Ed,
Good points, thanks.
Paul

"Ed Price" wrote in message
news:8Z85c.7613$uh.3623@fed1read02...

"Paul Schilter" paulschilter@comcast,dot,net wrote in message
...
Doug,
Thanks for the correction, I was referring to AC breakers. Although
I
wasn't aware there are specific breakers for DC. In machine control we
don't
have breakers that are specific to DC or AC.


Not exactly. When a contact opens, either in a switch or a circuit
breaker,
the metallic conductors move from a condition of forced mechanical
compression to a condition of separation so large as to preclude a voltage
breakdown of the (maybe air) gap.

In the first condition, the resistance of the connection is very low, so
the
Ohmic heating (I*I*R) is very low (due to very low resistance). In the
last
condition, the Ohmic heating is also very low (due to very low current).

But in between those two conditions, all hell breaks loose. As the
contacts
just begin to lose touch, the resistance goes up, and the heating goes up
as
its square. A bit of the contacts melt, then vaporize, and the current is
carried by a (relatively) high resistance plasma (very hot). As the
contacts
move further apart, a visible arc (metal & air heated till they glow) can
be
seen. The heating is so severe that if the contacts were reclosed at this
point, they could very easily weld together. If the contacts aren't pulled
apart quickly, the arc may melt the contact surfaces, melt the supporting
structure, or char and burn the nearby insulating materials. As the gap
widens, the distance eventually becomes too wide to sustain the arc, and
it
finally extinguishes.

All this happens to even the smallest switch contacts, even if the arcing
is
on a microscopic scale.

So what is the difference between switching AC & DC currents? The DC
current
is being driven by a constant voltage, so the arc extinguishes only when
the
gap gets wide enough. But the AC current is being driven by a voltage that
approaches zero twice every cycle (for 60 Hz, this is about every 8
milliseconds). So, with AC, the arc can be extinguished by the voltage
dropping to zero even faster than the contacts can swing open far enough
to
otherwise extinguish the arc.

Simply put, when you switch DC, it's always a "hard" interruption. But the
AC current is only strong during part of the sine cycle, so your odds of
switching at the maximum are less. And, the voltage and current drop to
zero
every 1/120 of a second, providing a "self-extinguishing" effect too.
Switching DC is just harder duty than switching AC, so many switches and
breakers designed for AC duty don't even have a published DC capability.
(And switching DC on an inductive load, like a motor, is even worse.)

Ed
wb6wsn