# Measuring current in phone lines

Discussion in 'Electronic Design' started by [email protected], Sep 17, 2007.

1. ### Guest

Hi!

Do you guys know how to measure the current flowing in our telehone
lines using a multimeter? I really need it badly.

What's the difference between AC Current and DC Current? What will I
use in measuring the current in phone lines?

I'm doing my research regarding the "Electrocution through phone
lines." Have you heard of that urban legend?

Thanks a lot.

2. ### PhattyMoGuest

Urban legend eh?
Well,okay,It's probably NOT fatal,but I can assure you it hurts like
Hell when you're fiddling with the phone wires,and someone just happens
to call at the very second that you are touching the phone line,and are
also (unintentionally) well grounded.(wearing only socks on wet

3. ### Joe G \(Home\)Guest

If your specific topic is electrocution via telephone lines then ..... only
the AC ring voltage should be considered. as it can be as high as 90 Volts
AC.

So do a search on Telephone line ring voltages.

This does not include lighting strikes.

Joe

4. ### Tam/WB2TTGuest

The peak voltage will be 48 + (90 SQRT(2)). The ~90 volts will be at 20 Hz.

Tam

5. ### HapticZGuest

you arent going to try to sap enough juice from the telco to power your
house are you??

6. ### JeffMGuest

No. Obviously, no one here has ever used a meter.
(An oscilloscope would be more useful.)
you should use Google's search engine.
AFTER that, If you have SPECIFIC questions,
they belong in sci.electronics.basics.
Battery voltage can be as high as 56VDC.
Ringing voltage can be as high as 135Vrms.
Peak voltage could be as high as 245V.

7. ### DaveMGuest

Unless you have a current probe for that oscilloscope, it would be the wrong
instrument to measure current in a phone line; a scope is normally a
voltage-reading instrument. Also, the instrument used to measure phone line
current needs to be totally isolated from the AC line and safety ground.
Otherwise, the line becomes unbalanced and the phones don't work.
If you do have a scope with a current probe, you're in business. You just have
to make sure the combo is calibrated for meaningful measurements. Also, make
sure the instrument ground (chassis) is isolated from the power line safety
ground. To use a current probe, just clip the probe's sensing loop around one
side of the phone line.

For measuring the DC loop current, a (digital or analog) multimeter having a DC
current range of 10ma - 50 ma. For measuring the ring current, a (digital or
analog) multimeter having an AC current range of 100ma - 200ma. Frequency
response of the AC current ranges is a factor because the ringing signal
frequency is 16Hz to 20Hz in most areas. Just insert the meter in series with
one side of the phone line (doesn't matter which side) and switch to a range
that gives you a reasonable reading.

The difference between DC current and AC current (is that redundant?) is that DC
flows in only one direction, while AC alternates direction (hence, the term
Alternating Current) periodically.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra

8. ### Robert BaerGuest

Obviously you do not know much (if anything) about electricity or
simple electronics.
Therefore, please win the Darwin Award by licking all of the power
outlets.

9. ### mpmGuest

There's a show called "Mythbusters" where they approached this very
question and as I recall, their results suggested it was indeed
possible to be electrocuted through a phone line by lightning. Not
likely, but possible under the right conditions.

Google the show and maybe they have that episode online?

As for ringing voltage (ie, no lightning strike), I also recall
(1.5VDC) in Compliance Engineering magazine many years ago (1998-
ish?).

It only takes about 100mA or so, under the right conditions, to stop
the heart. You have to work at it (skin resistance, etc...), but if
you can do it with 1.5VDC, it stands to reason 90VAC (or 90 volts
pulsating DC to be more percise - although by the time it reaches your
phone the edges are worn off quite a bit..), will do the job too.

Finally, when I was younger, I once played around with a Secode rotary
encoder. You know, like the old rotary telephone dials? Only this
one still had the batteries in it. Like an idiot, I was somehow
connected to the leads when I decided to "dial" it and got the crap
shocked out of me!! And yes, in complete accordance with Murphy's
law, I had in fact dialed the "0" for maximum effect.

I immediately set it down, recovered my senses (what little I had
left), and opened the test set to find two old, but apparently well-
charged 45-volt batteries. They looked like 9-volts, only longer.
Lesson learned: Don't assume it's safe, just because it's not plugged
in!

About a month later, I got a refresher in that exact subject while
repairing a foot switch on a Pace vacuum extractor - where the
schematic clearly indicated no line voltage on the foot switch......
Yeah, right.

-mpm