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Author Topic: Double fusing  (Read 3782 times)
K2OWK
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« on: November 26, 2012, 09:51:50 PM »

Hello, I have been in ham radio a long time, and an electronic tech. for about as long. I have always wondered about using a dual fuse in a DC circuit? I notice most modern transceivers use dual fuses one in the positive leg and one in the negative leg. I am going to build a power supply cord for one of my radios, and I am curious if the dual fusing is really necessary or not? In the old days we just fused the positive line.

Thank you.

73s,

K2OWK
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K8AXW
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« Reply #1 on: November 26, 2012, 09:54:54 PM »

OWK:  Suggest you check W8JI's website for the answer and reasons to that question. 
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WX7G
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« Reply #2 on: November 27, 2012, 01:34:48 AM »

Dual fusing is useful in mobile installations where the negative lead could become a current path for other devices.

Here is one senario: The radio negative cable is connected at the battery. The battery-to-chassis cable connection becomes resistive and the lowest resistance path is through the radio negative cable, along the coax shield to the antenna chassis connection.
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K1CJS
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« Reply #3 on: November 27, 2012, 03:49:45 AM »

Exactly what WX7G just mentioned.  That is the reason for dual fusing.  For a radio that isn't mounted in a mobile, there isn't much point in including the second fuse--unless you're going to use that radio and that particular power harness in a mobile installation in the future.
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KE3WD
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« Reply #4 on: November 27, 2012, 06:09:44 AM »

One bad scenario is the case where the Starter Motor should lose its ground strap connection strap from engine block to vehicle chassis, it can try to sink to chassis ground through the rig's negative lead but that amount of current would heat up the typical 10 or 12AWG rig wiring to fire-starting capability very rapidly if not for that negative lead fuse. 

No, it does not happen all that often, but why tempt fate? 


73
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K2OWK
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« Reply #5 on: November 27, 2012, 02:34:43 PM »

Thank you for the responses, and excellent information. I checked the W8JI website, and he seams to agree with the rest of these answers, that the dual fusing is necessary for mobile use in automotive battery circuits. I am going to use my transceiver as a base station, but will duel fuse it as recommended in case I decide to move it to my car.
Better safe then sorry.

73s

K2OWK
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N4NYY
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« Reply #6 on: November 27, 2012, 02:44:52 PM »

Thank you for the responses, and excellent information. I checked the W8JI website, and he seams to agree with the rest of these answers, that the dual fusing is necessary for mobile use in automotive battery circuits. I am going to use my transceiver as a base station, but will duel fuse it as recommended in case I decide to move it to my car.
Better safe then sorry.

73s

K2OWK

This is good info, as my HF rig was the first rig I noticed dual fuses. Now I know why. If I get a VHF mobile, it will make sure it is fused on both sides.
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K4KRW
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« Reply #7 on: November 27, 2012, 04:56:50 PM »

You may want to re-read the 'Mobile Radio Wiring and Grounding' page on W8JI's web site.

http://www.w8ji.com/mobile_ground.htm

I had read that page in the past and what I saw in this thread didn't match what I remembered.

(Partial Summary: Don't fuse negative lead.  Don't run negative lead to battery. - But, don't take my word.  Please read his article carefully)

Richard - K4KRW
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AA4PB
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« Reply #8 on: November 27, 2012, 05:09:16 PM »

Thank you for the responses, and excellent information. I checked the W8JI website, and he seams to agree with the rest of these answers, that the dual fusing is necessary for mobile use in automotive battery circuits. I am going to use my transceiver as a base station, but will duel fuse it as recommended in case I decide to move it to my car.
Better safe then sorry.

73s

K2OWK

There is no need for a negative lead fuse when operating from a base power supply. When you are operating from a vehicle you need to have the fuses (both positive and negative lead) located near the battery end connections of the power cable in order to protect from possible wiring shorts. You are not likely to be able to use the same DC wiring cable for both mobile and fixed station use.

If your vehicle provides a negative grounding stud located near the battery (intended by the vehicle mfg to be use for grounding accessory equipment) then you SHOULD NOT use the negative lead fuse for the vehicle installation either.
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AJ4WC
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« Reply #9 on: November 28, 2012, 10:09:18 AM »

Some of the ideas in the article are pretty far fetched...  Commercial radios are typically not fused on the negative lead, simply because you don't want a situation where you loose the grounding.  Same in home wiring, you don't see breakers on the return because it would create a dangerous situation to have power to a circuit with no ground return.  (I have seen return breakers on ships, but it's done for different reasons.) 

I have seen radios loose ground and pull current thru the antenna shield.  In those cases, the radio was not damaged since it's the same point electrically.  If the radio is properly grounded, either at the radio or where the battery ground connects to the frame, the vehicle cannot pull current thru the radio as described because if one gets disconnected so does the other.  The ground wire on the radio is also directly connected to the frame of the radio and the vehicle body thru the mounting bracket so I don't see how any internal radio circuits are in that return path either.  Now if someone that doesn't know what they are doing is jacking around with the vehicle wiring, then yes, probably all sorts of problems could happen.

I personally don't like non-removeable fuse holders, especially on the ground, because it makes the wires more difficult to run thru the firewall.  I always cut the ground ones off.
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AA4PB
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« Reply #10 on: November 28, 2012, 10:34:42 AM »

The negative lead fuse came about because people were connecting the negative power lead of the radio DIRECTLY to the negative battery terminal. If the vehicle-to-battery negative terminal gets a poor connection then vehicle currents (worst case - starter currents) can flow from other radio grounds back through the radio power line to the battery negative terminal. On some radios that ground connection can even be via PC board runs in cases where the negative power lead connects directly to the PC board. The idea of the negative lead fuse was that if this happens, the negative lead fuse would blow and protect the radio and its power wiring from very high vehicle system currents. The down-side is that if the fuse blows, the radio may continue to work by drawing current through the other grounds such as a metal mount or coax shield and you may not even realize it. Still, that is better than sending several hundred amps of starter current through the radio wiring.

Connecting the radio negative lead DIRECTLY to the battery negative terminal is a poor choice, in my opinion. If you insist on doing that however, a negative lead fuse is a good idea.
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WX7G
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« Reply #11 on: November 28, 2012, 11:44:10 AM »

And I should point out that the negative lead and where to connect it on the vehicle was nicely explained recently by W8JI.
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K1CJS
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« Reply #12 on: November 30, 2012, 04:00:32 AM »

Connecting the radio negative lead to the stud on the body that the negative battery cable is connected to doesn't provide all the protection that some people think it does.  If there is a ground fault, there is still some chance that the negative lead to the radio would be overloaded, no matter if it's connected there or directly to the battery.

I'm not going to go any further--except to say that the dual fuses are put into the transceiver leads by the manufacturer for a reason.  Just about every manufacturer--and harness maker for that matter--puts that second fuse holder in the negative lead.    Ignore the intentions of the manufacturer if you wish--and just maybe you'll be sorry for doing so later.    
« Last Edit: November 30, 2012, 04:03:26 AM by K1CJS » Logged
KB1LKR
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Posts: 1899




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« Reply #13 on: December 01, 2012, 11:45:50 AM »

(Partial Summary: Don't fuse negative lead.  Don't run negative lead to battery. - But, don't take my word.  Please read his article carefully)

OTOH as I recall both Ford & GM engineering bulletins call for protecting both leads.
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W6EM
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« Reply #14 on: December 01, 2012, 04:26:41 PM »

Quite a while ago, I got into a rather vociferous argument over on QRZ about reasons for fusing the negative lead.  While I support it, for the scenarios presented so far, a couple of commercial two way radio techs countered with the fact that mother /\/\ never has followed such a practice.  Not for trunk mount decks, or for dash/floor mount units.

Their argument for trunk mounts was that the ground return is/was usually via the body/frame connection inside the trunk.

Dash mount radios all had at least an AWG No. 8 copper negative lead, so the coax shield from a roof or rear body antenna mount would likely have a much higher return resistance to the battery than would the body of the vehicle.

Anyway, one thing that has not been mentioned so far is this:  Since the fuses in both leads are usually the same value, the one in the negative lead will have likely been damaged if the hot leg fuse blows.  Replace both with new fuses if only the positive lead fuse blows.  Fault current from the radio can return to the battery not only through the negative lead, but also via the mounting bracket if connected to dash or body metal.  The return current in the negative lead could be high enough to damage, but not melt the link in the negative lead.  Once damaged, it could open at much lower than desired short circuit values.  Perhaps even at normal values while transmitting for lenghty periods.


73,

Lee
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