On Wed, 19 Jan 2011 17:03:46 -0500, "Ralph Mowery"
wrote:


wrote in message
.. .
On Wed, 19 Jan 2011 10:48:56 -0600, bud--
wrote:

Magnetic breakers operate on "ampere turns" - the coil has X number of
turns of wire on it, and a portion (depending on the design) of the
load current goes through that coil. When the current through the coil
gets high enough the breaker trips. A 600 volt 20 amp magnetic breaker
will trip reliably on a 24 volt system as well as on 600.

A 120 volt thermal breaker will trip at a lower current on 240 volts,
and will require an extremely high current to trip on 24 volts.

Most American residential breakers, being "hybrids" work well on 120
volt circuits but would be useless on say a british 240 volt system.
Not totally useless, because the instantaneous magnetic trip would be
fine, but the thermal would trip too fast.

Both magnetic and thermal breakers only operate on current. It does not
mater what voltage is being used. You are fine on the magnetic, but wrong
on the thermal breakers.

A magnetic breaker operates almost instantly on an overload. The thermal
breakers (almost the same as fuses) will operate on lower voltages than the
maximum ratings just fine. They have a small amount of resistance material
in them that heat up depending only on the current. I just looked at a
single breaker that is normally used in houses. It is rated at 20 amps and
states it is for 120/240 volts.
The thermal breakers are sort of time delayed. They take time to trip
depending on how much over current is put through them. If say a 20 amp
rated breaker has 22 amps going through it, it may take 30 minuets to trip.
If it has 25 amps, it may trip in 1 minuets. If it has 40 amps , it may
trip in less than a second. The exact trip curve will depend on the
breaker.

Well, my experience using a heavy duty thermal/magnetic breaker on a
low voltage DC application flys in the face of your knowledge.
The breaker in question has an adjustable magnetic (fast) trip and a
fixed thermal(slow) trip. On 24 and 48 volts 450 amps would not trip
the thermal trip in 2 minutes. I could adjust the magnetic to trip at
any current I wanted, from about 50 amps up - and it tripped very
reliably at that current on both 24 and 48 volts (DC).
On 120 volts 80 amps would trip the thermal in less than a minute.
This was a 3 phase breaker designed for use on a standard commercial 3
phase system (115/208/whatever) and was used in an electric car as the
main circuit protection.