While I don't truly know the answer, I have seen this many times, on many electrical components. I have always speculated that for very sensitive components, this provides a higher level of protection. Since DC current is actually believed to flow from negative to positive, a fuse on the negative wire is actually first in the circuit. In addition, for a simple circuit, a fuse on the negative wire will provide the exact same protection as a fuse on the positive wire as an equal amount of current flows through each. Maybe an electrical engineer will chime in (hint, hint).
Ground for CB Install (1 Viewer)
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While I don't truly know the answer, I have seen this many times, on many electrical components. I have always speculated that for very sensitive components, this provides a higher level of protection. Since DC current is actually believed to flow from negative to positive, a fuse on the negative wire is actually first in the circuit. In addition, for a simple circuit, a fuse on the negative wire will provide the exact same protection as a fuse on the positive wire as an equal amount of current flows through each. Maybe an electrical engineer will chime in (hint, hint).
I think the concern is that if the main negative battery cable is disconnected or vibrates loose while the CB's negative wire directly to the battery remains attached, then all of the electrical devices in the vehicle will attempt to use the CB's negative power wire to complete their circuit to the battery now that it is the only path to the negative terminal. If this current exceeds the CB negative wire's current capacity, it will cause burnt wiring, a dead/burnt CB radio and perhaps even a vehicle fire.
psfpilot has it correct. (Electronics degree... Controls Engineer by trade) Now how do we lube those windows again???? JK.
OK, time to chime in on this one...
I'm not seeing any way the main cable can come off without taking my little ground wire with it. ...plus it's not good to run it back to the battery. Look at all the factory grounds, the radio, amp, etc., they're are all grounded to different points on the chassis. The more long ground wires you have running all over the place through fuses the better chance you have of something coming loose and having a bad ground. ...very bad for electronics.
1. The reason for the fuse in the positive side is to protect if there's a short to ground. If there's a dead short inside the radio (amp, inverter, light bulb socket, etc) or the wire rubs through and makes a short to ground the fuse will blow and not melt the wire/start a fire. ...aka "thermal event"
2. If there's a short anywhere that draws enough current to blow the fuse the circuit is broken. No need to break the same circuit in 2 places.
3. When I used to be into car stereos a long time ago the best practice for high current amps was to put a fuse within 12" of the battery on the positive side (to protect that big wire if it shorted, trust me it's not pretty) and connect the ground to the closest solid point so it's as short as possible.
So my non-EE opinion is..
-for something small that won't overload the factory circuit like a 1 amp CB grab some radio power and find a good screw that goes into metal under the dash preferably with a factory ring terminal already attached.
-for something that draws some real current like an inverter or subwoofer amplifier run a big wire to the battery+ and put a fuse within 12" then find a good solid bolt on the body/frame as close as you can.
I'm not seeing any way the main cable can come off without taking my little ground wire with it. ...plus it's not good to run it back to the battery. Look at all the factory grounds, the radio, amp, etc., they're are all grounded to different points on the chassis. The more long ground wires you have running all over the place through fuses the better chance you have of something coming loose and having a bad ground. ...very bad for electronics.
1. The reason for the fuse in the positive side is to protect if there's a short to ground. If there's a dead short inside the radio (amp, inverter, light bulb socket, etc) or the wire rubs through and makes a short to ground the fuse will blow and not melt the wire/start a fire. ...aka "thermal event"
2. If there's a short anywhere that draws enough current to blow the fuse the circuit is broken. No need to break the same circuit in 2 places.
3. When I used to be into car stereos a long time ago the best practice for high current amps was to put a fuse within 12" of the battery on the positive side (to protect that big wire if it shorted, trust me it's not pretty) and connect the ground to the closest solid point so it's as short as possible.
So my non-EE opinion is..
-for something small that won't overload the factory circuit like a 1 amp CB grab some radio power and find a good screw that goes into metal under the dash preferably with a factory ring terminal already attached.
-for something that draws some real current like an inverter or subwoofer amplifier run a big wire to the battery+ and put a fuse within 12" then find a good solid bolt on the body/frame as close as you can.
Some vehicles run accessory wires separately and directly to a negative battery post (my marine Diehard PM-1 has this) to eliminate the EMI and RFI that comes from using the chassis as a negative "ground" point. Another way this situation could occur is if the chassis end of the negative battery cable becomes loose or corroded. Small odds, but I don’t want to take that chance.
They get their negative feed from the chassis, which comes through the negative battery cable when it is connected. Once it is disconnected, if there is any other wire available such as CB negative wire direct to the battery that bypasses the negative battery cable, these items will all seek to complete their circuit through the CB radio, which is grounded to the chassis both by the braided outer wires of the coax feedline and sometimes also by any screws in the CB case that connect to metal chassis components.
It is true that if the CB is merely grounded to the chassis like everything else, then it loses its “ground” connection to the negative terminal when the negative battery cable is removed from the terminal. But if its negative wire bypasses that by being connected to the compression fitting at the battery end of the cable, or is wired separately to a negative post on the battery, the CB negative wire becomes a fire hazard.
Good point, if there's a way for a small ground wire to stay connected it's a problem, you'll feed through all kinds of things you don't want to. I do think it's pretty rare and a bad practice to make multiple small connections to separate battery posts.
So I agree with pfspilot and still recommend...
-for something small that won't overload the factory circuit like a 1 amp CB grab some radio power and find a good screw that goes into metal under the dash preferably with a factory ring terminal already attached.
-for something that draws some real current like an inverter or subwoofer amplifier run a big wire to the battery+ and put a fuse within 12" then find a good solid bolt on the body/frame as close as you can.
hicruise55
IH8MUD VOYEUR
Thread revial...
So... I just installed my CB into the ashtray, love it there! My question is, why can't you use the ashtray light pigtail? It has a ground and fused power? I know I would want to run a smaller fuse at CB, but makes sence to me as its already there?? I don't think it's necessary to run to batt and fuse ground as I have CB's in my other rigs with spliced in wiring?? Am I wrong?
So... I just installed my CB into the ashtray, love it there! My question is, why can't you use the ashtray light pigtail? It has a ground and fused power? I know I would want to run a smaller fuse at CB, but makes sence to me as its already there?? I don't think it's necessary to run to batt and fuse ground as I have CB's in my other rigs with spliced in wiring?? Am I wrong?
I actually run my CB off my ashtray light. 
I put the CB into the ash tray (it's the little Radio Shack unit), so the power was handy. It's worked fine, not really ideal as it requires my lights to be on and the dimmer at max. You can "dim" the CB, I'm sure it's not a good idea but it's been dimmed a few times without issues (accidentally). I only run my CB on runs so it's not on a ton.
Depending on the CB, it might not have enough juice. I can see a bigger CB pulling too much power, so that might be a concern.
It's not ideal, and some day I'm going to change mine.
I put the CB into the ash tray (it's the little Radio Shack unit), so the power was handy. It's worked fine, not really ideal as it requires my lights to be on and the dimmer at max. You can "dim" the CB, I'm sure it's not a good idea but it's been dimmed a few times without issues (accidentally). I only run my CB on runs so it's not on a ton.
Depending on the CB, it might not have enough juice. I can see a bigger CB pulling too much power, so that might be a concern.
It's not ideal, and some day I'm going to change mine.
hicruise55
IH8MUD VOYEUR
I didn't think about the dimmer... Prob just better to source elsewhere I guess? I like the thought of the ext speaker in the under dash speaker pod! Mines a little Cobra, looks like an updated Uniden
edit... thinking i'll use the cigarette lighters power, fused again at the CB, then rewire the cigarette lighter for more power and always on...
edit... thinking i'll use the cigarette lighters power, fused again at the CB, then rewire the cigarette lighter for more power and always on...
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