Re: Battery Boxes...What One Company Designed

[ddl@danlan.*com (Dan Lanciani)]

In article <ehopi1lk9nh29a8u247sbu1o1tt5cm65la@xxxxxxx>, MFHult@xxxxxxxxxxxxxxxxxxxxx (Marc F Hult) writes:
| On 18 Sep 2005 02:52:12 GMT, ddl@danlan.*com (Dan Lanciani) wrote in message
| <1331585@xxxxxxxxxxxxxxxxxxx>:
|
| >In article <as8pi1da45o1psehe0s983ql7439j8k3ek@xxxxxxx>,
| MFHult@xxxxxxxxxxxxxxxxxxxxx (Marc F Hult) writes:
|
| >|
| >|
| >| Inteli-Power PD9180 and PD9160 (now Progressive Dynamics) as I wrote in my
| >| previous post. See
| >|
| >| http://www.progressivedyn.com/power_converters.html
| >
| >Interesting.  I had no idea that you could get class 2 supplies with such
| >high power output.  Maybe I'll have to reconsider my low-voltage distribution
| >analysis...
| >
|
[...]
| IMO, Key here is UL and internal transformer isolation. But I haven't probed to
| see whether it is specifically UL for Class 2. I am satisfied that it means
| engineering needs.

Hmm.  But if it isn't UL listed as a class 2 supply then it can't supply class
2 circuits.  Class 2 supplies have some pretty stringent requirements for
current limiting (not just over current protection--source impedance is a
consideration).  Last time I checked the most power allowed for a class 2
supply in the lowest voltage range was 100VA.  (At higher voltages the power
is reduced disproportionately IIRC.)

Another issue is that even if the supplies are listed as class 2 you can't
interconnect them unless they are also listed for _that_.  (721-45(b) in
the 1999 NEC)

| The other important part of a distributed DC system is use of isolating DC-DC
| converters. There are plenty of NOS devices to be had in a variety of Vin and
| Vout, nominal telco 48vdc being especially prevalent on the resale market.
|
| I generally assume that all equipment that came with a wall wart and is located
| more than  ~10m from the primary battery bank should be isolated (again) unless
| known otherwise.

I assume that anything that came with a wall wart needs to be isolated
regardless of location unless it has no other external connections.
Sometimes the negative power input is not common with the "common" of
any other connection.  Very annoying...

| DC-DC converters also provide the current limiting to 'convert'
| the battery into Class 1.

You probably want to "convert" to class 2, but I don't know if DC-DC
converters are generally listed for that.  (If you convert to class 1
then you are stuck with class 1 wiring materials and methods on the
load side, making the whole thing more trouble than running high voltage
to a wall wart. :()

|The NEC specifically states that a plain old zinc dry
| cell is Class 2 (not 1) because the potentially high current capabilities.

It's sort of the other way around.  A dry cell (provided it is < 30V and has
<= the capacity of a series string of No. 6 carbon zinc cells) is considered
an inherently limited class 2 power source.  That is, the power available from
such batteries is *low* enough to be considered class 2.  Any more power and
it would be class 1 (or worse).

|I
| wonder how many door bells of yesteryear, connected with 20AWG bell wire, were
| in violation ? ;-)

Bell circuits have used current-limited class 2 transformers for a long
time (though I don't know when they actually started calling them that).

				Dan Lanciani
				ddl@danlan.*com