From What’s Old is New Again…Why Gundrum’s Law Still Applies to VDSL2/G.fast and IPTV Services, by Russ Gundrum, published by ISE Magazine
in September 2016.
“When the book, abc of the Telephone Volume 14 on Power Line Interference; Problems and Solutions was first published in 1982, my recommendation was this: ‘As a general rule of thumb from a maximum safety, equipment malfunction and noise reduction standpoint, one Induction Neutralizing Transformer should be required for every five miles of cable that is exposed to power lines.’”
“This configuration usually yields the best overall results and represents the most economical approach in controlling inductive interference on loops up to 5 miles (8 Km) in length. I also recommended placing three mitigative devices on the copper loop, most notably the Induction Neutralizing Transformer (INT).”
“Later, as a result of my trip to the Far East in 1983, I amended that advice for loops exceeding this 5-mile (8 Km) length. They should have multiple INT’s.”
“Though I had never previously referred to it as Gundrum’s Law, my experience in the jungle outside of Bangkok, Thailand changed that. I was asked to meet with one of the telephone company’s managers on a hot, humid Saturday, where he had brought his technicians together. Though I thought I was going to give a training session, I was surprised when the manager did instead. During this training, he kept describing a variety of electrical laws with names like Ohm and Kirchhoff.”
“Since the training session was held near the notorious area known as “The Golden Triangle” and gunfire could be heard in the distance, I was anxious to get back to Bangkok as soon as possible. In frustration, as I was leaving, I couldn’t help but write a message on the manager’s white board. Very simply I wrote, ‘This is Gundrum’s Law. If you just place an INT in your cable every 8 Km then you won’t have any more of these problems!’”
“After returning home, I decided to expound on the law, which led to the illustration below.”
“When I wrote the Bell System Practice (BSP) 873-505-107 on low voltage neutralizing transformers in 1974, I shared that if a cable could have more than 100 volts to ground (Vg) induced on it then two INTs would have to be installed in order to keep the induction below 50 Vg. This was the maximum voltage level allowed for personnel safety reasons in the Bell System.”
“Later, in my own book I amended that advice. I recommended 30 Vg was a better safety criterion to use, especially when public safety situations were considered. I also felt this was more of a reasonable design criterion for manufacturers of electronic circuit cards to meet, even though to this day, since there is still no such requirement, many devices won’t even tolerate 5 to 10 volts of induction!”
“I also knew from field experience that from a maximum power influence (noise-to-ground) suppression standpoint, greater circuit noise reductions could be obtained with more INTs being placed in a long cable route than just one.”
“So why is Gundrum’s Law still relevant after all these years, especially since we now have much shorter copper loops? Part of the answer is in an article the author wrote for ISE Magazine five years ago (then known as OSP Magazine) called ‘The Secret Lives of INTs—Bringing the INT into the 21st Century to Solve AC Interference Issues Affecting IPTV Services’”.
“But what about the last 3 Kf of copper, that is now in a less built-up suburban or rural area and the power loads can be very unbalanced as they may just be single phase power lines? Might not just one INT, placed at the VRAD, help reduce possible equipment failures/blown ports due to power and lightning surges, as well as help suppress higher frequency impulse noise that could cause quality of service (QoS) issues with the video signal?”
“That was partly behind the idea I gave SBC (now AT&T) in January 2002 for a lower voltage specially designed Digital Subscriber Loop (DSL) INT, because the original units that could handle up to 600 volts were no longer needed for such short loop applications. I said it should also be constructed of at least a Cat 5 or 6 twisted pair cable, because the original INTs that were made of Cat 3 cable could create a crosstalk problem at the higher DSL frequencies.”
“I hoped this research would help AT&T and other service providers justify the cost of this new network element as part of their engineering design in existing and planned deployments of IPTV services. Later field trials would document the increased revenues and cost savings that could be expected if this patented design element were included in these IPTV networks. The field trials would also show the potential cost savings by reducing VRAD line card failures and RG or set-top box replacements. That doesn’t take into account the expense associated with technician trouble reports or the long-term benefits of retaining more satisfied customers over time.”