[geeks] Re: 48v

[James Fogg]

For the unfamiliar....
It began with the Telco's. The phone system runs on negative 48v DC. Because of
this, telco equipment centers (POPs, etc.) have copious quantities of -48vDC
available from truly uninteruptable sources (huge lead-acid battery banks, not
finicky inefficient dc-ac inverters). The current is "reversed" because it
prevents corrosion (specifically, galvanic action) on the lines (that is what
the trickle sealing current is on your copper digital circuits). High end
servers and network equipment frequently gets installed in such centers, so
they make -48v supplies for the purpose.

If you ever have the pleasure of working in such an environment, pay attention.
The -48v will hurt worse than 110v AC. Also, the red = + / black = - color code
most people learned doesn't apply, its reversed (black doesn't actually equal
negative, it equals "return" (I think), so therefore the color code is actually
correct in a Telco facility). In some facilities, they use red and green.

If your powersource (or your digital circuit) is called "wet-span", it is
powered by batteries (prefered). If it is called "dry-span" it is from power
company mains (converted to -48vDC).

With the death of the mechanical crossbar phone switch I thought -48v would go
away, but its more popular than ever.

btw, I recently moved from a town that has a crossbar. The telco refused to
give it up and installed dtmf "converters" to provide touchtone service. They
provided "pheatures" like call forwarding by switching the calls through a
nearby Nortel DMS100 digital switch.

On Fri, 20 Apr 2001, THOU SPAKE:
> What's this 48v thing people are talking about?
> 
> --
> Joshua Boyd
>