Unless DC breakers are designed for a certain polarity (and usually
marked), it really doesn't make any difference. Also, they do not know
how much voltage is running through them while they are closed. when
they open, it matters, as there will be an arc. This will determine the
max rating. Polarities are another subject and some contactors have a
magnetic field, when collapsing, which must be in a certain direction to
extinguish the arc. But that is usually on the more powerful ones, like
the 400 amp 2000 volt Kilovac ones used where I work.

Bottom line: look for a polarity marking, and if you really want to be
sure, find the spec sheet for the breakers, then obey it. But you want
to know what? At 12 volts on a breaker rated for 80, it will be very,
very difficult to do any harm no matter which way you connect it.




Rasputin Novgorod wrote:
>
>
>
> Gentlemen:
>
> Besides test gear, I'm also a Ham (radio amateur) so have
> a bunch of 12 dc powered radios. I recently got a bunch of
> (Telco surplus) DC breaker switches.
>
> I mounted two 75-amp and twenty 30-amp breakers in a
> 19 inch rack panel. I ganged all the breaker line (inputs)
> to a common copper buss bar.
>
> The load(!) side of the two 75-amp breakers go to a 75 amp
> 12vdc power supply, and battery (for backup), respectively.
> The load side of the 30-amp breakers go to various loads.
>
> By reversing the line and load connections on the two 75-amp
> breakers, the breakers could all use a common buss, making a
> tidy, elegant installation.
>
> ~BUT~ a fiend (sic) of mine saw what I'd done and had a fit.
> He said that you cannot reverse the load and line connections
> on a DC breaker switch; terrible things will happen. When I
> look inside a DC breaker, there is just a coil in series with
> a spst switch. If enough current flows thru the coil, it
> magnetically trips the switch.
>
> I can't see why polarity or line/load could make a difference.
> So I query the group:
>
> 1) Does it matter if I reverse the load and line connections?
>
> 2) The breakers are rated at 80vdc and I'm using 12vdc;
> will that change the trip rating of the breaker?
>
> The breakers are Heinemann 30 amp, trip 37.5, volt: 80vdc.
>
> Actually, the protection is redundant, as the power supply,
> battery and loads are all fused. I'm using the breakers as
> switches (because I had them, cheap) to control various loads.
> The breaker protection is a nice bonus protection/isolation
> of a dead short in a load line.
>
> Sincerely
> /blair
>
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