Hi Guys,
I stand corrected for my incomplete posting regarding multi gaps for DC.... Here is my experience and a little of my own home brewed "folk lore" to clarify....
AC breakers generally have a fairly weak return spring on the lower voltage, lower current stuff, the 110 to 380V, 5 to 80A, in particular..... The intermittent arc caused by AC has natural 0 point crossings where the arc falls away and the plasma stops/slows in forming.... This allows for a slower opening contact to be effective and breaking the arc/plasma gap...
Once a plasma bridge has been formed, it can sustain fairly easily on DC, much more so than on AC. Glort's playing around, mine too, for example creating a mini arc at 30V DC with a 10A panel is a fair example.... This is quite difficult to sustain to the same gap with an AC supply...
If you use an AC breaker for switching Highish current DC, albeit at a reduced voltage, in general I have found to get a decent life from the device, a rating of between 2 to 4 times the current capacity is required in the contact area side of things. Lets face it, 20A is 20A, the actual carrying capacity of the device is not affected, DC or AC. The difference in life is caused by the switching time... If you use a slowly opening set of contacts that allow an arc to draw on DC, the life is diminished drastically. Multiple contacts opening simultaneously on a slowly activating device will help to a small degree, but not a dramatic one (my experience here) .... A somewhat more "beefier" contactor, with a faster opening time, makes a huge difference in longevity. Whilst the contact and conductor sectional areas are well above what is required for the carrying of the load, they are not what is of consequence... The opening time and opening gap is....
But hey, feel free to agree or disagree, I am simply relating what I have found and what has worked for me... (With my own twisted logic involved to explain it.... Lol...)
Incidentally, I was forced into using an AC contactor that was only just within rating for carrying current capacity some years back on a DC panel...(Size restraint, availability and budget...don't ask).... My solution was quite "different".... Stripped the contactor down, pulled out the return spring and put in one that was so hard that the contactor could only just pull in on the coil.... Opened much faster and way less arcing after that.... Downside: it buzzed a bit when held in, but not too badly....
Multiple gaps will improve matters slightly, as will compressed air blown between the contacts during opening, or even oil bath contacts. Another easy and simple way is to put a capacitor across the contact to reduce the opening arc, a la points and condenser...
Regarding opening gaps - the "sustainability" of an arc in free air, is (if I remember correctly) roughly inversely proportional to the square of the distance the arc has to travel.... (Way out of my comfort zone her, so I might be talking crap)... If I understand this correctly, founded on my "zero-based stochastic estimate", in a 6 contact arc break of 1mm travel, we end up with a factor of (1/(1^2))/6 = 1/6..... A single pair of contacts opening 6mm would give us a factor of 1/(6^2) = 1/36 likelihood of sustaining an arc.... Granted, probably mostly BS and a lot of rambling.... but hey, hasn't failed me yet!
Regarding DC dangers and my BS relating to it.... AC lets you know from a much lower starting voltage that bad shit is about to happen from a discomfort POV due to the oscillating cycle and nervous reaction thereto... because DC doesn't have the oscillating effect on the nerves, much less discomfort is felt initially. When skin resistance meets its match in the DC arena, muscular spasm causing one to grab hold tightly onto the exposed conductor, thereby lowering resistance still further and the "solid" contraction on the muscles involved....well, I don't think we need go much further, especially if the body forming the bridge has the current flowing across the chest area.... There is one vital organ that needs intermittent nervous stimulation to keep pumping.... block that stimulation out with more than about 30ma and suddenly it all goes dark...
The most likely injuries to be found, I would think, in DC engineering would be of a "burn" nature... Most of the "WTF was I doing" instances come from lower voltage, high current short circuits - the wedding ring that cauterizes a finger, the metal watch strap that forms a neat bridge on the DC buss, the bling chain making a neck worn gold fuse, the 10mm spanner that forms a wonderful high current fuse while held in the palm of your hand, or, maybe even, that bit of gaseous copper that coats your safety glasses (we all wear them right?) after snipping off that last sticky-outy-piece which homes like a guided missile towards and between the buss bars yet again.... Simple, easily occurring injuries that could easily be avoided...
Regarding the arcing, or lack thereof when disconnecting or connecting parallel banks: Easy to understand - For an arc or spark to form, there must be enough voltage differential across the gap to initiate it. The current alone would not be sufficient at the levels we are concerned with... When a parallel bank is disconnected from the array, yes, the buss voltage does drop slightly, but not to 0. A typical voltage drop would be in the region of a few volts, probably less than 5. The free panel voltage would rise a few volts, but probably not too much either. The nett voltage across the contact would probably be less than 10v... Lower voltage, less arc, littler bang! .... Again my (il)logical explanation that keeps me going....
Glort, are you crazy? 300V of DC into a Hydrogen generator!! Hydrogen will kill you! So will 300V! The world is going to end!! Let me get my will organized! When are you going to do it so I can turn on my seismic monitoring plant? Have you let NASA know what you are doing so they can reschedule their launch windows?.... .... ... ... . . . Uhhhh.....Nope! Have fun and let me know how it goes!! Are you doing separation of H2 or simply generating Browne's gas?
Ah well.... Enough rambling and BS (from me anyway).... Those in the know, please comment, correct, and set me straight!
Keep it shining....
Cheers
Ed