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Author Topic: Fuse in Negative lead  (Read 8187 times)
WB0KSL
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« on: September 29, 2010, 11:43:50 AM »

I'm installing an Icom IC-880H in my Honda CR-V.  I have 8ga power leads going to the rear of the car where the main radio unit is mounted.  Both leads are fused at the battery, 40 Amps each lead.  The 8ga power cable plugs into a RigRunner in the back of the car, the radio plugs into the Rigrunner. 

Icom's (long) power cable for the 880 is fused in both leads, at the far end from the radio connector.  I assume that Icom's use of fuses (20amp)in both leads is in keeping with current good practice of preventing the radio negative lead from being the only path to ground in the event of a major wiring failure (such as car starter ground lead failure etc.)  I do hate to have more fuses, connectors etc. and their inherent failure points (both mechanical and electrical) than necessary.  If I clip the fuses out of the radio power cable and plug it directly into the RigRunner, I'll still have the RigRunner's 20amp feeder fuse, its 40amp buss fuse, and the 40amp fuse at the battery, all in the positive lead.  The negative circuit will have the single 40amp fuse at the battery.  Is this adequate, or should I be putting a smaller, 20amp fuse in the negative cable lead between the RigRunner and the radio?

73 de John - WB0KSL
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AA4PB
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« Reply #1 on: September 29, 2010, 04:51:24 PM »

The 40A fuse in the negative lead at the battery should be fine. If you put a 20A fuse there then it would limit the total RigRunner load to 20A. I assume you are using the RigRunner because you want to power other devices from it in addition to the Icom.
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KE4DRN
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« Reply #2 on: September 29, 2010, 06:29:23 PM »

hi John,

I'd leave the stock fuses in the power cables
just in case  you need to operate the radio
where there is no rigrunner available.

73 james
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K7RBW
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« Reply #3 on: September 30, 2010, 05:05:25 AM »

I had a similar situation and ended up clipping the fuses out of the power cord for the reasons cited above. To cover all the bases, however, I then put connectors on the clipped section with the fuses so I could re-insert the power-line fuses for use in cases where there were no other fuses (e.g.directly off a battery when portable).

Best of both options.
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WB0KSL
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« Reply #4 on: September 30, 2010, 08:31:45 AM »

Good suggestions, all.  I think I'll go with K7RBW's last post.  By clipping the cable just long enough to comfortably reach the RigRunner, I cut down on unnecessary connections/fuses in the positive lead, but still have the 40amp fuse in the negative lead at the battery.  I'll put Anderson Power Poles on the remaining 6 feet or so, which will have Icom's dual fuses in it.  Save that for portable, etc.  Thanks, guys.

73 de John - WB0KSL
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VE3ZXK
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« Reply #5 on: September 30, 2010, 06:57:50 PM »

I never understood the reason for the negative fuse, this makes sense.

I suppose the answer is to preserve the negative fuse if you're wiring direct to the battery. In lower-power radio applications, less than 50W, I usually wire power through the vehicle fuse panel in the cabin of the vehicle. In this case, I don't think preserving the fuse would have any benefit, and would likely be a point of contact failure before limiting any current. In such case, I remove the fuse, wiring the negative to ground. Recall that your radio bracket (if mounted on metal) and shield of you antenna coax are connected to vehicle ground, and there is no protection there. Both of these contact the radio chassis directly, as much as the negative lead.
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AA4PB
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« Reply #6 on: October 01, 2010, 06:36:20 AM »

The fuse in the negative lead is recommended when connecting the negative lead directly to the battery. If the starter motor negative lead were to open (or become corroded and high resistance) then starter motor current (which can be a hundred amps or more) would flow through the radio's other grounds (antenna, mount, etc), the radio chassis, and the radio's negative lead back to the battery. This high current is not a good thing for any of the hardware or wiring involved. If you have a 20A fuse in the negative lead then the fuse will blow and protect the equipment and wiring once the current reaches 20A.

Yes, if the negative lead fuse is blown then the radio will draw its current through the other grounds (antenna, mount) but this doesn't have nearly as much potential to damage anything as the hundreds of amps of starter current. Depending on the resistance of the other ground connections and the size of the radio, you may notice low power output or dimming of the dial lamps when you talk (SSB) or key the radio (FM) if the negative lead fuse is blown.
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K0BG
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« Reply #7 on: October 01, 2010, 06:37:03 AM »

PowerWerx carries OEM parts to make up just about any power cable. If nothing else, it save you from cutting the stock cable.

I shortened the feed to my IC-7000 by disassembling the stock cable, but that's just me.

Edit: I agree with Bob. I received an e-mail several months ago, about an esoteric problem one of my readers was having. The radio in this case would shut down when he tried to transmit with the engine off. He'd been working on the problem for several weeks. I suggested he measure the voltage at the back of the radio, which he did. Discovering it was low, took him to the power cable. There wasn't even a fuse in the negative lead. Irksome to be sure, but no damage done.
« Last Edit: October 01, 2010, 06:41:35 AM by Alan Applegate » Logged

AA4PB
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« Reply #8 on: October 01, 2010, 06:41:52 AM »

Me too Alan. A pair of dikes will do wonders  Grin  I made the factory cable just long enough, took out the fuses, and terminated it with a PowerPole. The 20A fuses are back in the main power cable near the battery connection. With the PowerPole termination I can always make a fused adapter cable if I take the radio out of the car.
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N1DVJ
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« Reply #9 on: October 21, 2010, 11:11:53 AM »

The negative fuse is a hot topic. 

I NEVER use the negative fuse, but then, I never ground to the battery.  Why?  Because the battery is NOT necessarily at vehicle ground potential.  It's always slitghtly off.  Even worse, it can be off in a MAJOR way during vehicle starting.  Hundreds, maybe even a thousand, amps are needed to start a vehicle engine.  During that time, the block and the battery negative can be separated by over a volt.  And since most of the items in the dash area are motor referenced, do you want to be at the battery or at the vehicle for your ground?

IF you want to be at your battery, by all means, make it with a fuse.  But I prefer to use the high end audio ground points available in most vehicles.  Chryslers with Infinity sound systems always grounded in the front of the trunk, behind the back seat.  When I mounted my Alinco in the trunk, there's where I picked up my ground.  And I ran a single 10ga stranded wire run from the battery (with a fuse) back to the power plug in the trunk.
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AA4PB
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« Reply #10 on: October 21, 2010, 11:36:11 AM »

On my Mazda (Ford) PU I didn't connect either power lead "directly" to the battery. The vehicle has a heavy duty power connector within a foot of the battery where all power except the starter is taken. It also has a heavy duty ground lug on the chassis connected by a short, heavy wire to the battery neg terminal. Both + and - starter connections are via separate wires directly to the battery terminals. In this case I connected my radio pos wire via a 20A fuse to the vehicle power terminal and my radio neg terminal directly to the vehicle power ground terminal.

The best way to connect the power depends on what the manufacturer has provided.
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K0BG
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« Reply #11 on: October 21, 2010, 12:25:50 PM »

Quote
Because the battery is NOT necessarily at vehicle ground potential.


Gee Mike, how can that be? Everything in the vehicle is referenced to battery ground.

Quote
Hundreds, maybe even a thousand, amps are needed to start a vehicle engine. During that time, the block and the battery negative can be separated by over a volt.


Except for some very large OTR engines, most starter motors draw about 300 to 600 amps, and then only as inrush current. Run current is typically less than one half inrush. Depending on where and how you measure the voltage drop during starting, it may be as much as 6 volts, but not for very long. In any case, there won't be a voltage drop through the chassis unless there is a serious wiring problem.

Quote
...I prefer to use the high end audio ground points available in most vehicles.

I've never seen such a thing. Really good sound people always wire to the battery for very good reason; they don't want to set up a ground loop, or create a Pin One problem. If you were to measure the voltage drop from the battery to the point you connected the ground to your Alinco, you just might be surprised how much it is during transmit. There is another issue too. It is very possible to upset a data sensor by creating a ground loop. Most folks think that vehicle grounds are just haphazardly, but they're not. Any device sharing power or interconnections, are always grounded at the same point. Again, this is to prevent a false signal as a result of a ground loop. A data disruption can occur if there is enough of a drop between the true battery ground, and where you transceiver is connected. This is one of the reasons vehicle manufacturers are very specific about how to wire their vehicles for both transceivers, and high power audio system.

I suggest you down load the Chrysler's wiring guide, and give it a good read.
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N1DVJ
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« Reply #12 on: October 21, 2010, 01:24:06 PM »

Quote
Because the battery is NOT necessarily at vehicle ground potential.


Gee Mike, how can that be? Everything in the vehicle is referenced to battery ground.


Really?  There's no I^2 R differentials with hundreds of amps flying around in a vehicle?   That's a new one to me!

You've never seen a dash fire in a vehicle because of a ground strap failure too I suppose.
Quote

Quote
Hundreds, maybe even a thousand, amps are needed to start a vehicle engine. During that time, the block and the battery negative can be separated by over a volt.


Except for some very large OTR engines, most starter motors draw about 300 to 600 amps, and then only as inrush current. Run current is typically less than one half inrush. Depending on where and how you measure the voltage drop during starting, it may be as much as 6 volts, but not for very long. In any case, there won't be a voltage drop through the chassis unless there is a serious wiring problem.

Ever see a start stall?  Ever see what happens to measurement shunts when that happens?  Yeah, USUALLY the current is lower than 1000 amps, and if you'd stop 'picking' and actually read, you'd see that I said that.  To me 'hundreds' sure overlaps with your 300 to 600.  Geesh, why do you feel you have to pick that apart?

And as much as 6v but not for long.  Come on!!  How long does it take before bad things start to happen at hundreds of amps!
Quote
Quote
...I prefer to use the high end audio ground points available in most vehicles.

I've never seen such a thing. Really good sound people always wire to the battery for very good reason; they don't want to set up a ground loop, or create a Pin One problem. If you were to measure the voltage drop from the battery to the point you connected the ground to your Alinco, you just might be surprised how much it is during transmit. There is another issue too. It is very possible to upset a data sensor by creating a ground loop. Most folks think that vehicle grounds are just haphazardly, but they're not. Any device sharing power or interconnections, are always grounded at the same point. Again, this is to prevent a false signal as a result of a ground loop. A data disruption can occur if there is enough of a drop between the true battery ground, and where you transceiver is connected. This is one of the reasons vehicle manufacturers are very specific about how to wire their vehicles for both transceivers, and high power audio system.

I suggest you down load the Chrysler's wiring guide, and give it a good read.

I have Chrysler and other wiring guides, and I've read them.  I've also installed many high end systems in vehicles over the years, and yes, I've connected directly to the battery more often than not.  IT DEPENDS ON THE SYSTEM.  If the system I was installing was 'isolated', that is, the 'audio' ground was AC coupled to the chassis and NOT DC coupled, and I wanted a low impedance feed, then the battery was my first choice.  Unless I went with an aux battery in the trunk with the biamp setup.  Or nowadays I suppose the big cap arrays that people seem to use.  But an aftermarket setup is not always the case.  Infinity, for example, put amplifiers in the rear.  In the Dodge Intrepid they were trunk installed, and referenced to the chassis for ground.  As such, they used the low impedance ground point in the trunk.  If I still have an old Intrepid factory manual I'll see if I can make a pdf of the page and send it to you. 

If you have a radio that is DC isolated at the chassis and the antenna (a lot are) then there could be advantages to hooking at the battery.  Lower impedance power source.  Higher noise immunity.  And others.  But if you have things TIED to that radio that reference to ground, like a data link, or a speaker that you mount in a dash panel, and suddenly your radio sees that ground tied to it's circuit ground.  When the dash ground varies by even a fraction of a volt from the battery, you now have currents running through the chassis that are not conducive to a good quiet signal, and certainly not to long life of your radio.

Think of it this way if you can...  You have a radio in an insulated chassis, and power and ground that you take to the battery.  Battery ground is usually at 0.00v to the chassis.  You screw your radio down and run a speaker (or mic interface, APRS, or whatever) that happens to touch and tie to vehicle chassis ground, say behind the dash.  When the two are equal, fine.  But when the battery isn't at the dash ground, that current will flow through the speaker ground (or whatever) THROUGH the radio to the battery and try to 'equalize' things. 

The most violent example of this is what happened to about 3 years of Fords back in the 70's.  They had a run of bad straps in the engine compartment.  When they corroded, the battery ground to the chassis and the engine block ground could get separated by a few volts when starting the motor due to I^2 R losses in the battery to engine block cable.  But all the idiot lights and sensors were connected to the engine AND through circuitry to the chassis ground of the dash area.  Current flowed.  Ford had LOTS of dash fires. 

OK, don't believe me...  Hook a voltmeter up to your battery ground and and a ground at the dash.  Start the car, and watch it.  If it STAYS at zero the whole time, then you have good connections and everything is great and all the excitement of this topic is a moot point.  Today.  IF your systems STAYS in good condition.

Connect to the battery?  Sure, if you fuse both sides AND your radio is isolated from chassis on the ground.  Most now are.  Even the antenns shield isn't connected to 'power ground', and that's what makes it possible.  And most of those manufacturers are the ones providing double fused leads.  If you manufacturer provided only a single fused lead, then you need to really check out the radio before you ground to the battery.  If the antenna connector is DC coupled to the power ground, that means the radio has a hard connect to whereever on the chassis you ground your antenna.

And...  You bring up the point that there are single ground points for sensors and other devices.  That IS the point.  If you rig forces you to consider the ground connection at the antenna as part of the DC power ground, then you shouldn't be trying to fight it by connecting to the battery ground.

Ok, you say the battery ground is quieter and lower impedance.  But then you ignore that this very atribute of the battery ground is WHY you can't use it in some cases.
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K0BG
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« Reply #13 on: October 21, 2010, 01:55:57 PM »

It is obvious that I hit a nerve. But, when I read your comment about the Ford problem, you lost me because you're mixing metaphors. Yes, Ford had dash fires, but that wasn't a grounding issue, it was a chaffing issue. And yes, they had battery to starter cable problems due to the aluminum wire that was being used at the time. However, the two issues have nothing to do with one another. Oh! By the way, during circa 70, Chrysler and GM had their own problems with aluminum wiring.
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N3IDG
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« Reply #14 on: October 21, 2010, 04:59:50 PM »

i disagree with a fuse in the negative cable when i blew that fuse the radio grounded through the coax and would go dim on transmit , found problem quickly and did away with that fuse , no problem since
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