Date: Sat, 4 Dec 2010 23:26:46 +0100 (CET)
From: Ingo Flaschberger <if@xip.at>
> There's also a telco oriented 48V inverter rack system thats escaping
> my mind at the moment. It can be setup with A/B 48V strings, and you
> plug in inverter modules up to IIRC around 8kW. Not parallel capable
> between racks AFAIK.
48V (and some more when batteries are full) are slightly below the limit
of non harmfull voltage.
Thus you have a voltage with less power loss at short transports and a
secure voltage. (creating a short is still not a great idea).
Saying that 48V is not a harmful voltage is a very dangerous
statement. It is unlikely to be a threat of electrocution (though even
that has exceptions), but people have lost fingers to 12V systems.
Lead-acid batteries can deliver way over 100 amps of current and a
conductor across "safe" voltage will get hot and, if not heavy enough,
will vaporize. The temperatures attained can cause major burns and,
should the metal vaporize, can damage tissue so severely that fingers
have been lost when the blood vessels were cauterized.
While safety rules often list voltages under 50 as being safe, it is
still important to exercise caution like removing rings, bracelets and
the like.
Kevin Oberman wrote:
Lead-acid batteries can deliver way over 100 amps of current and a
conductor across "safe" voltage will get hot and, if not heavy enough,
will vaporize. The temperatures attained can cause major burns and,
should the metal vaporize, can damage tissue so severely that fingers
have been lost when the blood vessels were cauterized.
While safety rules often list voltages under 50 as being safe, it is
still important to exercise caution like removing rings, bracelets and
the like.
I can't remember what I was trying to accomplish, but when we were building a telco office, and after making sure I was completely "demetalicized", I had to climb up the ladder and sit on one of the 48V 1/4"x4" (2-sandwiched) copper buss-bars and lay out accross the others, everything being already 'hot'. Unnerving to be sure.
I can also recall one morning at the S.P. Railroad when they called all us 'Diesel Electricians' together and showed us a wrench from graveyard shift.
Most of one end was burned off, and the other end was welded to the thick, gold, wedding-band which had been cut off the guy's finger on the way to the hospital.
They reiterated the mantra, 'when working with batteries, always disconnect the grounded/carbody side first'.
At IBM, we had a ritual before working on -anything-. Take off rings, watches/bracelet, tie-clasp and put into pocket. Tuck tie into top opening of shirt (white) so your neck doesn't get broken when tie catches on all the spinning crap. Even after the 360/370 came along you could always tell the old hands...the guys with their tie tucked in.
Our smallish 540Ah -48VDC plant has a 35,000A short circuit rating; important to know when sizing the disconnect breaker, as 50,000AIC breakers are required for that. The A and B side rectifiers are Lorrain 200A three phase units, built like tanks.
We have a secondary 12V plant at one solar location that is using six 2,320Ah cells which required two disconnects in series to meet AIC ratings, since the nearly 100,000A short circuit current makes it difficult to get small (<100A) breakers with 100,000+AIC ratings.
We're doing the solar thing for our optical telescopes, using Xantrex inverter/chargers and Outback solar charge controllers, 24VDC nominal strings. Works great; DC input switches make it even nicer, although you then need low voltage cutoffs to prevent battery damage when there have been several days in a row of dark skies.
At the 5ESS in Buckhead/Brookhaven I recall seeing an operating A buss current of >20KA years ago; the AIC on that plant has to be huge (of course, that's been 25+ years, and that's my memory, which could be mistaken as to the exact current value). A technician there told me he had seen an 18 inch adjustable wrench totally vaporized when it bridged from B- to ground.
Yeah, not something to play with.