Rule of thumb when replacing a fuse with a breaker?

[Chris Bardon]

I've used a mini circuit breaker in the past soldered to a burnt out fuse in order to track down problems. This is a really valuable tool, except that I haven't always used the same amperage breaker as the fuse. Last time I went with a 4a breaker for a 5a fuse just because that's what I had around, but is there an argument to be made for always going lower than the fuse you're replacing? Are breakers likely to take more current to trip than they're rated for (or take longer to trip than a corresponding fuse), or are you safe replacing a 5a fuse with a 5a breaker all the time? To me, it'd make sense that a 5a breaker and 5a fuse would behave exactly the same way-is there a reason that you could damage anything replacing a fuse with a breaker?

 

[Menace]

Not a bad question. Normally you would want to use a breaker that is rated the same as the fuse you are replacing, BUT the condition in which you'd like the breaker to trip will depend on the type of fuse it's replacing. With a SB fuse, the current draw can actually exceed the rated fuse current for a short period of time (hence the slow blow designation) like on coils for example because of in-rush current. FB fuses react right away once the fuse rating has been exceeded, which is why they do not work for coils.

I use common sense when sizing breakers for testing. Where breakers react slowly to over-current situations, it's safe to use same ratings for SB locations. For FB circuits you'll want to undersize the breaker. I think I've got a 5A and a 10A in my toolbox and that has covered the range well in my ventures, although I should probably add a 3A.

D

 

[Vengeance]

That's odd, cause I've had times where I'm blowing fuses left and right, and then I under amp the same fuse holder with a breaker, and it stops.

 

[Menace]

Maybe you were using FB where you should have been using SB?

I just re-read what I wrote and I need to correct myself. Breakers don't act like FB fuses, they react more like SB fuses in that they are slower to react to short over-current situations. I have edited my original post.


D

 

[Vengeance]

*shrugs*

I don't remember the exact situation anymore, just know the couple times I've actually used breakers, it's stopped the actual fuse from blowing, so I started to under amp just to be sure

 

[Menace]

That's fine, but if the game is still blowing the correctly rated SB fuse there is still something not right in that circuit. Something is drawing more current than the circuit is designed to handle, and usually fuses are spec'd based on wire AWG and PCB trace sizes to avoid fires.

D

 

[Chris Bardon]

That's good to know-thanks! I suppose the way that you'd really tell what's going on is to wire up a meter to the circuit and see what the actual current is going through the breaker at any given time. If something is fused at 5a, is it typically going to run at 3-4a, or will it be up around 4.9 (with the exception of SB coil fuses that can handle the over current)? I would assume that it's usually well below the maximum just to allow for some fluctuation and to avoid blowing things too often, but I'm not sure how close to the limit the Engineers who designed these things got.

 

[superjackpot]

*shrugs*

I don't remember the exact situation anymore, just know the couple times I've actually used breakers...

Jungle Lord had one in place of a fuse.

 

[Menace]

Chris,

Rule of thumb with design is the avg load is 50% of rated fuse, so 5A fuse would normally run around 2.5A. This way you have lots of headroom for transient peaks. This isn't hard and fast rule, but usually how things work out.

D

 

[Alienagent]

Breakers react much slower than fuses, likely around 40 AC cycles to trip as opposed to about 1 for a fuse to blow, fuses provide much better protection for equipment than breakers.
A 5 amp breaker is a way different beast to 5 amp fuse, you are not providing near the same level of protection, also every time your breaker trips the point at which it trips changes, usually it takes more current to trip each time, as the breaker gets older the springs get weaker and it trips at a lower amperage, if it has tripped recently and it is hot, this will also change the set point. where I work breakers supplying important loads are usually replaced after a defined number of trips.
It doesn't surprise me at all that your fuse is blowing but your breaker isn't tripping.
Also- an expensive breaker is very different from a cheap breaker.
Bottom line, they are different beasts with different uses and are not interchangeable devices, the only time I would use a breaker is if I ran out of fuses, it's a hack