DC breakers [RE-wrenches]

Thu Jun 10 13:21:44 PDT 2004

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This is a little confusing with the Solar Converter's "DC
autotransformer" installed - but I think the problem is not being caused
by it and is just one of good old ohm's law and basic physics.  

The 80 feet of #12 wire has an overall DC resistance of 1.93 ohms per
1000 feet.  So the total DC resistance of the 80 feet is therefore o.154
ohms - so even at 70 amps DC you would get a voltage drop of 70 amp x
0.154 ohm = 10.78 volt and this is not the only losses in the circuit
since end of the 12 gauge wire doesn't end directly onto the battery
terminals and there is resistance in the outlet and the jumper as well.
So I doubt if there is even 70 amps flowing in the fault itself - its
probably mush lower.  

So - to really know what is occurring - you need to measure the actual
current flowing in the shorting jumper itself - not the value shown on
the E-meter (which is a combination of what the 24 and 12 vdc sides of
the system are doing).  Also - where the solar converter box is
connected needs to be considered.  

I will get 80 feet of #12 wire and a few breakers and will runs some
tests on both power supplies and batteries and will report them here.   

Christopher Freitas
OutBack Power Systems
cfreitas at outbackpower.com
19009 62nd Ave NE 
Arlington WA 98223 USA 
Tel 360 435 6030
Fax 360 435 6019
www.outbackpower.com 

>> -----Original Message-----
>> From: Kurt Nelson [mailto:sunwise at cheqnet.net] 
>> Sent: Thursday, June 10, 2004 5:27 AM
>> To: RE-wrenches at topica.com
>> Subject: DC breakers [RE-wrenches]
>> 
>> 
>>  
>> 
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>> 
>> 
>> 
>> Greetings all, 
>> 
>> I just came across a situation that concerns me a bit, and 
>> has the customer concerned as well.  I recently installed an 
>> off-grid, 24 volt system that also has a couple 12 volt 
>> circuits in the home, with a battery balancing Solar 
>> Converters auto-transformer (the 12 volt section of the 
>> battery is 350 AH).
>> 
>> The customer was concerned about having DC wiring in his 
>> house because someone told him it was dangerous and a 
>> possible fire hazard.  I told him that it should not be a 
>> problem with properly rated breakers.  He was still 
>> concerned so I suggested that we simulate a short circuit 
>> condition.  
>> 
>> The circuit I "tested" is to a DC stereo, and with the 
>> stereo running I shorted out the same outlet by inserting a 
>> short length of wire across it.  The stereo went dead after 
>> a second or two and I removed the wire and started to walk 
>> back to the Outback center.  I was about half way there when 
>> the stereo came back on.
>> 
>> Here's what I believe is happening (I have repeated this a 
>> few times). There is about 40 feet (length of run, 80 total) 
>> of 12 gauge wire between the battery and the single duplex 
>> outlet in the circuit.  Short the circuit at the outlet and 
>> the voltage falls below that needed to keep the radio on and 
>> the current through the circuit goes to about 70 amps 
>> (e-meter).  The CB remains closed.  
>> 
>> I can switch the CB off okay, but it, as well as another 
>> OBDC20 I tried in the same test, do not trip.  The longest I 
>> held the short (70 amps through the 12 gauge wire), is about 
>> 7 seconds, at which point the wire I was holding was getting 
>> slightly warm and still the breaker hadn't tripped.
>> 
>> I understand that there can be a time-delay in the tripping 
>> of such devices, but is this within it??
>> 
>> Kurt Nelson
>> 
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