[HomeBrew] AC vs DC Breakers

[rbethman]

AC circuits do VERY different things than you describe under short circuit.

I spent a number of years on distribution systems and their short 
circuit analysis.

The worst possible scenario is the circuit that has large motor loads.  
The circuit goes to zero voltage and zero current for a BRIEF moment - 
followed by a BIG reverse voltage AND current from the motor loads.

As to automotive 12VDC circuits, I've got a separate 60A fuse panel on 
the right hand side of my car.  Factory design and installation.  Along 
with a 150A+ alternator.  Also factory design and installation.

The spacing for fuses is the SAME as "ANY" 12VDC fuse.

Interesting!

Bob - N0DGN

James Kearman wrote:
> Interesting. I was thinking about the arc lamps I built as a kid, using 
> alkaline D-cell carbon anodes as electrodes. They arced pretty well, 
> with no inductance. Or the 20-A ac breaker I once used in an auxiliary 
> lighting system in a sports car. Guess what?
>
> Ac-rated circuit breakers depend on the source voltage and current 
> reaching zero very quickly after break. While an ac breaker may function 
> safely when the current slightly exceeds its rating, the same is not 
> true when the load is a short circuit.
>
> Bear in mind, under short-circuit conditions, the instantaneous current 
> is much greater than whatever the breaker is designed to carry. A large 
> battery can source several hundred amperes before it mercifully 
> overheats and starts to melt. When the breaker opens there is likely to 
> be some molten copper flying around inside as well, which you can see if 
> you can find enough pieces to inspect after the fire.
>
> There's a reason manufacturers build dc-rated breakers. I'd encourage 
> anyone interested in the subject to Google around and learn the 
> differences in construction between ac-only and ac-dc circuit breakers. 
> It might save you a lot of money. Or make sure your ARRL insurance is 
> paid up.
>
> 73,
>
> Jim, KR1S
> http://qrp.kearman.com/
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-- 
Bob - NØDGN