Ham Radio Fuses and Fire?

[modelbuilder]

So my Ham radio since it has been installed has had the fuses closer to the receiver than the battery. I have a friend who is telling me that unless the fuses are closer (like right near the battery) I could start a vehicle fire.

Had the thing for a year and never had a problem.

What gives? I don' think it's as dire as he is making it out to be.

 

[gary in ohio]

So my Ham radio since it has been installed has had the fuses closer to the receiver than the battery. I have a friend who is telling me that unless the fuses are closer (like right near the battery) I could start a vehicle fire.

Had the thing for a year and never had a problem.

What gives? I don' think it's as dire as he is making it out to be.

keep in mind the fuse is to protect the radio and the battery. Fusing at the radio does nothing if the short is before the radio. Fusing at the battery will protect the entire circuit. I have circuit breakers on the battery side and use the stock fuse on the radio side. That way if I every pull the radio to use it else where the fuse is still inline near the radio. I also fuse both the positive and negative lines.

 

[modelbuilder]

Hmm...All my wires in the engine bay are inside flex tube...soldered joints for the extension are in shrink tube, electrical tape, and flex tube. I'll put in another fuse this week.

Until then my car shouldn't catch fire right?


.

 

[cruiserlarry]

You should always fuse a circuit as close to the power source as possible. With 2-way radio equipment, both the positive and negative leads should be fused, too. And the radio's power and ground leads should be wired directly to the battery, not the fuse box or chassis - that way the radio is isolated from electrical interference created by the vehicle's wiring...:smiley_drive:

 

[chris snell]

You should always fuse a circuit as close to the power source as possible. With 2-way radio equipment, both the positive and negative leads should be fused, too. And the radio's power and ground leads should be wired directly to the battery, not the fuse box or chassis - that way the radio is isolated from electrical interference created by the vehicle's wiring...:smiley_drive:

Why would you fuse the negative lead?

 

[Mash5]

Why would you fuse the negative lead?

I have also wondered this... Radio guys tend to do it and many radios come with fuse holders in both lines so I'm guessing there is a solid reason.
My guess is that there is a possibility of the signal received on the antenna generated power that could be shorted to case ground if something was wrong and thereby power the ground wire that would also be connected to case ground... but that's just a guess.
If anyone has a better explanation I would love to hear it.

 

[chrismc]

The fuse on the ground line is because some commercial vehicles used to be wired with a positive ground. Since you didn't know what type of vehicle the radio would end up in, you would put fuses on both lines just in case. In a modern vehicle, the fused ground lead doesn't really serve any function, and can actually cause problems if the ground fuse were to blow (a large electrical path will stay energized, and the current may find an alternate ground path with higher resistance, ie fire).

More info

 

[SunTzuNephew]

Here's another explanation:

On the Technician class test there is a question which says, “What is the purpose of a fuse in an electrical circuit?” and the answer is, “To interrupt power in case of overload.” In the Gordon West Technician Class Q And A book it says, in the added notes, “Place the fuse as close to the battery as possible.” (Page 162). That is good advice because if for any reason the positive wire’s insulation is damaged and the wire touches the chassis or engine (a hot manifold is a frequent cause of this problem) then it will blow the fuse if it is close to the battery but not if it is between the radio and the short.

On that same page it shows a schematic of an installation and it shows a fuse in the negative line. Many hams think that is unnecessary because you will not cause a short if the negative wire touches the metal of the vehicle. So why place a fuse in this line?

The diagram shows the negative wire going straight back to the negative terminal of the battery. What I am about to say I have personally seen happen more then once so it is not hypothetical. If the battery cable develops resistance between the cable and the body of the vehicle by rust or corrosion or the wire itself corrodes to the point that it is not a good conductor this type of installation can cause problems.

When the engine is being started a lot of current is being drawn from the battery and the wiring to the mobile radio is not designed to handle any where that much current. Simple Ohm’s law will tell you that the maximum current will flow through the path of least resistance and if that path happens to be through the negative wire of the radio to the negative terminal of the battery then that is the where the most current will flow. Frequently the unit is not grounded well at the mounting bracket but the shield side of the coax makes a good ground by the antenna mount. In that case the current for the starter will attempt to flow through the coax shield to the coax connector on the radio then on to the negative wire to the battery. If that wire is not fused the coax shield will smoke. If the radio is grounded at the mount the negative wire to the battery is not big enough to handle the load and it will smoke. Either way there is a fire danger.

http://hamslife.com/?p=106

 

[DaveInDenver]

In a modern vehicle, the fused ground lead doesn't really serve any function, and can actually cause problems if the ground fuse were to blow (a large electrical path will stay energized, and the current may find an alternate ground path with higher resistance, ie fire).

This is not true unless you ground your load to some place other than the battery. If you run both wires back to the battery you need to keep the negative fuse. You can eliminate the negative fuse if you make a good ground close to the radio that's to your vehicle chassis. It is true that it is risky to the radio to run the negative all the way to the battery and leave it unprotected. If a winch or starter cable shorted there could be a pretty big current looking for ways home.

BTW, there are still valid reasons to wire both sides all the way to the battery. Having equal lengths of similar positive and negative sides might be necessary if the radio designer has a common mode choke on the power supply. If you change the routing or length of one leg relative to the other, then a common mode filter won't work anymore and so you might defeat one method of eliminating ignition or alternator noise or a filter to keep RF from getting into places you don't want it.

As a result of being microprocessor controlled, modern radio power supplies are somewhat sophisticated, so the way you supply power to them isn't necessarily the same as you might for a fridge or aux lighting. For example, if you check the schematic for the Yaesu FT-857 and FT-897, this is the reason for the T3002 coil in the PA unit, to filter the 13.8V supply for common mode noise.

 

[chris snell]

This is not true unless you ground your load to some place other than the battery. If you run both wires back to the battery you need to keep the negative fuse. You can eliminate the negative fuse if you make a good ground close to the radio that's to your vehicle chassis. It is true that it is risky to the radio to run the negative all the way to the battery and leave it unprotected. If a winch or starter cable shorted there could be a pretty big current looking for ways home.

I'm not quite following your reasoning. Are you suggesting that the ground wire is merely a potential pathway for a short and therefore must be protected? I can't see the likelihood of this. I see a much greater risk of a winch wire, etc., rubbing against a sharp spot and shorting onto the body. My winch and leads are encased in heater hose and my starter lead goes through a grommeted hole in my battery box. My radio (once it is reinstalled) will be wired into a Blue Sea Systems fuse box, which sits next to the battery.

Note: that picture does not include the grommets which I have since installed in the holes.

 

[crusader]

I'm not quite following your reasoning. Are you suggesting that the ground wire is merely a potential pathway for a short and therefore must be protected? I can't see the likelihood of this.

I think this is what's being said:

Imagine that the huge black wire going from your negative battery post to the frame/engine block breaks off one night. You go to crank up your car in the morning, sending lots of electricity toward the starter. With that ground wire to the battery broken off, that electricity can't complete the circuit back to the battery and nothing happens.

But wait! That enormous electric current DOES have a path to get back to the negative terminal of the battery--It can go through your radio mount, through the radio chassis, down that (comparitively) tiny black wire and back to the negative terminal of the battery, possibly frying the wire and your radio in the process. Frying the wire may cause a fire if conditions are right.

The fuse in the negative circuit will help prevent the above scenario by opening the circuit, and halting the flow of electricity through that particluar path.

 

[crusader]

crusader, that's one supposed scenario. It might be unrealistic because many vehicles ground the starter by being bolted to the bellhousing and don't have a ground cable. It might be the cable from the negative battery post to the engine block that breaks.

????

That is the scenario I described. At least that's what I THINK I described. Lemmie re-read it...


Yup...A situation where the negative cable from the battery breaks off the attach point to the frame/engine block, leaving the the radio ground as the only return path to the negative battery terminal.

 

[DaveInDenver]

????

That is the scenario I described. At least that's what I THINK I described. Lemmie re-read it...


Yup...A situation where the negative cable from the battery breaks off the attach point to the frame/engine block, leaving the the radio ground as the only return path to the negative battery terminal.

You are right. Yesterday was a l-o-n-g day, last post before heading home. Looking back this morning with my first cup o' joe and it's obvious I may have read the words but they sparked nothing close to coherency in my head... Sorry, sooo I deleted the post and corrected it...

crusader, that's a good scenario. It would not have to be the only ground path, just one that has a ground loop potential referenced to chassis for possibly damaging current.

Point is the negative fuse is not there as a mate to the positive side of the radio, but to protect the radio from being a return path for another circuit. IOW, the positive will open (maybe the negative, maybe not) if the radio circuit overloads, the negative fuse will open (positive probably not) if some other circuit shorts to frame or body ground. This is why you don't need it if you ground the radio to the frame, since there will be no ground loop created through the radio. But doing that might be counterproductive from a radio noise standpoint in some cases, particularly with our HF rigs that would be much more susceptible to it than an FM VHF rig.

 

[eugene]

First truck I owned (1988 Chevy S10) the previous owner had put on a cheap 3" body lift and the negative battery cable was too short so they never connected it. I had taken the body lift off and reconnected the battery cable but the mount for it on the engine block must have been rusty already as a couple years later the smaller negative cable to the body melted when I went to start it. I had to unbolt the main negative cable and scrape the block to get it to start. So those things can happen. I later put one of those internal toothed lockwashers between the cable and block to make sure it gets a good connection.

 

[Mash5]

I ran a pair of #8 (it was what I had) wires from direct connections at the battery to inside the cab. The Positive side has a 40A breaker 2" from the battery to protect it from a short, but could not think of any reason to do the same with the negative side. They connect to a distribution hub inside the cab where the positive sides are individually fused appropriately for anything I would run (only the radio as of now). The radio also has it's own set of fuses on both leads. That is the way it came so I'm going with it.
The way I see it, the 40A breaker is to protect the heavy wire during a short between the Battery and the distribution hub. The fuses at the hub are to protect the smaller wires running to appliances from a short between the hub and the devise and the ones that came with the radio are largely redundant but I would not want to take them out and void a warranty.
Anyone see a gap in this logic?