2 fuses in parallel, twice the rating ?

[Colas LeGrippa]

Hola amigos !


By curiosity, I was wandering  last night ( these days at night I think of electronics, I oughta see my doctor ) what would be the rating of two fuses in parallel. I was modifying a Bogen and have put temporarily a HT fuse of 100mA  that kept on blowing until I found a 125mA in a messy drawer that did the job...
So, two 100mA in parallel = one 200mA ?


Colas LeGrippa
Montreal, Canada (the other Trump country).

[sluckey]

So, two 100mA in parallel = one 200mA ?

No, doesn't work that way.

I could be wrong. 

[jjasilli]

I say yes, though there is confusion/debate on the issue: https://www.thirdgen.org/forums/electronics/11333-will-two-60-amp.html  I researched this before and found that electricians, e.g., may resort to 2X 10A circuit breakers in parallel if they have no 20A breaker on hand.


It's current flow -- amps -- that cause the fuse to blow.  If the fuses have unequal resistance value, then one fuse will be over-current and might blow.  If that happens, I might try matching the fuses.  But it's better to get the right fuse.

[hesamadman]

This is similar to a home with a 400 amp electrical service. They have two 200 amp disconnects. You're just paralleling the fuses. I don't recommend this in a tube amp application. Honestly I don't see a benefit.

[jjasilli]

There is no benefit, except I'm betting that Colas ran out of the fuses he needs. 

[PRR]

Unless the fuses are same exact model and same batch, they are unlikely to "match" and split current very equally. So the hi-R one blows, then the other one blows-- not much change in protection compared to one fuse.

Basement circuit breakers *may* be different. At least in US practice, they must be a specific type for the box they are installed in, which mostly means a specific maker, or a super-close licensed copycat. However I have *never* seen two in parallel (except by an idiot homeowner). A very large service may have two main disconnects, but they typically feed two sets of circuits. Maybe 200A just for the airconditioning, and 200A for the rest of the house loads. I have 100A for house and 50A for garage.

Are 200mA or 0.5A fuses *that* expensive??

[Colas LeGrippa]

Not at all expensive ! Jasilli got it right: I ran out of fuses...

[kagliostro]

This seems to be a particular kind of fuse (with diode in series)

but as you can see they are also placed in parallel by the factory

http://www.farnell.com/datasheets/1468450.pdf

http://it.farnell.com/bussmann-by-eaton/160fee/fusibile-160a-690v-tipo-t/dp/1701341




Franco

[sluckey]

Chewing gum wrapper or aluminum foil will increase a fuse's rating. Even a .22 caliber long rifle bullet can be used in a pinch! Be sure to read the Darwin Awards before trying the bullet! 

[Ritchie200]

Even a .22 caliber long rifle bullet can be used in a pinch! Be sure to read the Darwin Awards before trying the bullet! 

Don't laugh, that was one of my product lines.  PM me for intimate details.  Mythbusters didn't have all the details.

Many fuses have multiple elements inside the tubes that are essentially connected in parallel.  More elements, more amps.  However, all these elements are in a single tube package, these fuses are tested endlessly, and they must meet a very specific performance curve.  Also, it is not always 1+1=2.  There is so much more to the design - such as element spacing, heat sink, interior pack, connections, element composition, element design, on and on...  Sometimes these parameters change dramatically within the same fuse type to meet the amperage and performance requirements.  So while it was harmless in this case for Colas to do what he did, two 1/4" glass tube fuses in parallel will most likely not meet the performance requirements of a single fuse with a rating of the two added together.  The Cooper Farnell fuse in the link that Franco posted was designed and tested for that configuration and the footprint restrictions were driving the design process.

Jim

[sluckey]

Even a .22 caliber long rifle bullet can be used in a pinch! Be sure to read the Darwin Awards before trying the bullet! 

Don't laugh, that was one of my product lines.  PM me for intimate details.

If you are even remotely connected to that unconfirmed Darwin Awards story, please share with us. Don't hold anything back.

[trobbins]

The OP's issue is sort of akin to the 6L6G kept failing in my clone (I went through a few), so I found an old 6L6GC in the drawer and that is working, so should I just parallel 2x 6L6G as I have lots of them.

[Beezerboy]

you know the 6L6G and the 6L6GC aren't the same tube right?  19 vs 30 watts

parallel fuses.... its done in aircraft. theres a rule that says everything turns off with the master switch... unless it has a fuse, near the battery, thats rated 5A  or less. some systems need more amps so they use multiple fuses.... fire warning & control is one such system