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Can AC circuit breaker be used for DC?
    1) Can AC circuit breaker be used for DC?
It will "work" but if the breaker is not UL listed for DC operation then it becomes a safety / reliability thing.  I would never recommend it.
DC circuit breaker 'Direct current presents different problems than alternating current with regard to interrupting high value currents since arc extinction is difficult with DC."
Yes, that is the issue.  Arc flash going from closed to open is huge.  Magnetic blowout feature of certain relays helps deal with it.
"Alternating current, there is natural passage of voltage to zero with each half cycle as the current changes direction 60 times per second."
"With DC, the voltage is constant. DC does not oscillate like AC waveform. There is no passage to zero volts with DC.' Edited quotes from internet source
DC electrons have momentum.

DC roasts relays and contacts faster than AC because AC changes direction 60 times a second.  
Versus DC which is constant flow.   

Break a DC circuit and the electrons behind the break do what exactly ???
.... That's right.  They keep on going the direction and speed they were flowing. 
The electrons do not want to stop until they get to the break but then its too late. 
All the electrons behind them want to push them through the break.  
So they keep going and will arc over the gap.   DC has momentum.

AC on the other hand the electrons are more relaxed and just go back and forth in the wire. 
First they go one direction and then slow down, slow down more and then reverse and go in the other direction.    The vibration in flow keeps momentum from being built up so arc flash is reduced
Footnote: my pro electrician friend said that you cannot assume that AC will be at zero, and that arc is still present with AC. Yes, but arc will quickly end with AC. All electric points eventually get worn out because of arc
DC circuit breaker

DC circuit breaker
DC circuit breakers for Solar/ use 20-25 amp for designs on this page

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Not waterproof... mount inside Nema 3R enclosure
>Mount on din rail/ change fuse with flick of finger
Install 25-30 amp fuses
Use 10 x 38 DC fuses

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65 volt DC, 200 amp circuit
65 volt DC, 300 amp circuit breaker
Square-D QOU is rated AC-DC
Square-D QO is not rated for DC
Important to note that AC-rated breakers and relays will handle DC currents... but at much lower ratings. Choose devices that have both AC-DC ratings... then read spec sheet carefully to see the DC rating values. Do not exceed the DC ratings without assuming risk of property and casualty loss

Square D QOU circuit breakers at Amazon
Square D QOU 240V AC @ 30 Amp
..... is rated 48-60 VDC @ 30 Amp
Dave: Yes the QOU are limited when it comes to voltage. Less than 48 and 60VDC limits their usefulness. I last used them in a DC role on my old 1000Watt system. 
"Circuit breaker must have voltage rating greater than or equal to system voltage.
When circuit breaker clears an overcurrent, it does so in two steps:
1) First, the current sensing system identifies overcurrent and releases tripping mechanism.
This results in a parting of contacts.
2) Then the circuit breaker must extinguish voltage arc across the contacts.
If circuit breaker has correct voltage rating, it can efficiently extinguish this voltage arc." From Square D manual.

Each circuit breaker has specific rating to extinguish arc. DC has greater arc than AC.
This means arc extinction capability of breaker is challenged by DC voltage.
So the breaker rating is reduced for DC voltages, when compared with AC voltages

"QOU miniature circuit breakers are rated in the following UL 489 voltages:
•120/240 Vac •240 Vac
•48 Vdc •60 Vdc
Circuit breakers must be chosen with interrupting ratings equal to or greater than the maximum available short-circuit current at the point where the circuit breaker is applied in the system"
QOU circuit breakers
Attach breaker to din rail
Use mounting foot to install breaker
Mounting foot... use 1 pair for single-pole breaker
Mounting foot... use 2 pair for double-pole breaker
QOU breaker/ rated 48-60 Volt DC/ ... good up to 1000 watt DC array
square D circuit breaker
Square D QOU circuit breakers at Amazon
Data sheet on QOU breaker
Breaker subpanel
Larger image
Example breaker subpanel
Image shows a subpanel with circuit breakers mounted on din rails
Ordinary wire gauge will work with either AC or DC
Only the switchgear must be rated for DC, such as breakers, relays, switches, etc
DC arc extinction/ controlling high voltage DC
"the higher the number of contacts opening the circuit, the higher the breaking capacity of the circuit-breaker.
This means that, when the voltage rises, it is necessary to increase the number of current interruptions in series, so that a rise in the arc voltage is obtained and consequently a number of poles for breaking operation suitable to the fault level."
What this quote means: A DC circuit breaker or any DC switchgear must have multiple contacts that disconnect as illustrated in diagrams below
You want to increase the number of contact points, so the arc from DC power can be extinguished
DC arc extinction .pdf
Read about difference between AC and DC
Switchgear for DC::

Extinguish arc using contacts in series
AC contactor can have longer life-expectancy when switching DC if wired as shown,.
This puts 4 sets of contacts on the positive wire.
Obviously if all four contacts are disengaged at same moment, it will be more difficult for DC to arc across all four.

Or use a 3-pole for better arc extinguish using six contact points

Or use 4-pole contactor and 3 poles (six contact points) are used for positive wire, and 4th pole (2 contact points) is used on negative wire.
As we can see from this example, DC electrons flow from positive to negative because we are trying to extinguish the arc on the positive wire.

3-pole contactors
Difference between AC and DC
How to set up solar array
4-pole and 3-pile DC contactor
kA rating when using DC circuit breaker
kA rating is important when specifying AC-DC circuit breakers
kA rating is based amount of surge amperage from short circuit that will cause the breaker to fuse into closed circuit, and not trip.
Power remains ON. Wires overheat. Fire and electrocution danger. Equipment failure.
When using QOU breaker for DC, not only is voltage reduced, but kA rating is halved from 10kA for AC, to 5kA (5,000 amps) for DC
This means if short circuit on DC line causes more thant 5000 amps, then the QOU breaker will fuse closed and become a conductor letting voltage pass into wires that breaker was supposed to protect... causing overheat, fire, personal risk etc.
...indicating that DC poses higher risk of kA event than AC because of the lower 5000 amp rating ... especially with big array.
Does not apply to small solar array

Dave says: Not going to see 5000amps even with a huge PV array.  
However with a large battery bank like I have for off-grid setup it would be really easy to hit that.   That's why I have a 400 amp OSHT fuse right after the positive terminal on the bank.
If anything really bad happen the 400amp will pop.
Schneider CSA/UL?  Heavyweight
CSA/UL circuit breakers 500 VDC for ungrounded uninterruptable power source
The CSA/UL Listed thermal-magnetic molded case circuit breakers are specifically designed for use on ungrounded dc systems having a maximum short-circuit voltage of 500 Vdc or a maximum floating (unloaded) voltage of 600 Vdc.
The circuit breakers are suitable for use only with UPS (uninterruptable power supplies) and ungrounded systems
JGL37200D82 500 VDC
Eaton heavyweight
Eaton GL series AC-DC circuit breakers / 125 VDC 100-250 amp
GJ2-H3-U-0175-03-C breaker
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    3) Can we make DC electric meter using simple DIY?
Use one of these to convert DC amps to 0-5VDC.
  (Note the HCB version is 0-10VDC but I prefer the 0-5VDC version of sensors.

Then use the 0-10VDC version of to convert to 1-wire.

Which is digitized by ...

There are TWO other A2D boxes that will also work with the H970HCA and other 0-5VDC sensors.
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