[RE-wrenches] AC breakers used in DC applications

[Kurt Albershardt]

Small (QO or Airpax-type) breakers in a distribution bank should be front ended with a suitable DC master breaker or fuse that has the capability to interrupt the available fault current.  Ideally, the series combination should be rated but at a minimum, a coordination study should be undertaken.

I like to have primary protection on each string, as close to the battery as possible - preferably bolted to the battery terminal.  Cable limiters are nice, but a TPL style fuse is easier to get.



On May 16, 2010, at 16:13 , Kurt Albershardt wrote:

> Voltage is not the issue here, it is current, and battery fault currents can be staggering - far more than one would ever encounter in a typical AC system fed from a distribution transformer.  I have seen a 200A Class T fail to act on an 48V inverter fault which resulted in welding the metal tabs of the fuse holder (which had been greased with copper-laden Penetrox E) to their sockets.  That indicates a fault current well north of 200,000 amps, quite possibly several times that.   A fire ensued, which triggered the gas protection system and fortunately resulted in almost no peripheral damage.  It did cost about $10k to recharge and recertify the Inergen system.  The inverter manufacturer was quite cagey on warranty replacement until they saw the photos - at which point a replacement miraculously arrived by overnight air.
> 
> 
> 
> On May 15, 2010, at 9:43 , Drake Chamberlin wrote:
> 
>> I did see a QO breaker fail to trip on a 24 volt DC system with only battery voltage behind it.  It burned a #12 wire completely in two.  
>> 
>> 
>>> The 48v is a max rating not a nominal rating.
>>> So yes only good for 12 and 24v systems.
>>> 
>>> However with CBI breakers I don't understand why the need for QO for DC anymore?
>>> 
>>> jay
>>> 
>>> peltz power
>>> On May 15, 2010, at 6:28 AM, Rebekah Hren wrote:
>> Drake Chamberlin
>> Athens Electric
>> OH License 44810
>> CO License 3773
>> NABCEP TM  Certified PV Installer 
>> Office - 740-448-7328
>> Mobile - 740-856-9648
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