The New York City Fire Department (FDNY) relies on approximately 15,000 remote fire alarm boxes to alert them about fires or emergencies throughout this, the United States’ largest city (321 square miles; 8 million people). New York City is one of a number of cities in North America that still utilize the alarm box type of system to notify the fire department of fire or rescue situations. Among the other large USA cities using this type of system are Sacramento, San Francisco and Buffalo. In addition, a number of smaller cities and towns across the USA also still employ box alarm systems.

These alarm boxes are a key component of the first “line of defense” together with 911 telephone and central station notifications in FDNY’s ongoing battle to keep New York and its residents safe. Keeping the entire system operational is a critical challenge, as a failure at the time of a fire could result in severe injury or death, and the increased destruction of property. FDNY uses the Megger CFL535F Time Domain Reflectometer ( TDR) as part of its ongoing program to keep this system operational.

FDNY has a few thousand miles of underground cables throughout the five boroughs of New York City connected to Dispatch Centers in each borough. These centers monitor all activity on the system and orchestrate the response to an alarm. The cables used in this essential communication system are

telephone grade, twisted pair, copper cables similar to those found in residential telephone systems. As such, the system faces the same potential problems that can occur in a standard telephone system, only with much more grave consequences.

Cables of this type are prone to numerous types of problems that can hinder or prevent system performance, including shorts, opens, splits, water ingress and intermittent faults. All of these cable troubles can be located using TDR technology. These cable faults can be caused by a wide range of external factors, including splicing errors, construction damage (eg: backhoe trenching), standing water or simply general insulation degradation that occurs naturally over time. When faults of this type occur, they can render the alarm box location inoperative resulting in lost time in responding to emergencies, or cause false alarms to the dispatch centers, which cause resources to be dispatched in non emergency situations, wasting both time and money.

The fire alarm box system is maintained by FDNY’s Communications Division. While identifying the nature of the fault condition on the cable system is relatively easy using an insulation tester or even a volt/ohmmeter, the difficulty comes in pinpointing the actual location

of the fault so that remedial action can be taken to get the system operational again. After evaluating the issues faced by the Communications Division and the nature of their system, Mohawk Limited (Chadwicks, NY) introduced the Megger CFL535F Time Domain Reflectometer (TDR) as a solution to their fault locating challenges. A detailed explanation of TDR technology follows at the end of this article.

FDNY purchased a single CFL535F to use and evaluate before committing to outfit their maintenance team with the instrument. The evaluation was successful, as they were able to locate a number of faults with the instrument and reduce the time required to fix problems. After determining that the CFL535F was the right tool for their team, FDNY decided to equip the remainder of their technicians with the instrument. The next challenge faced by FDNY was to educate their maintenance staff to be proficient in the use of the instrument.

Megger, in conjunction with support from Mohawk Limited, provided the FDNY communications technicians with training on how to operate the TDR, how to recognize trouble conditions and how to pinpoint the distance to faults using the instrument. FDNY spliced several reels of spare cable together to simulate actual field conditions and situations, which allowed the technicians a hands-on experience during the training. Technicians were able to work with and learn to identify splices, bridge taps, opens and shorts, and to determine the distance to each event. By understanding these events, technicians can bypass splices and bridge taps, and focus on true fault conditions.

FDNY is taking advantage of the latest technology to maintain its alarm system.

distance to each event. By understanding these events, technicians can bypass splices and bridge taps, and focus on true fault conditions.

FDNY is taking advantage of the latest technology to maintain its alarm system. By evaluating and implementing changes in its maintenance program, and partnering with Megger and Mohawk Limited, FDNY is reducing the time required to locate and repair cable-based faults that would prevent the system from operating properly.

The dispatch center monitors the alarm system 24 hours a day, 7 days a week, 365 days a year, and when problems are detected, a communications technician is dispatched to locate and correct the problem in order to put the alarm circuit back in service. Prior to the introduction of the CFL535F, the technician relied on the old method of “divide and conquer” fault locating. This approach can be a very time consuming and labor intensive process, as it involves cutting the distance of several thousands of feet of cable sections in half, setting up the work area, pumping water out of the manholes, opening the cable, testing the circuit in each direction from that point, closing up the cable, breaking down the work area, then moving toward the direction of the fault, only to start the same process again at the next location. It can take as many as 10 or more set-ups before the fault could be located and repaired depending on the length of the cable route.

One of the features that proved very useful for FDNY was the dual trace capability of the CFL535F. By using L1 on a known good circuit and L2 on the bad circuit, and then comparing the two wave forms side by side, one or two possible trouble locations would stand out. By going to those locations first, the “divide and conquer” method could be reduced from 8-10 locations, to 1-2 locations, thus saving considerable time

and labor. As the crews become more familiar with the instrument, they will be able to reduce the time further.

Megger offers several models of TDR’s to choose from, including the TDR900 (digital display only; hand-held), TDR500 (full trace display; hand-held), CFL510F (full trace display; hand-held), and the CFL535F (full trace display; full featured; portable). FDNY chose the CFL535F because it offered features unique to their purposes such as intermittent fault find, better resolution, and trace storage/download.

Time Domain Reflectometer (TDR) Overview

A Time Domain Reflectometer uses simple transmission line theory and pulse reflection principals to detect impedance changes along a cable. The TDR transmits high frequency electrical pulses. When applied to a cable these pulses travel through the cable until a change in characteristic impedance is encountered. Depending on the nature of the impedance change either all or part of the transmitted pulse will reflect back to the TDR. A full-trace TDR will display a representation of cable and the various impedance changes encountered. The operator can then measure the distance to the impedance change using

the instrument’s internal distance calculation algorithm.

TDR Basic Operation

Impedance is the total resistance, inductive reactance and capacitive reactance encountered in a cable. A change in a cable’s characteristic impedance will cause one of two types of reflections: positive or negative. Positive reflections are caused by increases in impedance. Increased impedance results from increases in metallic resistance or inductive reactance. Negative reflections are caused by decreases in impedance. Decreased impedance results from decreases in insulation resistance or changes in capacitive reactance.

Characteristic Impedance Model

Reading a TDR is similar to reading a map. Before reading a map you must learn what the symbols mean and b efore reading a TDR you must also learn what the TDR signatures represent.

When measuring distances to an event on a TDR, the TDR measures the time it takes a pulse to travel down a cable, encounter an impedance change, and reflect back. By knowing the velocity of the pulse the TDR converts this time to distance. Pulses travel at different velocities on different cables much like a ball travels at different velocities through different liquids. The type of insulation and cross section geometry of a cable will affect the velocity of a pulse.

Common Telephony Velocity Factors

If the velocity factor of a cable is unknown it can easily be determined by connecting onto a sample cable of known length. Place the TDR’s cursor at the reflection representing the end of the cable. Simply adjust the velocity setting until the unit reports the correct length. This setting will be the velocity of propagation for the cable. If neither the velocity nor length of the cable is known an accurate locate can still be accomplished by measuring the distance to the fault from both ends of the cable. If an error exists in the velocity setting the TDR will either over measure or under measure from both ends of the cable. The fault will be between the two measurements.

For further information concerning this application, please contact Mike Lewis (mike.lewis@megger.com) at 214-330-3518. Mike Lewis is a Senior Application Engineer at Megger, specializing in communications-related applications. He has over thirty years of field experience working with Southwest Bell.