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Electrical Tester online
June 2008
Protecting the Big Apple

Protecting the Big Apple

02 June 2008

Jeff Jowett - Applications engineer

Around 15,000 manually operated fire alarm boxes are a key element in the strategy that the New York Fire Department (NYFD) has implemented to protect the city’s eight million inhabitants. Keeping these boxes in working order has in the past been a real challenge, but time domain reflectometers (TDRs) from Megger are making the job much easier.

Manually operated fire alarm boxes (call points in European parlance) are widely used in the USA as a means of alerting the emergency services about fires. The system in New York City is one of the largest, and it has thousands of miles of underground cables linking boxes in the city’s five boroughs with each borough’s despatch centre.

A failure anywhere on this extensive network is a critical issue, as it could result in increased risk of damage to property, severe injury, or even death. For historical reasons, however, the cables used in this essential communication network are ordinary telephone grade copper twisted-pair types, similar to those found in residential telephone installations.
Unfortunately, these cables are prone to numerous problems that can affect their performance. They are, for example, prone to splicing errors, damage during construction or roadway repair work, degradation caused by immersion in standing water, and also gradual deterioration of insulation over time. These issues, and others, lead to faults that render one or more alarm boxes inoperative.

The alarm box network is maintained by NYFD Communication Division, whose engineers found that identifying the nature of a fault on the cable system by using an insulation tester or even an ordinary ohmmeter was relatively easy. Pinpointing the fault was, however, an altogether more difficult problem.
Colloquially known as ‘divide and conquer’ the traditional method of cable fault location involved identifying the faulty cable span, which would often be several thousand feet long. Then this was divided into two and tests performed on each section. In this way, a faulty section half as long as the original span was identified.
The process was then repeated until the fault had been narrowed down to a sufficiently short section to allow remedial action – usually involving road works – to be taken. This process is inconvenient, time consuming and expensive. Frequently accessing the cable for testing involved opening and pumping out manholes, followed by the breaking and remaking of cable splices.

The shortcomings of this approach can be readily appreciated when it is noted that it was by no means unusual for as many as ten tests to have to be carried out at different locations in order to locate a single fault.
In an effort to develop a better solution, NYFD approached Mohawk Limited, who proposed the use of the Megger CFL535F (known as the TDR2000/2 elsewhere) time domain reflectometer. Essentially, this works by sending a brief pulse down the faulty cable, and showing the reflections of this pulse on a graphical display. Cable faults of various kinds produce characteristic reflections and, by measuring the time period between the injection of the pulse into the cable and the receipt of the reflected pulse, the instrument can calculate the distance to the fault.

To introduce the NYFD team to the benefits of the CFL535F, Megger worked in conjunction with Mohawk Limited to provide training, part of which involved splicing together several reels of cable and introducing simulated fault conditions.

 

Tags: fault location, TDR