Yes, you were clear. I was confused by the description on the manufacturer's site. With all the talk of sophisticated over-current detection I thought the supply had to interpose some electronics between the battery string and the output. I guess the protection is just for the wires to the battery. :( In case you care, I'm pretty sure that there do exist supplies that work the way I thought these work.
| Where do I find out about class T fuses ?
A Google search seems to turn up quite a few hits. One thing I noticed is that they aren't as absurdly expensive as I remembered...
| Searching the 2005 draft NEC came up | blank. Why do I need them?
To be honest, I'm not sure you do. This came up some years ago when I added a remote battery string to a UPS. To my annoyance I found that while inexpensive 100A automotive fuses could be used for similar applications in cars, boats, mobile homes, and even internally to said UPS and its packaged expansion batteries, I had to use a class T fuse because I was running the circuit in a building. This might have been a local issue and/or I might have overreacted to some of the requirements. After getting the correct fuse I sort of stopped looking for additional problems because, well, the box I was using wasn't a listed battery enclosure and the UPS wasn't actually listed for connection to anything but their packaged expansion batteries (though at least the string I used fell within the allowable range of capacities) and so on... :)
| Plain old Square D QO circuit breakers are UL'd to | 60vdc.
As I recall, a circuit breaker with the required DC arc interrupting capacity (20kA?) and response time was even more expensive than the fuses. But since a lot of this is listing politics rather than genuine engineering it's certainly possible that a commodity device would have been acceptable and I just missed it.
| >If they have added a genuine class 2 supply requirement | >I would think that many of the existing landscape lighting transformers are | >now out of spec since even the multi-output ones typically exceeded 100VA on | >each output. It may be that there is now a specific (and different) type | >of class 2 listing for such supplies that makes this all work out. There | >is an unfortunate trend in this respect to make listings extremely application- | >specific, thus thwarting non-standard or unanticipated custom assemblies. | | Right. ABIK, it has changed and become more restrictive/ less useful. That was | my point. | | Outdoor wiring low-voltage wiring was separated in the 2002(?). I haven't | pursued what the draft 2005 has to say. but I didn't stumble across anything | that clarified how to get from one t'other or made life easier.
I just checked the Juno Flex 12 system for which I happen to have a catalog. It's one of those exposed-conductor indoor low-voltage track systems. The transformers go to 600VA per circuit, so I think even indoor systems are going to have a problem with a class 2 supply requirement. Unless those were never under 411 to begin with (like your tracks?).
| >Here's the problem I had when I looked at the whole low-voltage DC distribution | >idea a while back. Whether you classify your circuits under 720 or 725's | >class 1 you still have to use the same Chapter 3 materials and methods that | >you would use for line voltage circuits. Plus you have to keep the two | >(or maybe all three) separate. Plus you have to deal with non-standard | >(and thus more expensive) ancillary components like DC-rated switches and | >fuses. Plus you have the inherent loss disadvantages of low voltage. As | >far as I can tell, the only thing you gain under class 1 is the ability to | >use No. 16 and 18 conductors, not that I'd want to. Am I missing something? | >
| | Yes and maybe not :-) | | Yes, from the perspective that you are analyzing the benefits as if the | installation is something to be commercialized -- which it is not.
I was really trying to frame some simpler questions. What is it that you are allowed to do by remaining < 30V that you would not be allowed to do if you used, say, 48V? Similarly for using DC instead of AC.
| Yes, from social reality that in a system with multiple, distributed components | that require electrical power, the conventional approach of providing that | energy with Class 2 wallwarts quickly becomes untenable. Spousal factor -- the | answer to "Can I put three _more_ wall warts over here so that the open-close | and tilt drapery motors and the controller can be powered?" is "No". So what | to do ? One part of the answer is distributed DC power in my case.
Don't the DC/DC converters take up about as much space as the wall warts they replace? Or do you have enough consumer devices that require neither voltage conversion nor isolation nor class 2 current limitation that you come out ahead?
Dan Lanciani ddl@danlan.*com