Fused Ground - Why is it there?

[Joe Z]

I'm in the process of replacing the 12v coach fuse panel in my motorhome and noticed that there is a 50amp fuse on the old panel in between the battery ground and converter ground. This fuse is attached directly to the old panel. Both the battery ground & converter ground attach at this point.

My question is "Why is the 50amp fuse there"? Is it just installed to make sure you don't fry your converter if the coach batteries are hooked up incorrectly?

[Tinsel Loaf]

Well for one thing, if that 50 amp fuse blows you know you have a problem.
I would think it is to protect your batteries if the converter should go crazy while charging. Rather a lame answer but you do have a good question.

[Frank S]

Hi Barkingspider--I have one also, in my trailer. Always wondered why it was there.--Frank S

[sos]

Hello Folks
This Came Up The Other Night Talking To Dad ,just Bought (11-26-2006) A 31 Excella 500,,1973 ,,reason It Came Up See The Picture And The Penny!!!!!! Posted Picture In "sos Menbers Photos"
He's Been A Electrician For At Least 60 Years ,in His 90's Now
The Idea Behine A Fused Ground On Battery Side
Due To Dc Volts/amps (if You Have Every Seen A Short Across The To Battery Post
It Will Melt The Metal Or In Case Of A Heavy Bar It Will Turn Red Hot
The Fuse On The Ground Side Was To Cut Off The Ground To The Short ,
If The Plus Side Can't Find The Ground (neg Side ) There Is No Short
Some Time In The Late 60"s Circuit Breakers Came In Which Are More Sensitive
And Just Worked Better Than The Old Glass ,paper Tue Fuses
Along With Over Load Protection Circuits
Dad Had About 2 Hours Of Imfo And More This Is About The Jest
Of What He Had To Say Of Course Thats Just A 90 Year Old School Guy Talking .

[sos]

One Other Item He Told Me About ,,I FORGET To Write ::
If One Of The Hot (plus Side ) Starts To Short Out And That Fuse Does Not Blow (or Takes To Long ) OR STARTS TO SHORT OUT ANOTHER CIRCUIT then Some Or All The Other Ciruits That Are Still Power Up (hot )could Start To Short Out ( Cause Fire ,melting Of Wires Ect Of Those Ciruits Too)so End Up With More Damage Then Just The One Problem Ciruit ( And What Is Connected To That Circuit..tv ,pumps,lights Ect )if The Short Is Bad Enought It ..The Short COULD Cause Damage To what Ever Is Hooked To THAT Circuit..so Then With a Fuse On The Ground Side It Helps Protect The Rest Of The Elect. System From Damage ,by Blowing ,CUTTING ALL POWER TO ALL CIRCUITS ,,SO THE SYSTEM DOES NOT HAVE TO RELAY ON EACH FUSE IN EACH DIFFRENT CICURT ,,THE THOUGHT WAS TO BE SOME KIND OF FAIL SAFE ,,MASTER CUT OF FOR OVER LOAD ..ELECTRITY (PLUS) IS ALWAYS GOING TO GO TO THE BEST GOUND IT CAN FIND ,,
Best I Can Under Stand Dad .....Is The New Ground Fault --breakers,plug Ins --ONES WITH THE LITTLE BUTTONS ON THEM--"TEST-RESET'' (110-220 Volt )work This Way BLOWING THE GROUNG SO TO SPEAK OR THE COMMON WHITE WIRE ,.....

THANKS FOR THE SPACE AND TIME
STEVE
NEW OWNER OF A 1973 EXCELLA 31' WITH A PENNY FOR A GROUND FUSE HOLDER CONTACT AND MANY OTHER THINGS ,,WILL POST PICTURES AS I FIND THEM LITTLE GUYS ..

[FrenchBern]

I heard about a fuse in the ground side of AS many times before and I verified on the schematic and physically on my 1970 airstream that it is false for that model. After 50 years in the electronic field I have never seen a fuse on the ground side of any circuit as this would be a safety hazard.
I really would like for someone to prove me wrong and send me a schematic or some proof where Airstream has done that.

[azflycaster]

Originally Posted by FrenchBern

I heard about a fuse in the ground side of AS many times before and I verified on the schematic and physically on my 1970 airstream that it is false for that model. After 50 years in the electronic field I have never seen a fuse on the ground side of any circuit as this would be a safety hazard.
I really would like for someone to prove me wrong and send me a schematic or some proof where Airstream has done that.

I have a service manual for 1976 (it's close to my 75) and it clearly shows a 50 amp fues on the line from the battery to the 12 Volt Fuse Pannel. I have not verified that this fuse does exist in my 75. In the pictue bubble 3 is the line to the negative battery terminal.
BTW, I have been in the business for 35 years and I have never seen this before.

[ticki2]

Originally Posted by azflycaster

I have a service manual for 1976 (it's close to my 75) and it clearly shows a 50 amp fues on the line from the battery to the 12 Volt Fuse Pannel. I have not verified that this fuse does exist in my 75. In the pictue bubble 3 is the line to the negative battery terminal.
BTW, I have been in the business for 35 years and I have never seen this before.

I have a '76 and it definately has the 50 amp neg. battery fuse. The reason for it is explained by the 90 year old electrician above , and is right on . If you have ever had a vehicle electrical fire you will know how a 12v battery can melt wires , even the heavy pos. and neg. from the battery. I have had 2 throughout the years and it isn't pretty , usually takes out an entire harness , plus fire . I think it is a good feature .

[Janets Husband]

I have been installing and maintaining large battery banks used to power communications sites for a number of years. We have always had 2 pole breakers (fuses) on the battery bank for safety. The way I see it is that you do not want to have a direct short across a battery because a cell will deliver many thousands of amps of current instantaneously which could cause it to erupt and spew acid all over the place or or in the worst case blow the cell apart. One of my fellow workers found that out last year when he accidentally shorted a single cell with a wrench. He was lucky and was not hurt. A deep cycle 12 volt battery has enough energy to do the same thing. The fuse is a "just in case measure". You would size the fuse to allow for the maximum expected draw, but it would be less than the carrying capacity of the battery wiring you are trying to protect.
Is it necessary? A prudent designer would put it in just in case something might happen. It would protect you and your equipment in the very rare case that a short would happen on the primary side of your wiring. Is it used? I don't know. Would I install a fuse? You bet, I've seen the results. and it is cheap protection. Just in case.

[RoadKingMoe]

A fuse in the line from the battery negative terminal to the grounded negative bus protects the battery, as well as the cable from the negative battery terminal to the chassis, in the event the battery positive terminal, or a tool on it, contacts the shell/chassis. As said above, it would also protect the battery and cables in the unlikely event the positive cable from the battery positive to main fuse/breaker shorts to the shell/chassis.

I've never actually seen this used in an RV or marine application and would not likely use one unless the primary cables are long.

Just a tip... never put a wrench on the battery positive terminal until the negative terminal is first disconnected.

[azflycaster]

Originally Posted by RoadKingMoe

Just a tip... never put a wrench on the battery positive terminal until the negative terminal is first disconnected.

Great Tip! I saw a few guys who welded their wrenches to the frame of an aircraft (T-38) when I was working for my Uncle Sam (USAF).

Remove negative & Install positive

[FrenchBern]

Thanks a bunch AZFLYCATCHER. That's exactly what I was asking for. Now I know for sure that sometime after 1971, AS did fuse the ground side of the battery.