DC neg bus bar why unprotected ? - Fiberglass RV
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Old 08-15-2022, 08:13 AM   #1
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Name: Andy
Trailer: Keystone
Wisconsin
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DC neg bus bar why unprotected ?

I have a 12v DC negative bus bar wired per the manufaturer of the converter installed on the vinyl floor behind the fuse panel

Can someone explain to me why this negative bus bar does not need to be covered / protected ?

Given that it is direct current doesn't it carry current ?

If I change the converter to an inverter & wire power into this DC fuse panel does this change my question above in regards to the negative bus bar ?

Any thoughts or your shared knoweldge is appreciatted
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Old 08-15-2022, 09:39 AM   #2
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I suggest you contact an electrician.
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Old 08-15-2022, 10:30 AM   #3
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Our trailers are usually wired with the positive DC line insulated and the negative DC line connected to the trailer chassis. Because the negative line is connected to the chassis, there is no voltage differential between the buss bar and the chassis. No current will flow from the buss bar to anything attached to the chassis, so no insulation is needed.
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Old 08-15-2022, 10:31 AM   #4
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The entire trailer frame is a negative terminal. As are the entire frame and hitch on the truck. While it is true that you have to keep anything connected with the positive side from touching the negative buss, and causing a short, you have to do the same with any positive connected wires from touching the frame. The positive side is the one normally protected with insulation, fuses and circuit breakers. Both sides don't need to be protected. Imagine how you might insulated the entire trailer frame, for instance. Not practical and not needed, but it is the same thing as the negative buss and probably has a wire running from the buss to the frame to connect them. The trailer frame, for instance, is usually used as a negative conductor.
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Old 08-15-2022, 11:13 AM   #5
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Thanks for the explanation

So should there be a wire from the negative DC bus bar to the chassis ground buss bar?
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Old 08-15-2022, 11:44 AM   #6
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^Yes, they're both connected together somewhere.
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Old 08-15-2022, 11:45 AM   #7
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Originally Posted by Agboett View Post
Thanks for the explanation

So should there be a wire from the negative DC bus bar to the chassis ground buss bar?
I am a belt and suspenders kind of guy. I would insulate everything. A DC power system is not required to have a "ground". That is the AC system. This is a safety thing. If a loose hot, (120VAC) wire were to contact anything that would be bad to have at 120VAC, then it would blow a fuse. This reasoning doesn't apply to 12VDC.

Grounding the DC system is a bad idea, in my opinion.
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Old 08-15-2022, 11:47 AM   #8
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Thanks for the explanation

So should there be a wire from the negative DC bus bar to the chassis ground buss bar?
There usually is, in some form, to complete the brake circuit and/or complete the charging circuit from the truck 12v plus wire in the seven pin plug. Often the ground wire in the seven pin plug, goes directly to a bolt in the frame. The brake ground wire will go to that same bolt, or another one closer the the axle. Then a separate wire will go from one of those bolts to the ground buss near the battery. This completes the charging circuit and the brake circuit. It can also be used to allow a one wire tail light circuit, that uses the frame for the ground.

I discovered a mistake in the Oliver wiring system on my Oliver where there was no wire between the ground buss and the frame, even though the lugs were there. I found the same problem on another Oliver as well. The problem was, the trailer batteries would not charge from the seven pin plug. I called Oliver about it and they said "the trailer is wired to code". Wrong answer. Apparently, they were relying on the trailer ball as the ground connection, which is a terrible idea. I dug deeper and found the guy who was either designing the systems, or doing the trailer wiring was let go. I'm sure it wasn't just because of that one thing though, as they had other wiring problems that were unacceptable too.
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Old 08-15-2022, 02:48 PM   #9
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The 12V negative side of the house wiring must be electrically connected to the neg- (ground) side of the house battery.
I prefer to use a wire in the 12V house wiring to accomplish this for reliability purposes.


The DOT light wiring must return the neg-(ground) back to the TV.
On these trailers, I prefer to hard wire back to the plug when there are no brakes or to a common ground on the frame when there are brakes.(conventional)

If you have brakes and a break away switch then the battery and the brakes are usually grounded to the frame. check these grounds for corrosion when ever you service the brakes.
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Old 08-20-2022, 10:50 AM   #10
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I just replaced my power supply/converter. Both the new and the old had a unprotected buss bar built in. That buss bar is for ground wires. It does not need to be protected. I’m sure the buss bar you are seeing is also for ground and is ok unprotected.
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Old 08-20-2022, 08:40 PM   #11
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Thanks for the explanation

So should there be a wire from the negative DC bus bar to the chassis ground buss bar?
There needs to be a wired connection from your DC negative bus to your AC ground bus and to your chassis. And no, your dc negatives do not need insulating.

Your DC negative bus at your charger (converter) needs to be connected to the chassis ground - the DC devices in the trailer (lights, fans, radio, etc) need a return path not only to the charger, but to the tow vehicle as well. While “trailer wiring” (brakes, signals, etc) is normally connected back to the chassis and from there to the battery and tow vehicle, those other loads likely are not. Areas where those devices are mounted are not usually conductive - wood, fiberglass - so there has to be a return (typically a black wire in this part of the DC world).

As for the connection to the AC ground (NOT neutral!!), this is a safety requirement. Should you have something plugged into one of your AC receptacles while connected to shore power, such as an extension cord, and if that extension cord were to have a bared black wire (black is hot in the AC world), and that hot lead were to come in contact with one of those white DC returns, there must be a continuous COPPER path back to the AC source - you want full fault current to flow unimpeded so that the breaker trips.
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Old 08-20-2022, 10:07 PM   #12
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There needs to be a wired connection from your DC negative bus to your AC ground bus and to your chassis. And no, your dc negatives do not need insulating.

Your DC negative bus at your charger (converter) needs to be connected to the chassis ground - the DC devices in the trailer (lights, fans, radio, etc) need a return path not only to the charger, but to the tow vehicle as well. While “trailer wiring” (brakes, signals, etc) is normally connected back to the chassis and from there to the battery and tow vehicle, those other loads likely are not. Areas where those devices are mounted are not usually conductive - wood, fiberglass - so there has to be a return (typically a black wire in this part of the DC world).

As for the connection to the AC ground (NOT neutral!!), this is a safety requirement. Should you have something plugged into one of your AC receptacles while connected to shore power, such as an extension cord, and if that extension cord were to have a bared black wire (black is hot in the AC world), and that hot lead were to come in contact with one of those white DC returns, there must be a continuous COPPER path back to the AC source - you want full fault current to flow unimpeded so that the breaker trips.
DC house wiring doesn't need to be wired back to the TV. DOT wiring does (of course).
The house battery needs a common ground with the TV and the DOT wiring only if you plan to charge the House battery from the TV while towing, since there is only one ground in the RV plug. The DOT wiring doesn't need a connection to the converter or the house battery to work properly and the house battery doesn't need a connection to the TV in order to work properly.
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Old 08-20-2022, 10:34 PM   #13
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DC house wiring doesn't need to be wired back to the TV. DOT wiring does (of course).
The house battery needs a common ground with the TV and the DOT wiring only if you plan to charge the House battery from the TV while towing, since there is only one ground in the RV plug. The DOT wiring doesn't need a connection to the converter or the house battery to work properly and the house battery doesn't need a connection to the TV in order to work properly.
Agreed! My point was for the case where I want my TV to charge my house battery while towing.

You’re quite right that they don’t need to be connected to each other to work individually; but they do if you want them to work together.
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