Do fuse ratings change with voltage? Why are fuses rated in voltage?

[m-mman]

 

 

When you buy new battery cables you are not asked whether your car is 6 volt or 12 volt.  Cables are not marked for their voltage. Cables are identified by their size, their diameter, the amount of copper contained per length of wire.

 

Many questions about hard starting are posted here and a common problem is to find “12 volt” cables on a 6 volt car.

12 volt cars use smaller cables and can’t carry the amps needed by a 6 volt car.  The cable gets hot because it is being asked to carry too big a load.

It is confusing because cables are not marked “12 volt” or “6 volt” but maybe they should be?

 

But what about fuses?   Those glass fuses that are ubiquitous on our vehicles.  The books and manuals specify the proper fuse in an Ampere rating. ("Install a 10 amp fuse")

 

I recently replaced a blown fuse on an Overdrive relay. I believe it was specified as a 20 amp fuse.

Digging into my boxes of fuses I found one of the correct length and it was stamped 20 amps BUT it also said 32 volt(?)  I began wondering, what is the actual ampere rating for this fuse? 

It is identified as a 32 volt fuse then does that ampere rating change if used in a 12 volt system?

If it is a 32v x 20 amp fuse, then should it not blow at 640 watts(?)  (V x A=watts)  

 

If installed in a 12 volt system then a 640 watt fuse should blow at 53 amps(!)  (watts/volts = amps)  Wow, this seems like putting a penny in the fuse box.

 

I have a fuse marked 32v x 15a that seems like it would not interrupt the circuit until it exceeded 40 amps

 

I have a 250v x 10a fuse that mathematically should offer 208 amps of “protection” at 12 volts.  Doesn’t seem like much protection to me.

 

I never considered the voltage listed on a fuse before. Now that I am thinking about it there must be something about it that I am missing. 

If it doesn't matter, then Why is a voltage listed on a glass fuse 

Could fuse ratings somehow be exempt from the laws of electricity?

 

Edited January 9 by m-mman (see edit history)

[TerryB]

Fuse and fuse holder ability to shield from arcing.  The higher the voltage the more prone the fuse can be to become just a spark gap or have enough current leakage to be useless.  Amps is amps, no doubt about that, but voltage is the variable that dictates physically how the fuse body is made in length and materials used.  Using a 10A 220V fuse in a 12V application still makes it a fuse that will do its job if 10A is exceeded.

 

Also in fuses you can get ones that are fast blow or slow blow depending on the type of load it sees. A slow blow is used where the load creates a momentary over current condition and allowing that short burst of extra current to flow is ok.

Edited January 9 by TerryB (see edit history)

[Fordy]

What TerryB said. 

The thing that beats me is Pommy fuse ratings. With my Triumph Spitfire I continually get told to up the Amp rating from that specified "as they use a different system" I stick with what the specs are because as an electrician I believe that an Amp is an internationally defined unit and not something to be messed with unless you want a fire.

Steve

[Bloo]

The fuse only cares about amps. Just use a fuse with a voltage rating higher than the voltage of the system. A 32 volt fuse is fine on any 6 or 12 volt system. So is a 250v fuse. Just don't use something with a voltage rating too low. Don't use that 32 volt fuse on 120VAC house power. Most commonly 250v fuses are used on 120V appliances etc. The issue is all about actually disconnecting the circuit and stopping current flow when the fuse blows.

 

When a fuse blows, a plasma cloud can form, plasma is much more conductive than air, and current might not disconnect, you are just left with an electrical arc where the fuse was. Since the whole purpose of the fuse was to shut the current down, this is bad.   Also, things tend to blow apart.

 

At 6 volts or 12 volts in a car, this is completely a non-issue, and you can use the cheesiest fuse you can find, though it might be hard to find anything cheesier than a 32 volt rated glass fuse. On 120V though, bad things could happen if you use an under-rated fuse. On 240V, there's a really good chance of something bad happening. On 480v, there is an extremely high probability of some sort of explosion if the fuse is not up to the task.

 

[joe_padavano]

[Frank DuVal]

Link does not work for me. I'll try on another computer later.

 

8 hours ago, Fordy said:

The thing that beats me is Pommy fuse ratings. With my Triumph Spitfire I continually get told to up the Amp rating from that specified "as they use a different system" I stick with what the specs are because as an electrician I believe that an Amp is an internationally defined unit and not something to be messed with unless you want a fire.

I've never hear of Pommy, but I do know British cars have their fuses rated different than American cars, or even American electrical systems in house, etc. The difference in amp ratings is what the "blow" or opening point is. Here we say a 15 amp fuse will let 15 amps pass through forever without opening. The British system rates the fuse in surge amps, or how many amps will open the fuse in so many milliseconds. A British 35 amp fuse opens instantly if 35 amps or more passes through it. It will also open with a smaller current passing through it, say 20 Amps for a time period. Just think of the difference as the circuit breaker curves you might have seen when adjusting those large industrial circuit breakers. There are published fuse curves also.  

 

You probably wondered why the British use a 35 amp fuse on small gauge wire! Way smaller than 10 AWG  (6 mm²). If you protect that 18 AWG wire with a 35 Amp American system fuse..... oops, there's smoke when something shorts.

 

Here is a writeup:

 

https://triumphtr6.info/Manuali/fuses.pdf

[Fordy]

7 hours ago, Frank DuVal said:

Link does not work for me. I'll try on another computer later.

 

I've never hear of Pommy, but I do know British cars have their fuses rated different than American cars, or even American electrical systems in house, etc. The difference in amp ratings is what the "blow" or opening point is. Here we say a 15 amp fuse will let 15 amps pass through forever without opening. The British system rates the fuse in surge amps, or how many amps will open the fuse in so many milliseconds. A British 35 amp fuse opens instantly if 35 amps or more passes through it. It will also open with a smaller current passing through it, say 20 Amps for a time period. Just think of the difference as the circuit breaker curves you might have seen when adjusting those large industrial circuit breakers. There are published fuse curves also.  

 

You probably wondered why the British use a 35 amp fuse on small gauge wire! Way smaller than 10 AWG  (6 mm²). If you protect that 18 AWG wire with a 35 Amp American system fuse..... oops, there's smoke when something shorts.

 

Here is a writeup:

 

https://triumphtr6.info/Manuali/fuses.pdf

Pommy is Australian slang for British. The rest of your explanation makes sense when stated clearly. Thanks Frank.

Steve

[dodge28]

I am subject to correction here. Fuses with ratings  of amps and voltages are for residential  AC radios. It will withstand  a certain voltage before blowing. A valve plate voltage can be as high as 370 volts but low ampere. 

In the old days the owner knew nothing about technical repair and was told to take the car to the dealer where the technicians were trained by the manufacturers to service their brand. Today those technicians  are  the owners. Train yourself. 

In late  1970 I visited a retired Ford Dealership at corner of Shanonville Road  and Highway 2 in Eastern Ontario. Hanging on the wall in his office was CERTIFICATE OF MOTOR MECHANIC BY FORD MOTOR COMPANY issued by a tech school in Boston dated 1919.  Mr. Ken Vivian closed his business because all the Manufacturers were asking  that the barn like buildings were to be upgraded like they are today. He could not afforded it financially. 

[Frank DuVal]

All fuses have voltage and current ratings. 😉

 

Voltage does not “blow” a fuse, excessive current flowing through it does. 

 

Exceeding the voltage rating means there may be an arc started by the fuse opening (“blowing”) that lets current continue to pass. Like the spark of a spark plug, current is flowing with an open in the circuit when the voltage exceeds breakdown voltage. Not a desirable operating condition of the circuit protected by the fuse.😲

Edited January 10 by Frank DuVal (see edit history)

[rodneybeauchamp]

On 1/10/2024 at 4:30 AM, Frank DuVal said:

 

I've never hear of Pommy, but I do know British 

On 1/10/2024 at 12:32 PM, Fordy said:

Pommy is Australian slang for British. 

 

Our Australian vernacular for Prisoner of Mother England. (POME) who were convicts transported out here. As a result anything from Britain including cars and migrants were all POMS or POMMY. 

 

Australia was settled as a convict colony and transportation was the punishment for offenders. The British chose Australia because the United States refused to take any further convicts.
 

Any way back to our regular programming 

Rodney 😀😀😀😀😀😀😀

[dodge28]

"GREAT EXPECTATIONS"  by Charles Dickens. 

From an old colonial from the West Indies.

[Tonz]

Ok, to retrack to the original question!

 

Talking about auto fuses!

 

I've owned 6volt and 12volt cars and bikes for decades and have completely  built harnesses and wired cars and bikes from scratch. These vehicles have never had electrical failure or even blown fuses. Probably because I never use crimp joints, everything is soldered and shrink wrapped.

Not being a qualified house or auto electrician scientific expert . .. common sense prevails.

10amp for light duty circuits, 20 for high demand circuits.

 

This brings me back to a famous quote from an old POMMY bike magazine...

"All bike wires are full of blue smoke. If the blue smoke escapes it won't go anymore"

 

[Ben Bruce aka First Born]

1 hour ago, Tonz said:

Ok, to retrack to the original question!

 

Talking about auto fuse

Not being a qualified house or auto electrician scientific expert . .. common sense prevails.

10amp for light duty circuits, 20 for high demand circuits

  Fuse sized to wire!

 

  Ben

 

 

 

 

 

 

 

 

 

 

 

 

 

1 hour ago, Tonz said:

 

 

[Fordy]

This is available for the Pommy cars.

Steve

[dodge28]

I am not a metallurgist  but I think the copper content in most European cars contain too much copper in there wires. After a while in the north American climate with salt on the streets in winter moister seeps in, Copper corrosion builds up.  Look at the connections where the green colour is apparent. Famous in British cars using Lucas type connectors. ( Lucas the prince of darkness) . Fires are frequent in Jaguars.  I stand corrected.

[Farmallregular]

On 1/10/2024 at 8:46 AM, Frank DuVal said:

All fuses have voltage and current ratings. 😉

 

Voltage does not “blow” a fuse, excessive current flowing through it does. 

 

Exceeding the voltage rating means there may be an arc started by the fuse opening (“blowing”) that lets current continue to pass. Like the spark of a spark plug, current is flowing with an open in the circuit when the voltage exceeds breakdown voltage. Not a desirable operating condition of the circuit protected by the fuse.😲

To add a little more info, fuses also have an "interrupting current" rating, which is indicative of the short circuit current the fuse can safely break.

 

In general, it's acceptable to use a fuse with a higher voltage rating than the circuit it's used on, but not a lower voltage.

 

If you really want to understand fuses, here's a good reference:  https://www.digikey.com/en/pdf/l/littelfuse/fuseology

 

Keith

 

[28 Chrysler]

From an old book about radio repair.

Murphy's Law points out the the circuit is there to protect the fuse.

Also the law of gravity is that a falling object will land were it will do the most damage.