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Thread: running dual charging systems 12V/24V

  1. #11
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    What is the theory behind keeping isolated grounds?

  2. #12
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    Tim, the "box" was a 15 amp, 24v to 12 v reducer. designed for the HEI.
    also, I remember the 12v battery was grounded to the frame and the 12v alt. was mounted in the oem Chevy mounts. I made a mount for the 24v alt. hanging off the 12v alt and they ran off the same fan belt. It wasn't rocket science or I could not have done it!......Al
    Last edited by osut362; November 24th, 2014 at 09:31 PM. Reason: spelling

  3. #13
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    From another forum, on Yachts, in relation to using a 12 volt system off of a 24 volt charging system, and ""Sharing a Ground"" ( DC Voltage , 12 and 24 volt )

    Quote:

    In theory, yes, all "ground" busses would be tied together.

    But in practice, this creates a new hazard on the boat. By tying the 12 and 24 volt grounds together, you now create the hazard that you can have a short or other unintentional connection between the 12 V+ and 24V+ systems, putting 24V on the 12V system or creating a 12V loop in the 24V side.

    That might be a small hazard and worth ignoring in favor of having one common ground--but I'd check out the system with that thought in mind, and make doubly sure the two system positives were kept isolated from each other to make sure it couldn't happen.

    Perhaps by using special connectors on the 12V system, to make Real Sure that someone couldn't accidentally tie it to the 24V system, or at least putting permanent warning tags on the 12V and 24V hot cables


    End Quote.

    Basically, you could fry whatever is connected in the 12 volt circuit, by inducing 24 volts into those items.

    There is also the possibility of a Ground LOOP occurring , when sharing grounds, and any electronics , such as radios or stereos MAY have induced NOISE from the sharing of a common ground.



    In theory, Yes you would tie the grounds together...
    But that invites at least three potential problems,

    And keeping everything separate, eliminates those potential "Murphy's Law", issues.

    I dont want to have to replace my truck.

    A Little more wire, used for grounds .. and keeping everything separate seems like the logical way to go.

    I may need to stand corrected on this, and if so... I will.

    I have the 2011 NEC book here, but have not looked up the NEC codes for DC voltages and sharing of grounds in different voltages.

    well , that was a waste of time...

    NEC 250 VIII
    NEC 250.162(A) and (B) and NEC 250.164

    No help for our uses... Deals with residences, and grounding Outside the structure... and SINGLE voltage,( 50 to 300 volts ) and not DUAL LOW Voltage.
    thats it for the NEC.
    hostis est intra portas tuas

  4. #14
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    NEC 411.5 Secondary Circuits on Low Voltage ( 50 volts or less)

    (A) Secondary circuits Shall NOT Be grounded.

    (B) The secondary circuit SHALL BE ISOLATED from the branch circuit by an Isolating transformer.



    That's pretty much it for the national electric code dealing with Low voltage circuits.




    The only other mention is Article 720:
    Circuits and Equipment operating at less than 50 volts ( DC or AC current )

    deals with hazardous locations enclosures ( Spark proof etc... )
    Minimum conductor size ( 12 awg minimum )
    10 AWG for a circuit sharing two appliances ( minimum )

    Lampholders must be 660 watts rated at Minimum.

    Receptacles must be 15 amps or greater.
    20 amps for kitchens, laundries and other locations where portable appliances are used.

    Battery Storage safety

    And 720.11 "Mechanical Execution Of Work"
    Circuits operating at less than 50 volts shall be installed in a
    "Neat and Workman Like Manner "
    ( Cables have to be secured as to not be damaged )

    Now, all the legalese is out of the way,
    lets get back to theory

    And I really want to hear some other opinions, as the info available on this subject is minimal, at BEST !
    hostis est intra portas tuas

  5. #15
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    Quote Originally Posted by Blitz View Post
    NEC 411.5 Secondary Circuits on Low Voltage ( 50 volts or less)

    (A) Secondary circuits Shall NOT Be grounded.

    (B) The secondary circuit SHALL BE ISOLATED from the branch circuit by an Isolating transformer.
    I can answer this one in particular. The "secondary circuit" is referring to the circuit that is derived from a transformer. The Isolation transformer referred to is to keep AC voltages separated between the primary and secondary side of the transformer. Isolation transformers have a special shield in them to prevent harmonic frequencies from being induced into the secondary circuit.

    The above section is strictly dealing with alternating current voltage not direct current voltage.
    Last edited by jeeper; November 25th, 2014 at 06:41 AM.

  6. #16
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    Thanks for that clarification.
    An 870 page code book is difficult to navigate, and not much at all is said about low voltage... especially DC current.

    After posting last night, I went back and read the entire section on Solar Photovoltaic systems.

    Nothing there will help us either. Its all pretty much about protectin structures, and not for mobile, dual low voltages. ( actually NO dual voltages I can find anywhere ! )

    I'm thinking the best best for information is going to be Yachts and Boaters ( and Sailboats )
    Folks and Forums that deal with these exact dual voltages all the time ( 12v, 24 v , 36v and 48 v, living in harmony )


    Is there a Maritime Electrical Code that I'm not familiar with?

    It seems like Our best bet, after researching for at least 3 hours.
    hostis est intra portas tuas

  7. #17
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    Here is a wiring diagram for a BOAT, with three batteries...

    notice that they did connect all the grounds together... BUT also notice the engines and all accessories seem to run on 12 volts only....
    EXCEPT for the starters on the boat motors ( controlled by a solenoid switch )

    The temporary use of a starter circuit on 24 volts, with a solenoid switch, might not be enough TIME to cause an issue mixing voltages with common grounds.
    ( the engines start, and then go back to 12 volts immediately)

    I'm sure Im missing something here...

    Someone else please take a look and see what you read on this diagram.
    it seems to be a parallel switch... NOT Series.... so not even 24 volts here right?

    Last edited by Blitz; November 25th, 2014 at 02:00 PM.
    hostis est intra portas tuas

  8. #18
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    Yeah,
    I'm still looking this stuff up.
    I really want to know what is theoretically correct as well as practically correct.

    In the interest of fairness and accuracy, I have found three instances where they say to connect all of the grounds in a dual voltage system in a boat.
    I have read more articles, Yacht wiring diagrams, and coast guard electrical manuals than i care to remember right now.

    "In a boat" is used for US, because, thats pretty much the only thing that uses dual DC voltage... besides integrated circuits and chips. which are less than > 2 volts. And the beloved M -715, and other M series vehicles .

    The same issues keep coming up...
    Mainly NOISE between appliances and especially radios, or really sensitive electronics such as LORANS, GPS, Depth finders, Radars , computers, etc...

    There is also the issue of inadvertently putting 24 volts onto a 12 volt appliance, either by cross wiring something, or by a direct short from one voltage to another on the HOT side.

    Also an issue with start up loads, drawing down the power from the "other" voltage source...
    ( Such as starting loads on 24 volt, pulling down the voltage enough to interfere with the other 12 volt appliances ( sometimes 1 volt in a DC circuit is enough to damage appliances, unlike AC current where a 1 volt drop is nothing)

    And finally.... Ground loops from the 12 volt side interfering with the 24 volt side.

    Grounding, it seems...
    Is very much like EMP protection....
    and is considered "a Black Art" or just "Black Magic".

    Its really hard to get the direct answer, as most never have to deal with dual voltages.


    But to be fair and accurate...
    I have found ONE diagram that shows the negative wires of a dual voltage system connected together... and two posts on Yacht forums, that say to connect all the grounds together. ( for a DC system )

    thats about a 5 % YES vote to a 95 % No vote... just from my observations.

    Here is the diagram, and you can decide for Yourself which is the best way to go.


    It seems that the consensus IS to Isolate the grounds, to avoid any issues, I should add.

    But there are some out there that say its no problem, and they just run a separate BUS for the 24 volt grounds, and a separate BUS for the 12 volt grounds to make wiring easier,
    THEN they tie those two together... to the immersed grounding strip, under the boat.


    So...

    until further information becomes available...
    I'm sticking with the way I've got it... Separate Grounds for everything.
    Everything Isolated, and Yes i used a little more wire, and have a little more voltage drop with long runs ( batteries to back bumper is the longest run )

    but it seems, it IS possible, with some risk, to tie them all together.

    You decide, but it seems the theory ... is out there...


    hostis est intra portas tuas

  9. #19

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    New member here, just bought my first M715, currently configured as a fire/brush truck. I'm looking forward to working on my new toy. While I am an M715 noob, I am a telecom engineer by trade and very familiar with electrical circuits and systems. It is fine to share grounds with 12 and 24 volt systems. Tying both 12 and 24 volt grounds together and to the chassis means you have a really big conductor for the ground "wire", which is a good thing.

    Ground loops are not really a concern in a typical automotive power distribution system. These are more troublesome in AC circuits, not DC.

    The concept of "floating" the 12 volt system would offer a little protection should you happen to accidentally connect either the 12 volt circuit hot or ground to the chassis or 24 volt line. But this only works for a single accidental connection. Once things get crossed up, you are no longer isolated.

    I prefer grounding both systems together with the chassis. I would use a different wire color for my 12 volt lines, or clearly tag them. And never tie a 12 to a 24 volt line any more than you would tie A 12 or 24 volt line to ground. All will result in bad things. .

    Most every electronic device you have uses multiple DC voltages inside and most always are referenced to a single common ground. All that said, an isolated secondary voltage system in a truck will work just fine.

  10. #20
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    Welcome to The Zone Texas Aggie,
    and Thank You for sharing in Your expertise .

    This is good to know, for saving the cost of copper wire, as well as for voltage loss throughout the entire circuit.

    A 1 volt drop in DC is a pretty big drop.

    What is your opinion of "Noise" being generated in radios and navigation equipment by combining grounds?
    Or electrical interference being generated by a sudden LOAD, such as a starting circuit, pulling lots of amps... and lowering the voltage on the "other" ( non starting voltage )
    DC circuit?

    I appreciate you joining in here and sharing your knowledge on this subject.
    The Dual Voltage, DC circuits are not covered most anywhere else... except electronics, boats and M vehicles.

    Everything I have learned is self taught, and through self study, especially in alternative energy , battery back up systems and photo-voltaic systems.
    I have some "required state courses" for my profession... But most of that is AC circuits for "appliance installation", but it covered the entire NEC codes.

    Electronics are another "Bear" with micro voltages and subtle differences that are critical in the functioning of components and chips, and thats a whole nother world to me entirely!

    So Welcome again to The ZONE Texas Aggie !

    We love photos here, and anything you need or run into with Your truck, I assure You, You can get Your answers here in 1/2 a day.
    advice, parts, sources... and just good conversation all the way around !

    Here is an interesting PDF I ran across while researching this grounding issue.

    you might find it interesting as well, as it pertains to your profession, and common grounds and galvanic isolation in low voltage chips and dual voltages.

    http://www.ti.com/lit/an/slyt211/slyt211.pdf
    Last edited by Blitz; November 26th, 2014 at 12:44 AM.
    hostis est intra portas tuas

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