Why should I put a fuse in both the positive AND negative leads of my CB radio?
Why fuse the negative lead? VERY good question. I'll try to explain.
Vehicles use the chassis/frame for some of their DC grounds. The negative lead of the battery is attached to the chassis and usually the engine block. The case of the starter motor is often times the ground connection so it gets its ground, back to the battery, via the block. This works fine.
What would happen if the chassis/ground connections between the negative lead of the battery and the starter became bad....or, more likely, started to get some resistance in it? The starter would have more trouble turning over the ending due to the voltage drop across that part that has a bad connection (aka., resistance).
Now...go one step further. You mount a radio in your vehicle. The power leads go directly to the battery (as they should) or to a nearby aux fuse block that was installed by you to handle your extra goodies.
The antenna is mounted on another part of your vehicle and its mount is grounded to the vehicle's body/frame (in order to work properly). The coax connects the CB radio chassis and the antenna together....and the outer coax braid is at ground also (but it is covered with an insulative coating).
OK...back to earlier in the explanation. You have a heavy current carrying device...like a starter....or maybe a light bar that is grounded to the vehicle body for the negative connection for that batch of off-road lights it is holding up. If the negative battery cable that goes to the block/chassis starts to go bad, the DC current flowing from one of these items...let's say the light bar), can find a ground through the body of the vehicle to the antenna mount and down the antenna coax and through the CB chassis/circuit board ground and through the CB negative power lead to the battery itself.
When that happens, you might very well have 30~50 amps of current flowing through the CB radio's chassis, trying to get to the negative terminal of the battery. If it were the starter motor that found its "other ground path" through here, you would have a couple hundred amps flowing.
The end result is that your CB chassis, with those tiny copper circuit board runs on it, can become a fusable link and burn open due to the excessive current flowing through the ground circuitry of your radio chassis. So...you burn out the guts of your radio.
By putting a fuse in the negative lead of your CB radio (or ANY other electronic device that is attached to the battery), it will blow open and stop the high current flow before it does any permanent damage. (just like the positive lead fuse will blow open if you shorted the hot lead to ground).
If you don't believe this, please feel free to stop by my house. If you volunteer your vehicle and radio, I shall be happy to do the "faulty ground wire" scenario on your vehicle. You can turn on the lights or crank over your engine and watch your CB radio smoke.
There should be a connection direct to the chassis of the car direct from the negative terminal of the battery. The starter motor is bolted hard onto the block (which also has a direct connection to the chassis) if the positive starter motor cable goes hi ohms, that wont affect the other circuits.
Fusing the negative MAY be an overkill. The positive should open if the fuse current is exceeded in THAT circuit! Each positive cable is an independent circuit, the only commonality is the negative return.
Thats my opinion anyway.
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I don't have a fuse on negative wire to battery .... should I ?
All of the commercially installed two way radios that I had in My vehicles were fused to both positive and negative. Those units were a lot more expensive than the equipment we are useing, but tis not difficult or expensive to install a second fuse.
I know Ford has used fusable links in their earth leads from batteries to chassis on their Transit vans in the past, (unsure if they still do).
I must agree that fitting an inline fuse on both pos and neg is a small cost and will protect the device it feeds if theres a problem.
Kezngaz.
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Thanks everyone for replying to post ... yes small cost to put a fuse on negative line .. just like most people I always thought that 1 fuse would be quite sufficient ...
The down side is every fuse adds voltage drop, so be sure to use good fuse holders and quality fuses. As for protecting the circuitry in the radio, it would need to be a very small fuse to do that. Fuses are for protecting the cable from melting and causing a fire, not to protect the appliance from damage.
T1 Terry
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Well to play the Devil's Advocate here I Suggest that the Cb Radio should rely on the Inline fuse being on the positive lead ONLY, One should not have a safety fuse ONLY on the negative line, Having a fuse only on the Negative line will not protect your Equipment and having a fuse both on Negative and Positive line can only lead to more problems and no added safety
Hi pinball never planned to put fuse on negative only ! cb already set up with fuse on positive which is a no brainer ... thanks for your reply mixed response from everyone appreciate it .
good reading all this and it brought back some memories of my early days when bmc minis were all the go.If the earth lead between engine and subframe broke,the choke cable became the earth and glowed red,didnt do the cable much good.
So here's a mind bender. We know electricity is able to do work via the flow of electrons. Yes! But there are two bizarre facts about electricity. Fact One - each of the individual electrons travel about the pace of a snail , yes, the event horizon travels near the speed of light, but each electron travels slowly. Thing of a tube full of ball bearings. You push one ball earring into the tube and one instantly falls out the other end, but it's not the same ball bearing. Add that to quantum dynamics and thibgs start getting hairy.
Fact Two (which I'm still looking for a third source to verify), the actual electrons travel from the negative to the positive. Negative is negative because it has excess electrons availabl and "pushes" them towards positive. So "electricity flows" from negative to positive!!. Ohhh my head hurts !!
The earth strap would be very unlikely to just break with no prewarning signs, such as slow starter turn over &/Or obvious broken strands & overheating
But in the event that the strap O/Ced or developed high resistance,electric current will take lowest resistance paths.
one of those path could well be the metal shielding of the antenna back to the radio & then through all common earths back to battery neg.
Provided the radio chassis is not directly earthed any other way ,then certainly a fuse in the radio neg would prevent the aerial feed shielding from carrying current
However I would suggest it would be much better to ensure the engine block to battery neg strap is kept in good condition. even using two!!
It is a very essential part of the system ,High resistance could lead to currents being shunted to places where they could do other damage,
The alternator could ,depending on set up ,also be damaged if it had open circuit in the neg line back to the battery.
So here's a mind bender. We know electricity is able to do work via the flow of electrons. Yes! But there are two bizarre facts about electricity. Fact One - each of the individual electrons travel about the pace of a snail , yes, the event horizon travels near the speed of light, but each electron travels slowly. Thing of a tube full of ball bearings. You push one ball earring into the tube and one instantly falls out the other end, but it's not the same ball bearing. Add that to quantum dynamics and thibgs start getting hairy. Fact Two (which I'm still looking for a third source to verify), the actual electrons travel from the negative to the positive. Negative is negative because it has excess electrons availabl and "pushes" them towards positive. So "electricity flows" from negative to positive!!. Ohhh my head hurts !!
No .NO You got it wrong ,the electrons flow into the neg hole left by the other electrons being pushed out through the positive pipe, just like an unending circle of ball bearings
not sure if they are the same shape but must be cause they can change direction with no problems, even millions of time per second
I can remember the days when it w,as simple.
Electricity flowed from positive to negative
Then along came the electron theory & electron flow
The world has never been the same But the old water anology still meets the challenge
You got it wrong ,the electrons flow into the neg hole left by the other electrons being pushed out through the positive pipe, just like an unending circle of ball bearings
