QO Breakers and Panels [RE-wrenches]

[Drake Chamberlin - Electrical Energy]

<x-flowed>Hi Bill,

I sure don't know why the breaker didn't open.  Unfortunately, I was unable 
to get any measurements of the actual current through the circuit.

I did calculations based on Table 8 of the NEC for the resistance of #10 
copper wire to see what the maximum current through the circuit could be.

The calculations are based on 1.26 Ohms/ 1000 feet and a 30 foot run (60 
foot circuit).  I used a battery voltage of 29 volts, which I am sure was 
over the charge of the batteries at the time.  The resistance of the run 
was adjusted to a temperature of 22 degrees C.

Using the equation  I = V/R, I came up with a maximum short circuit current 
of 463 amps that could theoretically flow through the run of wire.  Is 
there any way that the capacitance of the wire could allow for a large 
initial inrush?

What ever the reason might be, it convinced me that fuses are superior 
protection to breakers.  As you stated before, the QO breaker makes an 
excellent switch for low voltage DC.  I think it is good for PV array 
disconnects, as they are current limited.  On the battery side, I don't 
trust the QO as the sole protection.

Drake



At 05:55 PM 03/07/2001 -0800, you wrote:
>Cool, were you able to measure the initial burst of current. Not sure if it
>matters, but if you can get more than 5000 amps for a few milliseconds, it
>might be enough to destroy the breaker before it has a chance to react. Your
>assessment seems right-on about needing current limiting fuses. Good
>testimonial on why they are needed.
>
>Bill.
>
>
>-----Original Message-----
>From: Drake Chamberlin - Electrical Energy
>[mailto:solar at eagle-access.net]
>Sent: Wednesday, March 07, 2001 8:56 AM
>To: RE-wrenches at topica.com
>Subject: RE: QO Breakers and Panels [RE-wrenches]
>
>
>Hi Wrenches,
>
>I saw a QO breaker fail on 24 volt battery voltage.  The length of the run
>was sufficient to keep the maximum short circuit current well under 5000
>amps.  The breaker did not trip, but allowed a #10 wire to burn a fair
>chunk out of a cover plate and junction box, and finally burn the wire in
>two.  I always recommend using DC current limiting fuses between batteries
>and QO breakers.
>
>Drake
>

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