DC breakers [RE-wrenches]

Kurt Nelson sunwise at cheqnet.net
Thu Jun 10 06:26:37 PDT 2004


 

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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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