On Mon, 04 Feb 2008 00:45:17 GMT, Howard Eisenhauer
wrote:

re. 48 volts being dangerous I've worked around 48V systems for ~30
years. These systems actually tend to run more 52-54 volts due to a
higher voltage being required to keep the batteries float charged.

I've never ever felt even the slightest tingle by touching the + & -
leads at the same time, for most people you have to get up above 60
before they feel anything. I have however felt a mild tingle one
time, I was touching the hot lead while my arm was touching some
grounded metal, I'm guessing the skin on my arm is a bit thinner than
on my hands.

You never got bit by the -48 DC itself, it was always from the
inductive bounce from the line relays, or other Reverse EMF effects.
Or from the 130V 20Hz ring voltage on top of the -48DC.

The offices ran at 52V to 54V nominal float-charge voltages, but the
50V limit in the NEC was and still is the defacto dividing line
between Class II and Class I wiring. We had 130 VDC and 180 VDC
carrier repeater power circuits too, but it was treated the same as
other telephone circuits - though we did put special red ID sleeves
over binding posts to warn people not to mess with them.

The major danger with 48 volts, is, as it is with any system capable
of supplying more than a few dozen amps, is a short across the leads.
I've known people who have dropped wrenches across the buss bars in
telecom sites where the power plant is rated in the thousands of amps
on a continuous basis, generally the wrench flash vapourizes with a
*VERY* inpressive dissplay of light & sound. The person who does it is
invariably *VERY* carefull never to do it again .

Wrench? Feh, nothing! I was at the other end of the building, but
I sure knew something happened - Try an aluminum level, straight
across the Main Bussbars right over the batteries. (Name of the
guilty party omitted to protect the clueless, even though the clock
has long since run out.)

Sylmar CA, Stepper, around 1983-84. Five 24-cell 2400AH strings and
five 400A rectifiers online at the time, IIRC.

Impressive arc light wasn't the half of it - he dropped every call
in the building, and when the short cleared the voltage bounce blew
every 30A voice-battery filter capacitor fuse in the place. And all
the alarm fuses in parallel.

Whole lot of hummy calls till they all got changed, which took
hours. Had to call all the other switchrooms to send over extra 30A
cartridge fuses and bodies to change them - you had to use a 48V work
light as a mini load bank to charge the capacitors, then you could put
the fuse in.

That being said I'd expect in automotive systems an increase in
battery voltage would require less current from the battery.
Automotive companies being what they are (cheap) this most likely will
mean a battery with a corresponding decrease in plate area per cell,
with a higher resultant internal cell resistance, thereby actually
decreasing the battery's maximum current output to levels (1/4?) of an
equivalent 12 volt system (less sizzle).

They can cut the battery CCA requirements down a bit because of the
higher voltage, but they can't really reduce the overall battery
reserve capacity by much. You still have requirements on running tail
lights and/or emergency flashers for a set amount of time, to provide
emergency lighting.

I still think if they jump at all, 24V or 48V would be the more
logical points to pick, because the equipment already exists, motors
and lamps are readily available, etc. 42V is a *******ized level,
unless they're talking a 36V nominal battery and 42V is the float
voltage level.

-- Bruce --