Hot negative fuse?

[N9JIG]

In disassembling my scanner station's power cables so I could upgrade to a new PowerPole system I noticed that the fuse on the negative side of my power supply was very warm to the touch. When I removed the 30A fuse from the in-line fuse holder it was visibly cooked, the plastic was brown like cooked caramel on one side, although the fuse's link was intact. The rubber boot was partially melted.

The positive fuse on the red lead was fine and cool to the touch.

This had been in use for several years, it went from the power supply (currently a Samlex 30 amp) to a MFJ distribution block with 6 binder posts.

On my new set-up that I am still working on I will be using an MFJ-1128 PowerPole power block that includes a 40 amp main fuse and a dozen individually fused outlets. I will use some cable assemblies that have 4 coaxial power plugs each, to 5 amp fuses. Each of the 4-plug assemblies will power 3 scanners and an auxillary device such as a Stridsberg multicoupler or CSI tone decoder. The scanners are all current model Unide BCT15, BCT15X, BCD996XT, GRE PSR400, 410, or 600 type radios. There will be a total of 18 scanners, 4 decoders and 4 multicouplers. Total current draw is under 10 amps with all radios unsquelched at full volume so I don't think I am taxing the power supply too hard. It is usually left on 24/7 as 6 of the radios are used for data decoding and 5 are used for alert receivers.

(I have other radios but they are on other supplies.)

The questions here are:

1) Why was the negative lead's fuse so warm?

2) How can I prevent such a problem?

3) Should I even put a fuse on the negative lead? (I am not planning on it with the new install)

 

[jackj]

Why are both the negative and the positive leads fused?

Could the fuse holder have developed a bit of resistance and be generating heat? Just 0.5 volts drop across a bad connection will generate 5 watts of heat at 10 amps.

 

[DickH]

... a 40 amp main fuse ...Total current draw is under 10 amps with all radios unsquelched at full volume ...

If your numbers are right, I would certainly not use a 40A fuse. 15A Slow-blow would be more appropriate.

 

[majoco]

I'd be inclined to fuse the positive of each individual radio, not a humungous 40A supplying everything. Remember, a 40A fuse will PASS 40A, will blow within 5 seconds at 60A, and progressivly quicker as the current goes up. 1 or 2amps for each radio would be perfectly adequate.

 

[CCHLLM]

I believe you missed this part.....

On my new set-up that I am still working on I will be using an MFJ-1128 PowerPole power block that includes a 40 amp main fuse and a dozen individually fused outlets.

OK, so I read it twice, too.

Make sure there's a solid negative connection on the power supply that meters as close as possible to 0 resistance, and lose that ground fuse. Check voltage across each end of the ground cable under full load. If the connection is good, there will be only a few millivolts (how much depends on cable length, wire resistance, etc.) between the source end connection and the block end connection, and no heat created by the resistance/resulting voltage drops of poor connections. Simple long-term dissimilar metal corrosion between the metals of the terminals, the connectors, the wire, and the fasteners as well as the conductive mating surfaces of the fuse and the holder will create the heat that results in the distortion of the fuse and holder, so once a year, give 'em an inspection and cleaning.

 

[CoolCat]

The questions here are:

1) Why was the negative lead's fuse so warm?

2) How can I prevent such a problem?

3) Should I even put a fuse on the negative lead? (I am not planning on it with the new install)

A 1) - Because there was a bad connection at the fuse holder or a damaged wire close to it. The weakest point (where the current faces the most resistance) is where the most heat will be generated. What gauge wire were/are you using?

A 2) - You can prevent it by using a proper gauge wire to handle the maximum load you will put on it, use the proper size fuse for the gauge and length of the power wire itself, and make sure all wires are in good condition and all connections are solid (in-line fuse holders should be soldered on).

A 3) - No. Fuse only the positive lead.

Remember, a fuse at the start of a positive feed wire is there to protect the wire itself. The fuse rating should be no higher than the maximum ampacity rating for the gauge and length of wire you are using (but you can use a smaller fuse if you wish).

I highly recommend you use new wire for your powerpole setup, some of the old wires may be damaged. Even if the load doesn't require it, I would use at least 10awg wire for the power lead and ground line to minimize voltage drop; 10awg wire can handle a good amount of current and it's not too expensive.

good luck

 

[radioman2001]

Besides a bad connection, the - line could be handling all the returns back to the supply. That's why it's warm, it's returning more current from the other devices. A good test is to remove the - fuse and see what other devices stop working. In a lot of my mobile installations, especially fiberglass vehicles like ambulances I fuse the - as well as the +. I have had instances where the whole vehicle tried to use the radio wiring for a - return, because of some other wiring failure in the vehicle. It not only melted the - wire but the antenna coax,

 

[kb0nly]

Besides a bad connection, the - line could be handling all the returns back to the supply. That's why it's warm, it's returning more current from the other devices. A good test is to remove the - fuse and see what other devices stop working. In a lot of my mobile installations, especially fiberglass vehicles like ambulances I fuse the - as well as the +. I have had instances where the whole vehicle tried to use the radio wiring for a - return, because of some other wiring failure in the vehicle. It not only melted the - wire but the antenna coax,

This is why you should ground the radio's negative to the frame of the vehicle, then no negative fuse is necessary. If the ground from the battery to frame/engine fails then the radio is at the same potential as the factory electronics, it has no power and no return path to the battery so the vehicle will not use it as a ground.

When you ground the radio to the battery you create multiple grounds because the radio is also grounded through the coax to its antenna mount. That is why you would want a fuse in the negative, but if you just move the radio ground to the vehicle frame/body then you don't have the multiple ground problem and its safe without a fuse.

 

[radioman2001]

Easier said than done in some of these specialty vehicles. They are converted 40ft recreational vehicles used as Ambulances for multiple premature baby carriers. No matter how you connect there is a chance that the radio wiring may become the sole source of ground, that's why I fuse the -. Better to have the radio not work than cause the possibility of a fire from too much current through the radio wiring.

 

[KC0HQB]

an unbalance load with generate heat on the negative terminal. Have you put an SWR meter on your system as of late? Would be interesting if you lower the ampere rating on the negative fuse to see if you are measuring true power.

 

[jack103]

How do you "unbalance a load on a two wire circuit?
Dont you mean a multimeter not a SWR meter
Dont know where you are going regarding measuring power?

 

[kb0nly]

Easier said than done in some of these specialty vehicles. They are converted 40ft recreational vehicles used as Ambulances for multiple premature baby carriers. No matter how you connect there is a chance that the radio wiring may become the sole source of ground, that's why I fuse the -. Better to have the radio not work than cause the possibility of a fire from too much current through the radio wiring.

I have worked on a lot of fiberglass installs like this though, a simple frame bolt always does the job. Safer, and less chance of problems than fusing the negative. Just another item that can fail or cause problems in the future. I have done RV installs too, usually just go to the nearest ground, the AC system has a ground that is a frame ground as well. But anyway, a fuse will make it safe if you ground to the battery but its just not necessary if you ground to anywhere else.

I don't see a way for the radio to become the ground anywhere you connect on ANY vehicle. The only way this can happen is if the radio is grounded to the battery.

 

[KC0HQB]

Here is the message that has just been posted:
***************
How do you "unbalance a load on a two wire circuit? RX. with a resistor
Dont you mean a multimeter not a SWR meter Two SWR measuring watts
Dont know where you are going regarding measuring power? Yes

 

[kb0nly]

Here is the message that has just been posted:
***************
How do you "unbalance a load on a two wire circuit? RX. with a resistor
Dont you mean a multimeter not a SWR meter Two SWR measuring watts
Dont know where you are going regarding measuring power? Yes

The reason your post is confusing...

There is NO TRANSMITTERS involved here. Just receivers, scanners... So no SWR issues. I just re-read the first post, the OP only mentions scanners running from this.

 

[jackj]

There have been some really off-the-wall suggestions made in this thread. Your post implies that your scanners are NOT mounted in a vehicle. If that is so then just eliminate the fuse in the negative lead you don't need it and it serves no function.

If your scanners are mounted in a vehicle then yes it is possible to have vehicle current flowing through the negative lead (or coax braid) of your scanner, one term for that is "ground loop". But if your vehicle has an all metal body then your chances of that are very small. You can eliminate that as a possibility for your problem just by turning off your scanner(s) and checking to see if the fuse still gets hot. If it doesn't then eliminate the negative lead fuse for the above reason.

I suspect that your problem is a bad fuse holder that either has a loose or corroded connection.

 

[N9JIG]

Jackj has it right, this is on my home installation. The power cord I used to build the old power system had fuses on both the positive and negative leads, I just used it as it was.

I have not completed the PowerPole system to replace it. The main PowerPole distribution block has it's own main fuse, it came with a 40A, I swapped it for a 15. I might up this to a 20 or 25 if I decide to put my dual bander or CDM's on the same set-up, they are now on a separate power supply.

I did check the negative fuse holder, and there may have been some crud on it, it is kind of hard to tell due to the deformed and burnt appearance of it now. The new system is running very cool, and the built-in ammeter is showing just under 10 amps with all 20 scanners (full volume, unsquelched) and 4 CD-1 decoders as well as 3 multicouplers on and both the R7000 and R8500 (full volume, unsquelched) on.

It looks like I would have plenty of capacity for the dual bander and CDM's on the 35 amp supply, I might change over, or maybe not....

 

[JnglMassiv]

Rich, barring any exotic energy sources, the heat is from a poor mechanical contact or connection at the warm fuse. You would want such a connection to be at or near zero ohms. Say the bad connection was more like 2 ohms and you were drawing a modest 5 amps. You'd be 'making' 50 watts of heat (Power = I squared times R or (5^2)x2). This is comparable to a large soldering iron.

When you get your new system wired up, you can easily check the connections with your run of the mill DMM. Check the resistance from one end to the other of a given line. Any more than a few tenths could be trouble.

 

[N9JIG]

Rich, barring any exotic energy sources, the heat is from a poor mechanical contact or connection at the warm fuse. You would want such a connection to be at or near zero ohms. Say the bad connection was more like 2 ohms and you were drawing a modest 5 amps. You'd be 'making' 50 watts of heat (Power = I squared times R or (5^2)x2). This is comparable to a large soldering iron.

When you get your new system wired up, you can easily check the connections with your run of the mill DMM. Check the resistance from one end to the other of a given line. Any more than a few tenths could be trouble.

I will do just that, gotta bring home my meter from work tomorrow. I knew I forgot something last night! I have had the PowerPole system with all new wiring running for a couple days and everything is nice and cool.

 

[krokus]

The suggestion to NOT fuse the negative side of a mobile installation is in direct conflict with most knowledgeable sources.

Powering from directly from the battery is the best way, especially for cleanest power. Using this power method leaves you subject to problems, should there be a wiring failure on the vehicle, as others have pointed out.

Look at the mobile installation information provided from the ARRL and the vehicle manufacturer.

In your case, you are not in a mobile situation, so the negative-side fuse is pretty much a moot thing, but I am leery of suggesting the bypassing of a fuse.

 

[CoolCat]

I have had the PowerPole system with all new wiring running for a couple days and everything is nice and cool.

What gauge wire are you using for your power and ground lines?

And what type of fuse holder are you using on your power line? (use only an inline fuse holder with terminals or pigtail wires, never use the kind that pierces a blade into the wire)