DC power versus AC power

Unnamed Administration sources reported that Michael Painter said:

> >But, as Stephen already eluded to... Compared with an AC plant design, to
me, one of the biggest drawbacks of a DC plant is safety (I have had to
kick a fellow worker away from the rack before). <<

What was the worker doing? Is this 48 VDC?

Bet so.

And note, it's not just ISP's, of course. I heard that Sprint
PCS ha[s,d] a Dallas tech in critical condition and a dead switch
after a dropped wrench & resulting fire.

In the words of Phil Esterhaus:

  Let's be careful out there....

Is 48V DC at the amps present normallyin switch rooms etc enough to cause
electricucian? I have seen bad things with wrenches dropped across
batteries even 12 volt car batteries although in this case it was a large
battery bank in a submarine but I was curious about the 48V sources in
switch rooms.

but I was curious about the 48V sources in switch rooms.

Don't forget there's a quiet yet resourceful list over at
datacenter@shorty.com. They love talking about this kind of stuff.

Additionally I wonder why non-conductive tools wouldn't be the norm in an
environment where there's an "open" power grid? :slight_smile:

Charles

Is 48V DC at the amps present normallyin switch rooms etc enough to cause
electricucian? I have seen bad things with wrenches dropped across
batteries even 12 volt car batteries although in this case it was a large
battery bank in a submarine but I was curious about the 48V sources in
switch rooms.

I've laid across the buss-bars before...definitely an uneasy feeling, but never
felt it unless I was sweaty. <g>
Capability of thousands of Amps, but it's the old "power transfer"
deal...internal resistance of the source vs. internal resistance of the load (your
body).

--Michael

I'd bet the other way. CO battery has to supply ring current, dial
tone and voice current, not just run the switch itself, at least
in the Copper Age. I don't think -48VDC is an electrocution risk
unless you're sweaty, but a vaporized wrench sure can burn you, and
I don't think GFIs existed for DC.

Anyway, nukes don't need the battery capacity of the old diesels.

Thus spake "Scott Granados" <scott@wworks.net>

Is 48V DC at the amps present normallyin switch rooms etc enough to cause
electricucian? I have seen bad things with wrenches dropped across
batteries even 12 volt car batteries although in this case it was a large
battery bank in a submarine but I was curious about the 48V sources in
switch rooms.

2500-class devices only need 2A, but a GSR needs 60A -- and 60A DC is
significantly more dangerous than 60A AC due to the duty cycle.

If you _remember_ to defuse/disable the leads from the A _and_ B fuse panels
to the device you're working on, you'll be okay. The real danger is when
people are working on the "upstream" side of the power panel, which is often
difficult/impossible to defuse and carries significantly higher amperages.

S

Absolutely. Often, there are more than a few hundred amps available.
Remember, many of those switch rooms were built to specs to drive a very
large number of solenoids, relays, etc. All relatively high-current
devices compared to today's solid state stuff. Alot of the specs were
never reduced, the rooms just got bigger, and the number of customers.
There's a reason the backbone is multiple copper plates and not just
wire.

Owen

It only takes 30ma to put your heart into atrial fibrillation. In the usa,
gfi's are set to trip at 5ma.

Normally 48VDC wouldn't be considered a 'lethal' voltage (I've talked to
telephone technicians who said they used to play a game in the CO by wiring
a handle to 90V ring voltage and seeing who could grab it for the longest
time), but you've got to consider the extreme cases where the working
environment may be wet or a worker could get exposed through open wounds,
puncture wounds or mucus membranes.

Under the right circumstances, nearly any voltage can be lethal.

Joe

Thus spake joe mcguckin <joe@via.net>:

It only takes 30ma to put your heart into atrial fibrillation. In the
usa, gfi's are set to trip at 5ma.

Did you mean 5A, or am I misunderstanding GFIs?

Normally 48VDC wouldn't be considered a 'lethal' voltage (I've talked
to telephone technicians who said they used to play a game in the CO
by wiring a handle to 90V ring voltage and seeing who could grab it
for the longest time),

That explains so many things...

S

"Stephen Sprunk" <ssprunk@cisco.com> writes:

Thus spake joe mcguckin <joe@via.net>:
> It only takes 30ma to put your heart into atrial fibrillation. In the
> usa, gfi's are set to trip at 5ma.

Did you mean 5A, or am I misunderstanding GFIs?

it's 5ma. http://www.national.com/ds/LM/LM1851.pdf

                                        ---rob

Thus spake Robert E. Seastrom <rs@seastrom.com>:

"Stephen Sprunk" <ssprunk@cisco.com> writes:

Thus spake joe mcguckin <joe@via.net>:

It only takes 30ma to put your heart into atrial fibrillation. In
the usa, gfi's are set to trip at 5ma.

Did you mean 5A, or am I misunderstanding GFIs?

it's 5ma. http://www.national.com/ds/LM/LM1851.pdf

Perfect, thanks.

I learn so many interesting things on NANOG, it almost makes up for the
noise :slight_smile:

S