Positive Ground and 3 Phase AC System Used in Common Platform

[AHE Tom]

What could be the problems generated by having a Positive grounded DC system and a 3 phase AC system in/on a single motorized vehicle ?
I am concerned about the AC system grounding to the chassis of the vehicle ad the positive ground of the DC system could/would cause problems with electronics, power, etc.

Thinking that the DC system would have to be a 2 wire system in lieu of a single wire system utilizing chassis as the return path as most vehicles are

Thanks

 

[gar]

171102-0756 EDT

Can you clarify your question, and its purpose. Is this a car, truck, or the like? Where did the positive DC ground come from? It seems to me negative ground became common in the 1950s.

Whether DC ground (chassis not earth) is positive or negative would not seem important relative to your question.

What problem do you envision with both an external 3 phase AC, and a local DC using the same chassis as a common reference point? Why wouldn't problems be worse if they didn't share a common reference?

.

 

[kwired]

What could be the problems generated by having a Positive grounded DC system and a 3 phase AC system in/on a single motorized vehicle ?
I am concerned about the AC system grounding to the chassis of the vehicle ad the positive ground of the DC system could/would cause problems with electronics, power, etc.

Thinking that the DC system would have to be a 2 wire system in lieu of a single wire system utilizing chassis as the return path as most vehicles are

Thanks

There is no such thing as a single wire (conductor) system. The chassis may be one conductor - and is common in vehicles and other self powered machinery. For basic lamps and other non polarity sensitive items it makes no difference. Most DC motor designs and other items containing semiconductors may not operate the same way if you change polarity but otherwise don't really care if the chassis is at same potential as a supply lead.

Most cases with an AC system you probably are not using chassis as current carrying conductor - just for "equipment grounding" that only carries current during fault conditions and only long enough to allow operation of an overcurrent protection device.

 

[AHE Tom]

First I am not at liberty to disclose particular detailed information, so I am sorry about being vague

The DC systems must be positive ground by the design of equipment being provided

I am trying to identify if there could be a problem on a vehicle with a single internal power system providing both 3 phase AC and DC power.
The vehicle would use 3 phase AC power for particular equipment and a positive ground DC systems for other equipment and control systems.

I am not clear if there could be a problem with the AC system being grounded to the chassis of the vehicle, and the positive grounded DC systems.

I also understand that there is no such system that uses a single conductor, I just expressed it that way (my apologies on that), I do understand that the power systems on autos use the chassis as the common ground thus eliminating the need for a dedicated conductor for the negative connection.
This is why I am asking this question.

I am thinking there should not be a problem with any grounding issues if two dedicated conductors are used for the DC system, and not to use the chassis as a common for the DC powered systems

It is difficult to explain my concerns regarding grounding, I know without providing details it is difficult to get answers, but that is the situation I currently have


Thanks again

 

[winnie]

The key question is: are the DC and AC supplies galvanically isolated, or in other words are they 'separately derived systems'. When you 'ground' your system, you want to connect a _single_ terminal of each system to ground, and the other terminals must be isolated from ground and from each other.

If the DC and AC supply share a common connection (say the DC is derived from rectifying the AC), then adding the grounding connection would essentially cause a short circuit.

With a _single_ connection between the two systems, you don't have a closed circuit, and only minimal current flows (insulation leakage and capacitive coupling). This _single_ connection references both systems voltages to each other, but without a closed circuit you don't have a path for current flow.

Aircraft commonly use both 200/110V 3 phase 400 Hz AC and 28V DC. Both are referenced to the AC chassis.

-Jon

 

[kwired]

First I am not at liberty to disclose particular detailed information, so I am sorry about being vague

The DC systems must be positive ground by the design of equipment being provided

I am trying to identify if there could be a problem on a vehicle with a single internal power system providing both 3 phase AC and DC power.
The vehicle would use 3 phase AC power for particular equipment and a positive ground DC systems for other equipment and control systems.

I am not clear if there could be a problem with the AC system being grounded to the chassis of the vehicle, and the positive grounded DC systems.

I also understand that there is no such system that uses a single conductor, I just expressed it that way (my apologies on that), I do understand that the power systems on autos use the chassis as the common ground thus eliminating the need for a dedicated conductor for the negative connection.
This is why I am asking this question.

I am thinking there should not be a problem with any grounding issues if two dedicated conductors are used for the DC system, and not to use the chassis as a common for the DC powered systems

It is difficult to explain my concerns regarding grounding, I know without providing details it is difficult to get answers, but that is the situation I currently have


Thanks again

Two systems is commonly used on vehicles that also have AC wiring within them. Just like it is common in commercial/industrial buildings to have 480/277 as well as 208/120 systems. Any current from one system is not trying to get back to "ground" it is trying to get back to it's source. If both sources have one conductor bonded to the frame of the vehicle, and you depend on that frame as a current carrying conductor - the bigger risks are shock hazards should you have open circuit conditions on part of the circuit path involving the frame, or for high current carrying items (for either voltage) overheating of components that don't have enough cross sectional area of the "conductor" to handle the current imposed on it.