Jim Thompson wrote:

On Sat, 16 Apr 2011 08:49:53 -0700, Charlie E.
wrote:

On 16 Apr 2011 03:06:02 -0500, boB wrote:

On Fri, 15 Apr 2011 18:34:32 -0700, Jim Thompson
wrote:

On Fri, 15 Apr 2011 17:57:27 -0700, Charlie E.
wrote:

On Fri, 15 Apr 2011 17:48:07 -0700, Jim Thompson
wrote:

Telephone CO Question...

If I suddenly load a short on a telephone line (initial conditions
on-hook)... what is the PEAK current flow that could occur?

Is it the same as the "loop current"?

How is limiting done in a modern CO, series R, or an active circuit?

Thanks!

...Jim Thompson
\
Hi jim,
I don't know what they do today, but in the olden days there was a
coil that was activated by that current, so was limited by the coil
(and line) resistance.

In modern CO equipment, they would basically have a active sense
circuit to detect and measure that current. Too little current, and
they determine a line fault condition. Probably too much causes an
alert as well. Since there are multiple customer circuits on a single
line card, I would assume that they basically have a resistor in there
to measure current across, and maybe a relay to cut the line power if
it is out of service...

Charlie

I have sort of a weird line... almost textbook... 50VDC open circuit,
50mA DC when shorted, so a 1K source impedance (or an active circuit).

I going to switch on a "noise maker"... at first jolt, it's simply
just a small valued resistor in series with a zener and a diode.

I don't have any current probes so I was just trying to estimate the
initial surge. I guess just try it while keeping my face covered ?:-)

...Jim Thompson

How far are you from the CO ?
Don't forget to add the miles of copper resistance of the loop.

boB
And all the inductance/capacitance of that loop cable! But I wouldn't
worry too much, as it is pretty benign. However, consider lightning
surges from nearby strikes!

Charlie

All underground here.

...Jim Thompson

Famous last words. Lightning can (and seems to prefer) striking high value
underground equipment to some depth. And then there are plain old power
system faults that cause nearby, mutually grounded conductors (cables in
the same trench) to jump in voltage.

Invest in a couple of surge arresters (even spark gaps are better then
nothing).

--
Paul Hovnanian
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"Nothing Important Happened Today"
-- King George III, diary entry July 4, 1776