Q: How can the BT-10 aid in preventing damage to my telephone equipment?
A: It uses the principle of the Professional Lineman's trusty volt-meter, but with a twist. The BT-10 dynamically measures the battery voltage and gives you a consolidated indication with its Line/Ring Voltage display. The display will warn you of an over-voltage and potentially damaging over-current condition. Note that the BT-10 is easy enough for a novice to use, but it was designed with strict  Professional Guidelines and Specifications
Q: Why doesn't the BT-10 have a standard over-current indicator?
A: Because over-current in a telephone network is typically a function of the device under use and network source impedance. Quite simply, dynamically measuring battery voltage is more useful than over-current measurement techniques.
Q: Doesn't an over current indicator also tell you if the network voltage has exceeded operating limits?
A: No. As before, the likelihood of over-current (excessive loop current) is more a function of the device in use and source and impedance. For example:A 200 Ohm device load will meet the loop current requirements of an FCC part 68 approved device. Assuming a 200 Ohm test load, 56.5V battery (FCC maximum), and a 470 Ohm source impedance (FCC minimum), worst case loop current is 85 mA. An over-current tester could indicate a safe network. But a device can typically act like a 100 Ohm load. Under the same voltage and source impedance conditions the loop current would instead be 100 mA. Permanent damage can occur beyond 85 mA.Q: Line impedance, over-voltage, what does it have to do with over-current?
Now, let's say the source impedance is 510 Ohm and the battery voltage is 60V (over-voltage). You find that for the same 200 Ohm test load the worst case loop current is 85 mA. The over-current tester still indicates a safe network. Again, using a typical 100 Ohm device load, the loop current is 98 mA and potential damage to your telephone equipment can occur.
So the over-current tester's load would need to exactly match that of the desired telephone device under all conditions to accurately determine if an over-current condition exists. That just isn't ideal. Furthermore, the potentially serious battery over-voltage condition isn't detected.
What average testers perform is a line's source impedance check. But source impedance is rarely a problem. That's why Professionals can rely on a multifunctional BT-10 instead of difficult to use and inaccurate equipment.
A: As the previous example illustrates, if you just measure over-current as related to the line impedance, you still have not measured the capacity for equipment damage. All you have done is checked that a bad network can appear safe under an artificially unique test condition. Typical over-current tests assume that the line voltage is correct and load impedance is ideal. Acceptable source impedances can mask over-voltage problems, and actual device loads can vary considerably. Potentially costly mistakes.
Q: So over-current tests are invalid and all I need is a volt-meter, right?
A: No, a volt-meter is still not enough and satisfactory over-current tests are not found in ordinary pocket testers. But the BT-10's ability to dynamically measure a network can best warn you of a potential over-current condition. Also, a volt-meter is bulky and using one is too often cumbersome. You need the BT-10's modular RJ11 flexible connector and convenient receptacle to simplify the task.
Q: What about Ring Voltage, why does the BT-10 perform this test?
A: One of the most common indicators of an incorrectly installed or marginal network is poor ring voltage. The BT-10 is the only pocket test instrument that eliminates this uncertainty by indicating a network's ring voltage. Also, although an AC volt-meter can be used, in practice they are cumbersome and inaccurate since they are not optimized for telephone ring frequencies or duty cycles. The BT-10 is better suited for this task.
Q: My BT-10 shows that my Line Voltage and Polarity is good but my Loop Test indicator does not light. Have I done something wrong?.
A: Congratulations! You've found a problem with your network. This is quite common. We sometimes forget how a good telephone network sounds like; good volume and clear non-distorted conversations. What you are describing could be a central office or local problem.
First go straight to the incoming network interface and test the network. If the problem still shows then your provider's service is poor. However, if everything tests good at the network interface you may have a significant problem with what us engineers call "ohmic contacts." Easily put, a bad connection.
Here's what happens: When loop current drops the operating voltage of the phone equipment is reduced. The circuits get starved for power and can both distort or reduce the volume of your transmitter. Ohmic contacts are very common and usually occur in very new or very old installations. In old installations oxides build-up on contacts, and in newer installations contacts may have been poorly cleaned before terminating.
Engineers Note: (In talking with this customer over the same marginal phone line, his voice signal was evidently distorted and extremely low in volume. He solved his problem by cleaning and re-terminating his contacts. As a professional remedy we also suggested he use a contact preservative/cleaner. Another Happy Customer!)
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