By Jason Chesla / T3 Innovation
Utility equipment: Despite the growing demand for data and VOIP telephones, the analog telephone is still pretty much ubiquitous, especially in the residential market. These phones use a pair of simple copper wires that are usually bundled into a multi-cored cable and the service is known as the Plain Old Telephone Service or POTS.
The POTS service operates at a DC voltage of around 48 volts into a maximum impedance of 1700 ohms. This generally limits the maximum distance of the POTS service to three and a half miles from the exchange, although range extenders used in rural areas operating at 100 to 130 volts will increase that distance. When the phone rings, an AC voltage of around 90 volts is superimposed on the DC signal at a frequency of 20 Hz.
Telecom technicians use what is commonly known as a butt set for installing and maintaining these lines.
The Butt Set
A butt set, also known as a linesman handset, is a special type of handset that can be connected to a telephone line wherever there is a telecom connection point and is used to identify problems with voice lines. These devices have evolved from what were simple conventional telephone units adapted for field work into highly sophisticated testers that are capable of monitoring lines, measuring noise and identifying line faults.
Butt sets are intended for outdoor use, so are rugged, waterproof and able to withstand bumps and knocks. Butt set requirements are laid out in Telcordia's technical reference TR-TSY-000344, and only units complying with these requirements should be used. Some of the capabilities of butt sets include:
• Working on and identifying the characteristics of traditional analog POTS lines.
• Identifying a data line without interrupting data traffic.
• Differentiating between ADSL services and T1 lines.
• Testing line continuity and identifying faults, such as partial shorts and cross connections that degrade the signal.
• Measuring line voltage and current.
• Working with pulsed and tone-dialed circuits.
Butt sets may also have two-way speaker phone capabilities and can store commonly used numbers and recall recently called numbers.
Characteristics of a POTS Line
Because POTS lines are DC, the polarity of the signal is important for correct operation. Traditionally, these lines were connected using telephone plugs similar to those used for headphones. The tip of the plug was connected to the positive terminal of the battery, while the ring, which is separated from the tip by an insulating spacer, was negative. Consequently, the positive terminal became known as the tip and the negative as the ring. The positive terminal is usually connected to ground at the exchange. A diode bridge is often used to ensure that the polarity at the instrument is always correct.
The resistance of the telephone circuit is such that when the headset is off the hook, a current of approximately 22 milliamperes will flow through the circuit. If the DC voltage is low due to poor connections or excessive distance, the current will be lower, leading to a poor voice connection. It will also make the line susceptible to noise from nearby AC lines.
Traditionally, number dialing was accomplished by a series of pulses generated by a rotary dial. This was eventually replaced by tone dialing using a combination of two frequencies to represent a number. This is known as dual-tone multi-frequency dialing or DTMF for short.
Connecting to Lines
Because it's important to be able to easily connect the tester to telephone lines, piercing clip alligator clips were initially used that broke through the insulation of the telephone wire. However, these easily damage the insulation and cut through fine wires if not handled carefully. Newer butt testers generally use what is known as an angled bed of nails, which uses a clip fitted with a number of tiny spikes that pierce the insulation without cutting it or causing damage to the copper wire.
Using a Butt Tester
The butt set is used to measure the characteristics of the telephone line. Besides measuring the voltage, current and resistance of the circuit, a butt tester can monitor a circuit. In this mode it measures the frequency of the ringing signal and monitors the quality of actual voice calls that are being made.
Many butt testers are fitted with speaker phones so they can be used while the technician's hands are occupied, and the volume of the speaker can be adjusted to compensate for local noise levels.
Butt testers are designed for maintaining voice circuits and cannot be used to monitor and manage data lines. Although compatible with DSL and T1 lines carrying data, they are intended for setting up and maintaining basic voice circuitry. If used to tap into a line carrying data, they will interrupt the traffic. This is why the ability to be able to identify a data line is important.
Tel Scope™ Butt Set
One such tester is the Tel Scope from T3 Innovation. This unit epitomizes all the technical characteristics of a top-class butt tester along with the ability to measure key information and comply with Telcordia requirements.
This rugged unit is waterproof and is fitted with a large, easy-to-read backlit LCD screen. The screen displays the on-hook line voltage and off-hook current. It also displays the characteristics of the line and can measure the carrier frequency to identify if it is a DSL or T1 line. One particularly useful feature is the ability to identify issues with DTMF equipment when dialing out. The unit offers advanced data line in-use detection capability to prevent the loss of data.
T3 Innovation's Tel Scope is fitted with a glow-in-the-dark keypad to facilitate operation in dark locations as well as an adjustable output speaker and a noise-cancelling microphone. The unit comes standard with a 60-inch cable fitted with a bed-of-nails piercing clip cord set. Its body is ergonomically designed for comfort and incorporates a belt clip for easy use.