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#21089 - 01/27/03 07:59 AM Taking readings from a telephone line?
ThinkGood Offline
Member

Registered: 08/07/02
Posts: 1084
Loc: Milwaukee, WI
I know that a telephone line is supposed to read -48VDC.

Is it safe to use a multimeter to take other readings (resistance, etc.) while the line is live?

I know that there are test sets available specifically for taking line readings, but they are pricey.

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#21090 - 01/27/03 08:07 AM Re: Taking readings from a telephone line?
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6805
Loc: Brick, NJ USA
TG:
At a country club we work at, the phone lines are used for control for various irrigation pump controls. I believe the Verizon Tech said 175 VDC, (at source), and when it gets to the relay w/"coil load", its about 85VDC. Be Careful...
John
_________________________
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#21091 - 01/27/03 11:40 AM Re: Taking readings from a telephone line?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
With a 10-megohm meter, your are fine on DC-volt scale for a POTS {voice} line. Read about 52 volts across the pair, except about 6 volts "of hook" and about 90 volts flickering during ringing. Digital circuits operate under a different set of rules. “On hook” the ‘tip’ side of the pair to ground is only a volt or so, and the “ring” side or the pair is close the 52V reading.

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#21092 - 01/27/03 01:28 PM Re: Taking readings from a telephone line?
ThinkGood Offline
Member

Registered: 08/07/02
Posts: 1084
Loc: Milwaukee, WI
HotLine1:
The line for the irrigation pump control sounds like not-your-ordinary-phone-line

That's a nice amount of volts there...

Bjarney:
OK on the numbers, I was told that the ringing around here (NNJ) is over 90 volts now (that's AC).

In my original question, the word "safe" was meant for the meter. I suppose it depends on the meter and the range(s) supported...

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#21093 - 01/27/03 05:02 PM Re: Taking readings from a telephone line?
maintenanceguy Offline
Member

Registered: 12/02/01
Posts: 303
Loc: Southern NJ, USA
I think the standard for phones used to be 50VDC for on the line and 90VDC at 20Hz when ringing.

I worked for a college radio station a lng time ago. The university wouldn't give us a key to the building but we needed to get in at all hours to run the station.

So we added a relay to the existing magnetic lock on the front door. This relay was powered by the 90VAC signal from a phone ringing and a capacitor was used to keep it from opening from the DC on the line.

So to get in, you called the radio station from the pay phone next to the door. and when you heard the phone ring, the door unlocked. No body ever figured out how we were getting in.

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#21094 - 01/27/03 05:10 PM Re: Taking readings from a telephone line?
txsparky Offline
Member

Registered: 05/25/01
Posts: 578
Loc: Magnolia,.Texas U.S.A.
maintenaceguy or McGuyver?
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#21095 - 01/27/03 05:54 PM Re: Taking readings from a telephone line?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
TG—Using an overvoltage-protected meter should be default choice for all electricians. You should use one that is absolutely guaranteed to be unaffected by unintentional contact with energized circuits on the resistance ranges.

As for ring voltage, the standard answer from telco folks is "AC superimposed on DC." Set your meter on the lower end of the several-hundred-volt range.

maintenanceguy—that’s thinking on your feet! :-)


[This message has been edited by Bjarney (edited 01-28-2003).]

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#21096 - 01/27/03 08:49 PM Re: Taking readings from a telephone line?
ThinkGood Offline
Member

Registered: 08/07/02
Posts: 1084
Loc: Milwaukee, WI
 Quote:
So to get in, you called the radio station from the pay phone next to the door. and when you heard the phone ring, the door unlocked. No body ever figured out how we were getting in.


That's an expensive key if you add up all of the phone calls!

Thanks, folks, for the feedback.

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#21097 - 01/27/03 09:07 PM Re: Taking readings from a telephone line?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
TG—One last comment...if you could get the door open before the phone was answered, you could get your {sorry} dime back!

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#21098 - 01/28/03 05:57 AM Re: Taking readings from a telephone line?
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Yes, a regular POTS line reads a nominal 48V DC (North America) or 50V DC (some other parts of the world), with ringing at approx. 90V AC. Ringing voltage can vary considerably though, and with some switching equipment close to the exchange it may rise over 100V.

In all modern systems the AC ringing is sumperimposed on the 48/50V DC, so that the ringing can be "tripped" and the call completed as soon as the phone is picked up (without the superimposed DC you'd have to wait for the next burst of ringing, which in the U.S. could be up to 4 seconds).

Other voltages can be found on lines from time to time however. As has been noted elsewhere, coin-phone lines sometimes used higher voltage DC pulses for coin collect/return, and maintenance routines will sometimes briefly connect higher voltages to the line for test purposes.

A typical modern DMM will be set on the 200V range for measuring tel. line voltages, and the built-in overload protection on most modern meters should easily cope with anything the line can throw at it (lightning strikes and direct shorts to HV power lines excepted!).

I'm not quite sure what you're getting at with taking resistance readings on a live line though. If there's TelCo line current flowing,the meter reading will be meaningless. Or did you just mean would the meter be safe if you accidentally connected it to a live line?

Many modern DMMs will survive direct connection to the line on a resistance range, but a conventional analog meter might not do so well.

[This message has been edited by pauluk (edited 01-28-2003).]

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