[HomeBrew] AC vs DC Breakers

[James Kearman]

Bob Macklin wrote:


> An arc only occurs if you are switching an inductive load. And the arc will stop when the energy in the inductor is depleted.

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/