Solar protection system

Re: Solar protection system

Here is a quick report written by Midnite Solar (boB, Robin, and Ryan/"Halfcrazy" post here and have their own forum too at Midnite Solar) about their testing of DC Breakers for ETL (similar to UL):

http://www.midnitesolar.com/pdfs/MidNite_165vdc_ETLtest.pdf

The first test consisted of applying 165 volts of DC to the 63amp breaker while running 99 amps of current through it. The test was done turning the breaker on for one second and then off for 9 seconds. This was done successfully 35 times. That was 16,335 watts of high voltage DC the little breaker had to switch on and off. The first time we ran this prior to ETL being there, all we got was a melted down breaker fused solid in the closed position! It’s a good thing we keep a hand on the knife switch for cases just like this. The breakers are marked with words stating “+ for DC”. Now we better understand why this marking exists. We had it backwards and that doesn’t allow the breakers internal arc chute to work effectively. Engineers rarely read directions before applying power to equipment. The + wording on the breaker was added by CBI on my recommendation years ago.

More or less, DC Current is much harder to "break" than AC (alternating current has a zero crossing point every 1/2 cycle at 50 or 60 types a second).

From a welding company--DC vs AC current:

Q: What type of Stick welder works best for all-around use?

A: A welder with an AC/DC output, whether its an electric arc machine like Miller's Thunderbolt or a gas engine drive like Miller's Bobcat.
DC welding offers advantages over AC for most Stick applications, including: easier starts; fewer arc outages and sticking; less spatter/better looking welds; easier vertical up and overhead welding; easier to learn "how to weld" and a smoother arc. DC reverse polarity (electrode positive) provides about 10 percent more penetration at a given amperage than AC, while DC straight polarity (electrode negative) welds thinner metals better.
Q: Does an AC output have any advantages?
A: Yes, if you need to weld on material that's become magnetized from friction, such as when hay, feed or water constantly rub against a steel part. A DC output won't work because of "arc blow," where the magnetic field blows the molten filler metal out of the weld puddle. Because an AC output alternates between polarities, it enables you to weld magnetized parts.

So, DC arcs start easier and sustain better than AC.

Also, note a couple of effects here... First is that the effects of the DC arc are polarity sensitive. For a circuit breaker, as Bob (Robin?) notice saw, if a DC Breaker is connected "backwards" (positive vs negative), it can actually weld the breaker contacts closed.

And a second effect... The use of magnetic fields/permanent magnets affect the arc (plasma and flow of metallic ions). DC Breakers and Relays can use magnetic fields to "blow out" the arcs--Which can also be polarity sensitive (instead of blowing an arc away into the arc chute--an assemble to stretch and cool the arc), the wrong polarity can pull the arc into the switch gear itself.

In the end, many switches (and some breakers) are rated for both AC and DC use, and if you look at the specifications, you will see that DC ratings for such devices are much smaller than the AC ratings. (and the DC version of equivalent rated devices are much larger/heavier duty).

Depending on the size of your business (or your potential sales of DC switch gear)--You can see how easy it was for Midnite to connect a dozen mid size batteries in series to give them a high voltage DC source with ~2,000 Amperes of dead short potential.

You could, (if you have the desire, money, and are vary careful) do your own demonstration testing with heavy cable, a safety switch (large/high Amperage rated knife switch while wearing a welding glove/face shield) and make a simple video showing the use of AC vs DC breakers. You probably can get some pretty impressive photos--And you could keep a couple of test results (melted pile of AC breakers vs DC breakers with the same test) to show potential customers.

Obviously, this is a vary dangerous test--So doing it away from combustible materials and in an area with good ventilation and a couple of fire extinguishers is important (remember the batteries are their own source of acid/hydrogen gas (during charging) too.

I tried a quick look on the web for photos/videos that tested AC vs DC breakers and could not find any that would be a concise demonstration of the issues.

One last comment, I believe that somebody here (boB or Robin?) said that the agencies are working on releasing requirements for DC Breakers that are not polarity sensitive--There are many applications in solar RE where the direction of current flow through the breakers is bidirectional (charging/discharging)--And polarized breakers are not a great idea... But, in general, we always assume that the Battery Bank is the source of near unlimited current and that is what the breaker is usually protecting as the source.

-Bill