020929linetest

Is it feasible to test the phone line from home?

tinmoorthy
 
Disclaimer :: While it is believed that, at the time of going to the press, all the information contained herein is correct, the author does not assume any responsibility for damages -- primary or secondary. You may experiment at your own risk. The data contained is believed to be relevant for most of the networks, yet some may vary on details

Keywords : test, phone line, jig, Light Emitting Diode, Home Electronics, Hobby

Is it possible to test the telephone line from the home? Instead of booking a compliant and triggering a test from that big building in downtown??

But before all that, ask me why would you ever want to test a line and that too from the home. That is because the telephone lines seem to go faulty without notice. The current may also lose its way after it reaches your building. One look at the tangled wires outside your apartment complex would convince you of this. Another reason may be that you wish to be notified before a fault develops

As a matter of fact now that the cables are laid in a medium of jelly (and less often of air), there is no reason why should they fail at all (The power wiring shares a similar technology, but does not seem to be that fragile nor so fault prone). However the fact remains -- the phone cables fail. Tomorrow, they say, the new optical fibers (now being laid) wouldn't fail at all, however much you may try. But telephone cables fail primarily due to physical damages consequent to the never-seem-to-be-ending road dig. The fault invariably manifests as water penetrating through the damage following the first incident rainfall. The moral appears to be -- if there was no rain there would practically be no cable fault either -- which is almost true. It is to be noted that water penetrating in to the cables causes a short circuit of the lines. Unlike a power line short circuit, which is spectacular, the telephone line short is barely noticeable, save the loss of dialing tone in your instrument! You may for some time get, if lucky, some juicy cross-talk!!

A road dig can also cause cables to be snapped. The symptoms of this fault are different. There is no dialing tone, there is no ringing, there is no nothing on the line -- the line appears dead. In either case -- short or a snap -- you can neither originate nor receive calls

This article does not tell how to repair a faulted cable -- those privileges rest with specific monopolies. This only tells how to recognize a developing or a developed fault on the telephone line. It is merely going to tell you how to test that telephone line from home -- as opposed to testing from the big building in downtown
 

 

A telephone line has to be continuous from the big building. While being so it will carry a potential of some fifty volts as direct current and shall have the vigor to drive at least, say, 20 milli-amperes through your telephone instrument. Two observations seem relevant here. The first one -- almost all electronic apparatus enjoy working with a direct current -- including your PC. Physical continuity, secondly, is what has to be promised by the line -- the call routing or ringing has to be arranged by a complex switch from inside that big building

When your line is healthy -- one, it carries a potential of about fifty volts (not normally lethal) and two, it shall drive a current of at least 20 mA when connected to a telephone instrument off hook. These are the necessary and sufficient conditions to declare the line as healthy. In fact the line robots at the big building confirm only these while testing

How can you test the presence of fifty volts potential on an idle line? Simple, use a voltmeter. If you do not have a voltmeter handy, you can exploit a hook up as shown above. It uses a few components to achieve the same purpose. It is a good suggestion to have the various components mounted as a small jig with two probes to help quick testing.

A faint glow of the Light Emitting Diode (LED for short) annunciates the availability of some voltage on the line. Also observe there is a bell push in the element. When this is pushed, the high resistance is shorted, the current on the line increases and the glow of the LED improves -- demonstrating more current is flowing on the line.

All direct current devices are fussy about the polarity -- you ought to note. This jig is no exception. If it does not seem to react, just reverse the terminals and watch -- it might coax the machine to get in to the mood! If no glow of the LED is perceived even after the probes are reversed, a faulted line is indicated

The device may not be left hanging on your telephone line. So use it on a need-to-confirm basis. 
 

Index

 

Aside : Okay, you have obtained a nondescript Light Emitting Diode. It is understood it will emit light once energized. They are basically two issues about which one needs to be careful

The first one relates to the polarity of the LEDs -- you have to drive a current in a direction in which it likes best. That is from (-) terminal through the (+) terminal. Too bad. Convention is that the terminal through which current exits is called the positive one. Let us now re-state the issue. How to identify that positive terminal of an LED? The lead which is shorter is normally the Positive -- but this is not a reliable clue. Next the bottom of the LED is not round as one would like to believe. There is a section which is somewhat flat than being formally circular. The lead nearest to that section is positive. This clue also tends to be weak and vague. The best is to test the LED for its diode characteristics with an ohm-meter. The lead identified with the red terminal of the ohm-meter is the positive you are seeking. If this is not confusing enough, wait till you came to the later chapters!

The second aspect of the LED is that it does not have any control on the current that flows though it. So you need to have external control that normally takes the form of a simple resistor

An LED is also sensitive to the reverse potential it can tolerate. That is why a reverse biased diode has been suggested for protection. This diode -- that could be similarly identified for its polarities -- makes sure the reverse potential, if any, does not exceed the drop across itself. Notwithstanding the precautions envisaged the jig is not intended to be applied on a domestic power line. It will just explode, if you do so. So do not attempt. Can it not be protected against such a slip up? Of course it can be -- but then it makes the jig far too complex. Besides this jig is meant only as a starter -- not a panacea for everything!