the AC EGC. The grounding of the DC system would occur on the load side of the power supply output.
 
Besoeker said:
But what would you ground on the DC side?
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i wondering the same thing. what he is suggesting is that the DC gnd terminal be tied to the AC egc. unless he is solely talking about chassis grounding of the DC equipment. as example, a car, the whole thing is DC and DC neg is tied to the chassis. if i were feeding that car power from a AC source via power supply i would not tie car chassis back to AC egc.

if the dc equipment has isolated chassis then perhaps its ok to tie DC chassis to AC egc. seems more like a bonding (equi-potential) thing to me. but if the DC neg terminal to DC equip chassis has zero ohms, i would not tie DC gnd to AC egc. and, even if DC gnd was isolated from DC equip chassis, the question of what type of isolation is there, so still questionable if i would tie DC chassis to AC egc.
 
All grounds are ultimately tied together, you know, in the ground.
 
ggunn said:
All grounds are ultimately tied together, you know, in the ground.
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not really, i dont know.

AC to DC, or DC to AC. why would the DC neg "gnd" terminal be tied to earth, or AC egc ??

a AC to DC power supply would have AC egc tied to the chassis. there is isolation between AC & DC sides, etc. the neg from DC may or may not tie to the chassis of the DC gear. if it does then i would not tie any DC chassis to AC egc. if all the DC chassis are isolated from DC gnd "neg" then perhaps you bond them and then tie to earth ground rod, but i would not bond them to the AC egc. the AC egc is for the most part the center tap of Xph xfmer secondary coil. having AC egc tied into earth does not provide zero ohms back to source when that N lifts for any reason or becomes high ohms, etc.
 
Besoeker said:
Yes. But what do you ground on the DC system?
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Well, as jaggedben said, a negatively grounded PV system, as the name implies, has its negative conductor tied to ground. I'm not as familiar with battery/inverter systems, but I'm pretty sure they have the negative battery terminal tied to ground as well.
 
ggunn said:
Well, as jaggedben said, a negatively grounded PV system, as the name implies, has its negative conductor tied to ground. I'm not as familiar with battery/inverter systems, but I'm pretty sure they have the negative battery terminal tied to ground as well.
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but that just means "on the DC side". the common "gnd" for the DC circuits is the Neg side of DC source. i guess this is more about isolation and less about how the DC side is grounded.

is there NEC verbiage calling for the DC "gnd" (dc neg in this case) to be tied to AC egc ?
 
FionaZuppa said:
but that just means "on the DC side". the common "gnd" for the DC circuits is the Neg side of DC source. i guess this is more about isolation and less about how the DC side is grounded.

is there NEC verbiage calling for the DC "gnd" (dc neg in this case) to be tied to AC egc ?
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In a negatively grounded PV system there is normally a grounding electrode at the inverter that is bonded to the service ground, and both the AC and DC grounds are tied to it, along with the EGC from the array and the rest of the gear. Is that what you are asking?

The DC ground is tied to this through a fuse or GFD circuit in order to detect ground faults on the positive conductors, though the system is blind to a ground fault on the negative conductor.
 
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