DC breakers [RE-wrenches]

Christopher Freitas --- OutBack Power cfreitas at outbackpower.com
Sun Jun 13 13:39:14 PDT 2004


 

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Hey Kurt - here is a reply specifically from OutBack on this issue.  

I tested a couple of 20 amp breakers and found that they trip as 
described in the technical description by the manufacturer's spec 
sheets.  We at OutBack order all our breakers with a long time delay to 
reduce issues with false tripping caused by edge of cloud effect and 
other similar issues.  For overloads in the 50 amp range the breaker 
would take up to 25 or so seconds.  For overloads in the 30 amp range it 
could be as much as 35 seconds.  70 amps was around 7 seconds typically. 
 

I think there is a valid technical issue here that should be considered. 
 In the normal AC world - using a #12 AWG wire with 20 amp breakers on 
120 VAC circuits ensures that the fault current when a direct short 
circuit occurs will be in the several hundreds to thousands of amps - 
causing the breaker to trip on the instantaneous magnetic trip circuit 
instead of the time delayed thermal or hydraulic-magnetic mechanism (as 
in the case of the OutBack breakers).  When working with low voltage DC 
systems, the resistance of the wire can limit the current to the point 
that the fast response (magnetic) trip mechanism doesn't activate - so 
the slow time delayed circuit has to handle it instead.  

So the best solution here seems to be to size the breaker more closely 
to the actual load size (such as using an OBDC-5 breaker instead) or 
being sure to properly size the DC cable for the full current rating of 
the breaker to ensure sufficient current carrying capability of the 
circuit to cause enough current to flow to trip the breaker quickly.  
Using the standard "20 amp breaker use 12 gauge wire" rule does not seem 
appropriate for at least 12 vdc systems.  

The addition of a 20 amp fuse in the circuit will not solve the issue 
you have here - except that the fuse you use may not have a long time 
delay (most don't).  The only real solution is a smaller breaker or a 
larger wire size.  Fuses are also a potential problem in that it is very 
easy for someone to replace them with larger sizes when they blow - 
reducing the protection.  

I hope this helps and am interested in others opinions and perspectives 
on this issue.  

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  

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