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DT98 Offline OP
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I posted this topic in the alarm forum too but thought I would post it here since it involves phone lines...I can get the alarm techs to grasp the way phone companies see off hook line voltage. The alarm techs think the off hook voltage should be an exact or a minimum voltage. I keep telling them that the off hook voltage will vary and the LEC looks more at the loop current...here's the post on the page/alarm forum. Any ideas will be welcome...

The line cut monitor used on our alarm systems will think any off hook line voltage below 10 volts is a cut phone line.
All the phone lines used on the alarms are shared with phone system use via an RJ31X jack...pretty standard stuff.

Is there a line cut monitor unit out there available that is adjustable so it will trip at a lower off hook line voltage other than 10 volts?

Thanks, DT98

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I can get the alarm techs to grasp
"CAN" or "CAN"T" ?


Tutorial


Arthur P. Bloom
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I think you should be looking at loop current.
IMO a Loop current regulator installed at the proper place would take care of it.

Read Mike Sandmans bulletin "FIRE ALARM PHONE LINE TROUBLE ALARM"and see if you don't agree.


-TJ-
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DT98 Offline OP
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Arthur...my bad...some of my fingers are faster than my brain...some are slower...

CAN'T ...I can't get them to understand phone line loop current and voltage (ohm's law as it were) and why some phone lines will have a higher off hook voltage than other phone lines...

I need to find an adjustable line cut monitor unit that the alarm company will work with. The off hook voltage is all over the place...rightly so with over 20 LECs and a 1000 locations/systems.

Thanks...

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DT98 Offline OP
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Hi TJ,
I agree but the alarm company is not looking at loop current. If they did we would be OK...
Regards...

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Trouble is, I've personally seen 4 volts as an off-hook voltage! And 9 volts as an off-hook voltage. You are right though, current flow is what you want to monitor. And that can run from 15ma to 35ma off-hook!


When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.
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You need to invent a dial-tone detector, similar in scope and application to the stutter dial tone detectors that alert the user that a voice mail message is waiting.

But there are bigger issues than just detecting line current or voltage, neither of which guarantees that there is a live POTS line. The alarm companies with whom I deal have needed to be told about "left in" dial tone, which does almost nothing, and "left in" battery, which does even less.

Some LEC's, upon terminating service (generally for a seasonal subscriber) will leave an active dial tone at the premises. This dial tone identifies properly using ANA, but can only call toll-free numbers and 9-1-1. The reason is so that when the seasonal subscriber returns in the spring to reestablish service, they can just pick up the phone and dial the toll-free number for the business office.

Savvy alarm companies use a toll-free number for their panels to call the central station, so alarm protection is not compromised during the off-season, and the subscriber need not continue to pay for a line with full service.

So, my advice is to understand all the possibilities before even trying to narrow down a range of amperage that will be useful.


Arthur P. Bloom
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DT98,
I run in to line voltage problems often now days. Cablecos are the worst. What you will have to do is have a vendor meet with your provider and the alarm vendor. The provider should be pushing 45-50 VDC. I witnessed 50 volts last week and the calls would drop after 20 sec. I reversed the polarity (because the provider could not disable Battery Reversal) and it cleared the problem on POTS lines. Weird but true.

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The basic hurdle in finding a detector is that voltage can be measured passively, in parallel with the line, but current needs to be measured in series with a load, and with the line off-hook.

It is a tariff violation to seize a dial tone without the intention of making a call. The only exceptions are the afore-mentioned VM stutter dial tone detectors. There was a lengthy legal battle to allow that type of circuit to be used in the PSTN. I know one of the expert witnesses who participated in the proceedings on behalf of the manufacturer of one of the detectors.


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What the alarm panels monitor is Voltage AND Current.

Since an RJ-31 jack routes the phone line THROUGH the alarm circuity, it is a both/and situation.

If you go off hook AHEAD of the RJ-31 jack, there will be a voltage drop, but NO current flow THROUGH the alarm circuitry, thus it will trip.

If the phone line is cut, again there will be a voltage drop (zero volts), and NO current flowing through the alarm circuitry, the alarm will trip. If it is a Fire Alarm, there will be a second line that it will use to report the trouble condition.

If you go off hook AFTER the RJ-31 jack, there will be a voltage drop, AND there will be current flowing through the alarm circuitry, and it will be happy with that.

So the alarm just needs to be set for the typical ON-HOOK voltage, usually -48VDC, but could be -24VDC.

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