Re: (ET) fuses and breakers

[vic garza <vtr-garza worldnet att net>]

Thanks for your expertise on fuses and breakers!

At 07:39 AM 10/31/99 -0500, David wrote:
 So whether you have a 30 amp automotive
>fuse ( rated at 12 V ) or a household fuse at 30 amp ( rated at 120 or 
>220 V
>) they both blow at 30 amp.  It is just that the automotive fuse is 
>smaller
>than the household fuse because it only has to ensure an open circuit at 
>12
>V instead of 120 V or 220 V.

Do you know what the current limitation on interchanging ac and dc fuses 
and
breakers is? I have heard that because of much greater tendancy of DC to 
arc
especially above voltages of 30v or so, you must use a much higher rated ac
breaker compared to dc. For example a 120V ,30A ac breaker could not be 
used
to break 36vdc at 30A due to the arcing causing a current bridge. Instead
you might have to use say 220v 50a fuse to circumvent arcing and thus be
equivalent to the 36vdc 30A.

>    A fuse is a relatively  instantaneous device. Up to about 102 % of its
>rating a fuse will work.  Above that, the element inside will rapidly melt
>and separate, opening the circuit.

I have also heard that operating slightly below the rated current for a
prolonged period of time will cause a fuse to blow below its rating. For
example operating at 95% current rating for 24 hours CONTINUOUSLY would
cause enough heat buildup in the fuse for it to blow?
  
>
>    A breaker, on the other hand does not instantly open when its rating 
> is
>exceeded.  The most common breaker that we see is the Bimetalic element.  
>It
>works by having two different metals that heat diffently.  As they heat 
>they
>move and cause the circuit to open circuit.  As the metals cool they move
>back and the circuit can be made again.  When a breakers rating is 
>exceeded,
>the breakers opertate on what is known a an I^2 T curve.  That is current
>vs. Time curve.  The higher the current the faster the breaker trips until
>it is actully an instantaneous device also. 

Do you know if breakers and fuses cause substantial resistive losses below
their trip point? If there is an in line power drain associated with a fuse
or circuit breaker, about what percentage loss is there when operating at
say 50% of the trip rating?
 

>    There are different types of breakers and fuses.  Some are "quick 
> blow"
>and some are "slow blow".  depending on your application you might need to
>experiment with differnt types to find on that provides for turning the
>component on and still provide running protection.

Is there any rule of the thumb on what approximate values to start
experimenting with when protecting electric motors? For example the 24vdc
30a CONTINUOUS 1hp rated motor in my e-motorcycle routinely draws 100amps
when going up very steep hills for periods up to about 5-10 minutes, 
without
any damage visible upon motor disassembly. What circuit breaker/fuse rating
should I be using to protect the motor? Quick acceleration draws up to 175
amps but only for a matter of seconds, so I suppose something like 200amp
slo blo circuit breaker and a 1000 amp catastrophe(short circuit) fuse 
might
be called for?

Thanks for any help you can offer.

---Vic