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