Re: [TCML] using an AC breaker for DC

[FIFTYGUY@xxxxxxx]

In a message dated 9/9/08 1:37:07 P.M. Eastern Daylight Time,  
nicothefabulous@xxxxxxxxxxx writes:

>The question is, can you safely use 125VAC-rated breakers on a DC  circuit 
for 72VDC in a solar 
>power application as a battery disconnect?  I told my brother  that it was 
harder for a DC arc to 
>extinguish, but that I thought they would be okay as a  disconnect. 


    Any DC ratings for the breaker will be in the  manufacturer's 
specifications. This includes fault-current interrupting  capability, which will be much 
lower with DC than AC and may be important  with big batteries and big wire. 
Time curves may differ with DC vs. AC, although  I wouldn't expect by much.
    Some manufacturers use the frames of their  molded-case circuit breakers 
as switch bodies. They just don't put in the "guts"  that make the thing trip 
(and it will be conspicuously labeled as a "switch" or  "disconnect", and that 
it has not tripping mechanism), I've seen these from  Eaton/C-H for 120V DC 
applications (for UPS applications). 
    The rated voltage of a breaker is going to be  substantially lower for DC 
than for AC. From Square-D, I can tell you that the  QOx breakers for home 
switchboards or panelboards are not going to be rated past  60VDC. You pretty 
much have to get a breaker rated for 277VAC before it will  even possibly be 
rated for more than 48VDC. The larger frame breakers, including  the I-Lines, 
have some substantial DC voltage ratings, but always less than  the AC rating. 
    I presume your breaker will be a "standalone",  and not plugged into a 
bus feeding DC branch circuits? 
   What are the exact manufacturer and model numbers of the  breakers you're 
looking at?

 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities



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