Sharing common (ground) questions

Anything electric, AC or DC

Re: Sharing common (ground) questions

Postby H.A. » Tue Jun 20, 2017 11:16 pm

[qut.
Last edited by H.A. on Thu Jul 06, 2017 4:05 am, edited 1 time in total.
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Re: Sharing common (ground) questions

Postby kludge » Wed Jun 21, 2017 4:22 pm

Vspec wrote:
kludge wrote:7) Can I tie my DC returns (negative side) to the trailer chassis/truck wiring? Yes. You should absolutely do this. Bring all the DC negatives back to the fuse block and tie them to a negative bus and then one big fat wire from the negative bus to the chassis, or T it into the 7-pin wiring. Perhaps the best place to screw it down is the same place on the chassis that the 7-pin harness it screwed down.


Would that not cause an issue if both the 7-pin wiring and the DC source have their (-) tied together and both are providing power at the same time?



There are a couple ways to handle the +12V side: switches, isolation rectifiers (diodes). Probably some load panels/DC converters can handle this for you automatically. But if not, you can always disconnect the 7-pin from the T.V. before connecting the DC Converter to shore power. In fact you could rig up a 7-pin and connect it to the DC convertor. Then you HAVE TO unplug from the T.V. to plug into the DC converter. In fact, that would be a pretty simple setup.

Is there a storage battery in your future?

And no, tying grounds/negatives together won't hurt anyone's feelings. It's just a reference point. And it's always good to reference to the piece of earth you are standing on if the voltage is high enough to kill you. The closer the better.

Return current will always flow back to it's source, even if there are multiple sources in a system. Where you run into issues is when your return currents can share a conductor that isn't rated to handle the total current flow. That's why it's a good idea tie them together at the same point on the frame. both the TV and the Trailer will have the same reference point, but currents that don't belong there don't flow. On the positive side (+12V) you will want to isolate them and usually best to be able to connect to one at a time, as I mentioned in the first paragraph. You don't want current flowing from the +12V in your trailer to the +12V in your truck. Or maybe you do, but that's another discussion.
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Re: Sharing common (ground) questions

Postby Vspec » Wed Jun 21, 2017 9:04 pm

kludge wrote:
There are a couple ways to handle the +12V side: switches, isolation rectifiers (diodes). Probably some load panels/DC converters can handle this for you automatically. But if not, you can always disconnect the 7-pin from the T.V. before connecting the DC Converter to shore power. In fact you could rig up a 7-pin and connect it to the DC convertor. Then you HAVE TO unplug from the T.V. to plug into the DC converter. In fact, that would be a pretty simple setup.

That kind of set up crossed my mind at some point but that was really just a stop gap measure

Is there a storage battery in your future?

Yup. My guesstimations indicates with the future planned used of a DC fridge/cooler ~ Dometic/ARB kind, a 50ah battery should satisfy my needs. Definitely overkill right now but if I use my crappy 12v cooler and plug it once in a while to keep things cold, it should last a weekend before needing a recharge. I can also bring my honda inverter.

And no, tying grounds/negatives together won't hurt anyone's feelings. It's just a reference point. And it's always good to reference to the piece of earth you are standing on if the voltage is high enough to kill you. The closer the better.

For ease of trouble shooting (that's my day job), each appliance will have it's dedicated ground/negative tied back to the distribution fused panel. From there I was planning on
connecting directly back to the battery or converter.


Return current will always flow back to it's source, even if there are multiple sources in a system. Where you run into issues is when your return currents can share a conductor that isn't rated to handle the total current flow. That's why it's a good idea tie them together at the same point on the frame. both the TV and the Trailer will have the same reference point, but currents that don't belong there don't flow. On the positive side (+12V) you will want to isolate them and usually best to be able to connect to one at a time, as I mentioned in the first paragraph. You don't want current flowing from the +12V in your trailer to the +12V in your truck. Or maybe you do, but that's another discussion.

If my battery has sufficient capacity, I should not need the TV to power anything and can keep both system isolated from one another.



Any reason as to why I should not tie the negative of the DC source directly into the negative /common bus of the distribution block, using the largest wire possible?
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Re: Sharing common (ground) questions

Postby kludge » Thu Jun 22, 2017 5:15 pm

Vspec wrote:For ease of trouble shooting (that's my day job), each appliance will have it's dedicated ground/negative tied back to the distribution fused panel. From there I was planning on
connecting directly back to the battery or converter.


Sounds like a plan.

kludge wrote:Return current will always flow back to it's source, even if there are multiple sources in a system. Where you run into issues is when your return currents can share a conductor that isn't rated to handle the total current flow. That's why it's a good idea tie them together at the same point on the frame. both the TV and the Trailer will have the same reference point, but currents that don't belong there don't flow. On the positive side (+12V) you will want to isolate them and usually best to be able to connect to one at a time, as I mentioned in the first paragraph. You don't want current flowing from the +12V in your trailer to the +12V in your truck. Or maybe you do, but that's another discussion.


Vspec wrote:If my battery has sufficient capacity, I should not need the TV to power anything and can keep both system isolated from one another.


Do you think you will ever want to charge the trailer battery while driving? That might be one reason not to totally isolate the systems.

Vspec wrote:Any reason as to why I should not tie the negative of the DC source directly into the negative /common bus of the distribution block, using the largest wire possible?


I think you going about it correctly. Bring all the negatives back to a negative bus at the fuse block. Then tie the DC source (battery, converter, etc.) to the fuse block with one big cable - big enough to carry the entire load of the trailer, as if everything is running at the same time, plus a safety factor. You will probably want to use the same wire gauge on the positive and negative from the DC source to the distribution block.

If you are running totally isolated systems and no AC, there's no reason you have to connect the negative of the DC system to the trailer frame grounding point, but you may still want to from a static electricity point of view, just to keep it down to a minimum. And if you plan add an on-board DC converter it's a good idea to have the AC grounded to the trailer frame.
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